Calcium intakes and femoral and lumbar bone density of elderly U.S. men and women: National Health and Nutrition Examination Survey 2005-2006 analysis.

OBJECTIVES: This analysis was aimed at assessing the benefits of total calcium intake from diet and supplements on both femoral neck and lumbar vertebral bone mineral density (BMD) in a representative sample of older U.S. women and men. DESIGN: For 1384 women and men aged 50-70 and 71+ yr, quintiles of total calcium intake were tested for their association with hip and spine BMD after adjusting for body mass index. All data in this observational study were cross-sectional. DATA SOURCE: Subjects included elderly residents statistically representative of the United States, women and men aged 50 yr and older in the National Health and Nutrition Examination Survey 2005-2006 cohort. MAIN OUTCOME MEASURES: Calcium intakes and femoral and lumbar BMD were evaluated. RESULTS: Total calcium intakes ranged from means of 400+ mg/d in quintile 1 to 2100+ in quintile 5. Little difference in hip or lumbar BMD was found in relation to total calcium consumption in women and men across five quintiles, especially for those aged 50-70, in models adjusted for body mass index only. Femoral hip BMD in men 71 and older increased slightly with high calcium intake (3.6% higher density, P = 0.0391), whereas femoral BMD in women 71 and older decreased slightly with high calcium intake (-3.2%, P = 0.0132). Lumbar BMD remained fairly consistent across all quintiles, but greater variation within each quintile was found compared with the hip. CONCLUSIONS: A usual high calcium intake beyond the recommended dietary allowance of elderly women and men, most commonly achieved by calcium supplements, did not provide any benefit for hip or lumbar BMD. A dietary intake of calcium approaching or meeting the current recommendations was not related to higher BMD of the hip or lumbar spine in late life compared with lower intakes of calcium in older adults.

Using mixture models with linear predictors to identify incorrect gestational age in state birth records.

BACKGROUND: Birthweight distributions for early last-menstrual-period-based gestational ages are bimodal, and some birthweights in the right-side distribution are implausible for the specified gestational age. Mixture models can be used to identify births in the right-side distribution. The objective of this study was to determine which maternal and infant factors to include in the mixture models to obtain the best fitting models for New Jersey state birth records. METHODS: We included covariates in the models as linear predictors of the means of the component distributions and the proportion of births in each component. This allowed both the means and the proportions to vary across levels of the covariates. RESULTS: The final model included maternal age and timing of entry into prenatal care. The proportion of births in the right-side distribution was lowest for older mothers who entered prenatal care early, higher for teen mothers who entered prenatal care early, higher still for older mothers who entered prenatal care late, and highest for teens who entered prenatal care late. Over 44% of births were classified as incorrect reported gestational age. CONCLUSION: These results suggest that (1) including these two covariates as linear predictors of the means and mixing proportions gives the best model for identifying births with incorrect reported gestational age, (2) late entry into prenatal care is a mechanism by which erroneously short last-menstrual-period-based gestational ages are generated, and (3) including linear predictors of the mixing proportions in the model increases the validity of the classification of incorrect reported gestational age.

Micronutrient intakes in two US populations of older adults: lipid research clinics program prevalence study findings.

Dietary intakes of several minerals and vitamins were assessed in two US sub-populations of older men and women between 60 and 80 years as part of the Lipid Research Clinics Program Prevalence Study conducted in the mid-1980s prior to widespread fortification. Dietary intakes were analyzed from 24-hour recalls using the Minnesota Nutrition Coding Center. Descriptive statistics on the two diverse sub-populations were generated for the elderly subjects at the two clinic sites, southern California and Oklahoma. Regression analyses of specific micronutrients were performed while controlling for several variables, namely, age, sex, clinic (region), education, Body Mass Index (BMI), alcohol consumption, and smoking status. Compared to current (1999-2004) Estimated Average Requirements (EARs) and Adequate Intakes (AIs) for three micronutrients without EARs for the US and Canada, several micronutrients were consumed at or close to their EAR values. Exceptions include intakes of vitamin A, vitamin E, folic acid, potassium and calcium which were very low; intakes of thiamin, riboflavin, niacin, and vitamin C were low but closer to the published EAR or AI values. High intakes approaching cut-offs for practically all subjects were found for both groups of elders at the two clinic sites for iron, phosphorus, and sodium. In general, California elderly had somewhat better consumption patterns for the vitamins, but the Oklahomans, males at least, had higher overall mineral intakes. The micronutrient deficits found in this small study suggest that most elderly US citizens were likely to be deficient in five micronutrients and marginally insufficient in four others in the mid-1980s and, despite even greater fortification currently, elderly intakes seem not to have improved substantially since the 1980s, except for subjects who are regular multi-supplement users.

The multistate life table method: an application to contraceptive switching behavior.

In many demographic behaviors (e.g., those relating to marriage, contraception, migration, and health), people change among multiple statuses through time, sometimes leaving and then returning to the same status. Data on such behaviors are often collected in surveys as censored event histories. The multistate life table (MSLT) can be used to properly describe, in a single analysis, these complex transitions among multiple states measured in such data, but MSLT is rarely applied in the demographic literature because practical guidance is lacking on how to compute MSLTs with such data. We provide methods for computing MSLT quantities using censored event-history data: namely, transition intensities and probabilities, "state occupancy" probabilities and standard errors, average time spent in specified states, and average number of visits to specified states. Applying these methods to contraceptive use, we find high levels of switching back and forth, particularly between barrier methods and non-use, resulting in high rates of unintended pregnancy.

Macronutrient intakes of elderly in the Lipid Research Clinics Program Prevalence Study.

As part of the Lipid Research Clinics (LRC) Program Prevalence Study of coronary heart disease risk factors, nutrient intakes of two US populations over 59 years of age were determined by 24-hour recalls in the 1970s. Characteristics of the populations were (1) California: suburban, upper-middle class, 95% high school graduates, 10% blue collar occupations; (2) Oklahoma: rural lower-middle class, 75% high school graduates, and 40% blue-collar occupations. Macronutrients consumed by both populations were similar, except for alcohol. For both men and women, energy intake was approximately 25 kcal/kg/day (body weight) sources of energy were approximately 40% from carbohydrate, 16% from protein, 37% from fat, and 4% from alcohol. The Oklahoma population, however, consumed significantly less alcohol than did Californians. Percentages of calories from fatty acids were approximately 14% from saturated and 6% from polyunsaturated, which yielded a polyunsaturated: saturated ratio of 0.48. The intake of cholesterol for women was 190 mg/1000 kcal and for men, 210 mg/1000 kcal. Between the ages of 60 and 69, the Oklahoma men consumed more energy than did the California men. Both sexes demonstrated lower energy intakes with advancing age and with increasing body mass index. The higher energy intake of the Oklahoma cohort aged 60-69 was attributed to the greater physical demands of their occupations, but this difference disappeared after age 70. The California and Oklahoma women had similar caloric intakes beyond age 60. In summary, the LRC findings suggest that geographically diverse American populations consumed in the late 1970s remarkably similar intakes of macronutients and cholesterol, with the only major exception being energy from alcoholic beverages.

Using the overdispersed exponential family to estimate the distribution of usual daily intakes of people aged between 18 and 28 in Taiwan.

Assessment of usual dietary intake of a population is essential to understand the nutrition status of the population. Methods like 24-hour dietary recall and food records are commonly used for this task. However, within-individual variation exists in the food record data. Methods considering measurement errors have been used to evaluate the association between nutrient intakes and diseases. Few have been devoted to estimating the distribution of usual daily intakes. This paper proposes applying the formulation of the overdispersed exponential family to estimate the distribution of usual nutrient intake and applies the adjustments developed by Liu to reduce the variance of the distribution. The proposed method has the advantages of working on the original scale of data and implementation convenience. The adjustment of the variance is carried out by dividing the variance into within-individual variance and among-individual variance. The adjusted variance, then, is used to estimate the distribution of the usual daily intake. Sampling weights are considered in all the steps, except the estimation of the ratio of within to among variance. The data for this study are from the Nutritional and Health Survey in Taiwan conducted between 1993 and 1996 (NAHSIT, 1993-1996). An independent external set of data for people aged between 18 and 28 years (49 males and 20 females) is used to estimate the ratio of within to among individual variance. Due to the availability of data in estimating the ratio of within to among individual variance, 430 males and 422 females in the NAHSIT sample are used to estimate the distribution of usual daily intakes for people aged between 18 and 28 in Taiwan. The distribution also allows us to estimate the proportion of people who do not meet the recommended daily nutrient allowance (RDNA). The results show that the intakes of calcium and vitamin E of this group of people fall far below RDNA.

Influences of mother's work, childhood place of residence, and exposure to media on breast-feeding patterns: experience of Nigeria and Uganda.

This study uses data from the Nigerian Demographic and Health Survey collected in 1990 and the Ugandan Demographic and Health Survey collected in 1995 to examine the implications of mother's work, childhood place of residence, and exposure to the media for breast-feeding patterns (exclusivity and intensity) in Nigeria and Uganda. Nigeria and Uganda present an interesting contrast because Nigeria is more modernized and economically developed than Uganda, thus providing a good indication of the influence of modernization on breast-feeding patterns. Mother's work status is defined by considering whether mothers earned cash from work and took their children to work, hence emphasizing the compatibility of work with child care. Work least compatible with child care had a negative effect on breast-feeding intensity in Nigeria. The negative effect of mother's work on exclusive breast-feeding (that is, if the mothers used formula or milk instead) observed for some working mothers in Nigeria and Uganda was partly confounded by urban residence, exposure to media, and other socioeconomic factors. Mother's work did not have a negative effect on breast-feeding intensity in Uganda. The relationship between mother's work, urban residence, media exposure, and breast-feeding practice seems to be stronger in Nigeria than Uganda.

The paucity of effects in community trials: is secular trend the culprit?

BACKGROUND: The most recent major U.S. trials that evaluated community-level programs to influence risk factors and health behaviors identified secular trends in the risk factors and health behaviors among the factors that might have limited community-level effects. The research reported in this paper uses data from one of the trials to examine the secular trend explanation directly. METHODS: Data from the 22-community Community Intervention Trial for Smoking Cessation (COMMIT) were analyzed to test a hypothesis based on secular trend reasoning: program effects for smoking prevalence were larger for treatment communities matched to control communities with small declines in smoking than in treatment communities matched to control communities with larger declines in smoking. RESULTS: Consistent with the secular trend explanation, program effects were larger when control communities had relatively small declines in smoking prevalence. CONCLUSIONS: The findings suggest that secular trends masked community-level program effects in COMMIT.

Risk factors for horizontal transmission of hepatitis B virus in a rural district in Ghana.

Most hepatitis B virus (HBV) infections in sub-Saharan African infants and children are acquired through horizontal transmission, but the exact mechanisms of spread have not been documented. The authors conducted a study in rural Ghana which determined seroprevalence in a probability sample of 1,385 individuals of all ages, and evaluated risk factors for horizontal transmission of HBV in a subsample of 547 children aged 1-16 years who were not hepatitis B surface antigen (HBsAg) carriers. Most residents in this district live in compounds which typically contain 2-4 households each. Overall prevalence of HBV seropositives (any HBV marker) was 74.7% (95% confidence interval (CI) 72.5%-76.9%). Prevalence of HBsAg was 20.9% (95% CI 18.8%-23.1%). The data suggest a continuous nonuniform acquisition of HBV infection with advancing age predominantly through horizontal transmission in childhood, with the household, rather than the domestic compound, being the primary place for transmission. The behaviors most strongly associated with prevalence of HBV were sharing of bath towels (OR = 3.1, 95% CI 2.1-4.5), sharing of chewing gum or partially eaten candies (OR = 3.4, 95% CI 2.3-5.0), sharing of dental cleaning materials (OR = 2.5, 95% CI 1.3-4.6), and biting of fingernails in conjunction with scratching the backs of carriers (OR = 2.5, 95% CI 1.6-4.3).

PIP: Most hepatitis B virus (HBV) infections in sub-Saharan African infants and children are acquired through horizontal transmission. Findings are reported from a study conducted in rural Ghana to measure seroprevalence in a probability sample of 1385 people of all ages, and evaluate risk factors for the horizontal transmission of HBV in a subsample of 547 children aged 1-16 years who were not hepatitis B surface antigen (HBsAg) carriers. Most residents in the sample area live in compounds which typically contain 2-4 households each. The overall prevalence of HBV seropositives was 74.7% and the prevalence of HBsAg was 20.9%. These data suggest a continuous nonuniform acquisition of HBV infection with advancing age mainly through horizontal transmission in childhood, with the household, rather than the domestic compound, being the main place for transmission. The sharing of bath towels, sharing of chewing gum or partially eaten candies, sharing of dental cleaning materials, and biting of fingernails together with scratching the backs of carriers are the behaviors found to be most strongly associated with HBV prevalence.

Analysis of contraceptive discontinuation in six developing countries from durations of use at survey.

We conduct a cross-national study of contraceptive discontinuation among currently married nonsterilized contracepting women in Bolivia, Egypt, Kenya, Sri Lanka, Thailand, and Zimbabwe using the Demographic and Health Surveys (DHS). Since the DHS contains no true completed epochs of contraceptive use, the distribution of use times at survey is used to approximate the distribution of the completed epochs using the renewal theorem. Two techniques based on this approximation are used. The first technique uses local linear regression smoothing of a histogram estimate of the use time at survey pdf which is converted into an estimate of the discontinuation probability function. The second technique poses a proportional hazards Weibull distribution for the discontinuation probability function which is then converted into a model for the use times at survey. This second technique is used to model the observed variations in use across countries while controlling for other sociodemographic factors such as children ever born, age, and education, as well as a variable which encodes knowledge of the fertility cycle. Pill discontinuation probabilities range from 0.12 to 0.47 in the first year. IUD discontinuation probabilities range from 0.18 to 0.53 in the first year. Discontinuation probabilities in Egypt over all methods are in agreement with those reported in Ali and Cleland (1995). Logged relative risks of pill discontinuation range from -0.94 (Sri Lanka) to 0 (Kenya), while logged relative risks of IUD discontinuation range from -0.53 (Sri Lanka) to 0.41 (Zimbabwe). The ordering of risks of pill discontinuation among the six countries considered is in agreement with the ordering of total fertility rates excerpted from Westoff (1991).

Sudden infant death syndrome and local meteorologic temperature in North Carolina.

The association between meteorologic temperature and sudden infant death syndrome was investigated in the 1982-1983 North Carolina birth cohort. Maximum daily temperatures recorded at weather stations in the subject's county of residence for each day of the first year of life were entered into hazards models as time-dependent covariates. Risk ratios for a maximum temperature of < or = 53 degrees F (12 degrees C) 5 days before the event compared with a maximum temperature of > 53 degrees F were 2.3 (95% confidence interval 1.6-3.3) for blacks and 1.5(95% confidence interval 1.0-2.1) for whites. Similar results were found for minimum daily temperature. The analysis controlled for season of birth, sex, maternal age, maternal education, parity, and birth weight.

Some analytical models to estimate maternal age at birth using age-specific fertility rates.

"A class of analytical models to study the distribution of maternal age at different births from the data on age-specific fertility rates has been presented. Deriving the distributions and means of maternal age at birth of any specific order, final parity and at next-to-last birth, we have extended the approach to estimate parity progression ratios and the ultimate parity distribution of women in the population.... We illustrate computations of various components of the model expressions with the current fertility experiences of the United States for 1970."

The National Epidermolysis Bullosa Registry.

Since its inception in 1986, the NEBR has proved to be an excellent example of how a relatively small allocation of federal research funds for the development of a registry of cases of a single rare disease can have a major impact on the rapid expansion in the depth of knowledge of not only the disease itself but of a number of associated biologic principles, including keratinization and epithelial cell-extracellular matrix interactions. At present, the NEBR is generating extensive clinical, laboratory, and demographic data, both from cross-sectional and longitudinal perspectives, as well as establishing a centralized cell and tissue bank that will serve the scientific community at large as a valuable resource for future basic research on this oftentimes devastating genetic disease.

Smoothed breastfeeding durations and waiting time to conception.

Breastfeeding beyond the resumption of mother's menstruation plays a significant role in the proximate determinants of fertility. Breastfeeding and postpartum amenorrhoea data collected from retrospective surveys usually exhibit digit preferences. Here, these heaping errors were smoothed by B-spline and used in multivariate models of risk of conception to investigate the contraceptive effect of breastfeeding. The data used come from a 1987 Indian survey. Results show that lactation, after mother's menses resume, reduces the risk of conception. Heaping in breastfeeding data attenuates this relationship. When adjustment is made, breastfeeding reduced the rate of conception by 47 per cent; the reduction, with adjusted data, was 63 per cent.

PIP: Clinical evidence supports the relationship between breast feeding and postpartum amenorrhea (PPA). The effect of heaping errors (digit preference at months 3, 6, 9, 12 of breast feeding and PPA) in studying the relationship between breast feeding and PPA in the context of time-dependent hazard models is examined, with a B-spline smoothing technique to adjust for the heaping errors in the data. The relationship among PPA, breast feeding after menses resume, and waiting time to conception are also examined with and without adjustments. A retrospective survey entitled Breastfeeding and its effect on fertility conducted in 1987 by Banaras Hindu University, Varanasi, India, provided the basic data. Data (age at marriage, children ever born, lactation, PPA) were collected from 1100 urban and 900 rural households of Varanasi. Women were classified into low, medium, and high social groups. Univariate life table technique was used to calculate total breast feeding, for which median duration was 16.25 months and 13.35 months, respectively, with and without smoothing. Similarly, the median duration of PPA was 7.82 and 5.82 months, respectively. For closed (uncensored) intervals the time of conception was estimated by subtracting 9 months from the month of delivery. The overall median duration of waiting time to conception after menstruation was more than 15 months. The heaping of breast feeding PPA showed reduced effect on the chance of conception after menstruation. Without smoothing adjustment the rate of conception after menstruation was reduced by 47% if the woman was breast feeding at that time. With adjustment the reduction increased to 63%. Younger mothers had significantly higher rates of conception once menses returned, while literate women had significantly less chance of conception than illiterate ones after menses. For every additional month of breast feeding there was a 1.8% (3.3%) reduction in the risk of conception after return of menses without (with) smoothing breast feeding and PPA. Breast feeding had a significant effect on the duration of PPA.

Plasma triglyceride level and mortality from coronary heart disease.

BACKGROUND: Whether the plasma triglyceride level is a risk factor for coronary heart disease has been controversial, and evaluation of the triglyceride level as a risk factor is fraught with methodologic difficulties. METHODS: We studied the association between plasma triglyceride levels and the 12-year incidence of death from coronary heart disease in 10 North American populations participating in the Lipid Research Clinics Follow-up Study, while adjusting for the potential confounding effects of other risk factors for cardiovascular disease, including the level of high-density lipoprotein (HDL) cholesterol. All analyses were sex-specific, and separate analyses were performed in high and low strata of HDL cholesterol, low-density lipoprotein (LDL) cholesterol, fasting plasma glucose, and age. RESULTS: The rates of coronary death in both men and women increased with the triglyceride level. In Cox proportional-hazards models adjusted for age, in which the natural log of the triglyceride levels was used to give a normal distribution, the relative risk per natural-log unit of triglyceride (e.g., a triglyceride level of 150 mg per deciliter vs. a level of 55 mg per deciliter) was 1.54 (95 percent confidence interval, 1.19 to 1.98; P < 0.001) in men and 1.88 (95 percent confidence interval, 1.19 to 2.98; P < 0.007) in women. After an adjustment for potential covariates, however, these relative risks were not statistically significant. Analyses based on lipoprotein cholesterol levels revealed a positive association between the triglyceride level and coronary mortality in the lower stratum of both HDL and LDL cholesterol, but not in the higher stratum. Conversely, the HDL cholesterol level was unrelated to coronary mortality in the lower stratum of LDL cholesterol, but was strongly inversely associated with coronary death in the higher stratum of LDL cholesterol. The relative risk of coronary death associated with triglyceride level was higher at younger ages. The associations between the triglyceride level and coronary mortality in the lower HDL cholesterol, LDL cholesterol, and age strata were small and were further reduced by an adjustment for the fasting plasma glucose level. CONCLUSIONS: Overall, the plasma triglyceride level showed no independent association with coronary mortality. However, in subgroups of subjects with lower HDL and LDL cholesterol levels and in younger subjects, defined a priori, an association between the triglyceride level and coronary mortality was observed, although this association was small and was not statistically significant after an adjustment for the plasma glucose level.

Analysis of birth intervals in India's Uttar Pradesh and Kerala States.

Life tables of birth intervals and median birth intervals in two Indian states, Uttar Pradesh and Kerala, were computed for several subgroups of the study population. Multivariate hazards modelling technique was used to examine the net effect of each of the variables studied. The results show a substantial effect of socioeconomic variables in child-spacing after controlling for the major intermediate variables.

PIP: Child spacing patterns have been found to be reflected in the interaction between a variety of biological and social factors in India. Birth intervals differ from Western patterns: long lengths of postpartum amenorrhea and prolonged breast feeding. Cox proportional hazards models and descriptive statistics are used to examine birth intervals among 3514 households in the Varanasi district of Uttar Pradesh state in 1978, and among 3000 households from 3 districts in Kerala state in 1980. Women using contraception or who were sterilized were excluded from the analysis. Birth spacing among Urrar Pradesh women was 44 months from marriage to first birth and 32 months between subsequent births. In Kerala, the first birth interval was 20 months and subsequent intervals were up to 30 months. In Uttar Pradesh, the median birth interval declined as age at marriage increased; in the sample populations, age-at-marriage groups differed widely in their birth intervals. Birth intervals also varied with household economic status. Based on the social status index and educational status of husband and wife in Uttar Pradesh, differences in median birth intervals were minimal but large for the extent of childbearing. 28% of higher class women had a 5th child compared to 2% among low status groups. When controlling for marriage age and other variables, the proportional hazards model showed that all groups, other than the high status group, had a statistically significant higher risk of a birth. When using education as the status measure in Kerala, the findings on birth interval were similar, but the proportion having a 2nd birth was higher among lower status groups. Husband's education did not have much effect on birth interval. Religion did not strongly affect the risk of birth other than for first births in Uttar Pradesh. In Kerala, there were shorter birth intervals among Muslims and a higher probability of parity progression to the next birth. Muslims and Christians versus Hindus had a significantly higher adjusted risk of birth. Infant mortality shortened birth intervals for women in Uttar Pradesh by 9 months, and the adjusted risk of a next birth was 2 times higher. Kerala results were similar, but replacement was less likely with higher order births. Both states had slightly longer birth intervals after a male child.

Estimation of fecundity and secondary sterility from survey data on birth intervals in Egypt.

Data on the last closed and open birth intervals have been used to ascertain the current potential of childbearing for women in terms of estimating fecundity and secondary sterility by age, residence, and educational subgroup. Under the assumption that after a specific period from the last birth a certain proportion of women become secondarily sterile, we propose and apply an inflated model of open birth interval to obtain the proportion of women who are secondarily sterile. The data used for the analysis are extracted from the Egyptian Fertility Survey conducted in 1980.

PIP: Fecundity is estimated from last closed birth interval (LCBI) data using a steady state model. An inflated model of theopen birth interval (OPI) is derived on the assumption that the birth interval from first birth is marked by a certain proportion of women who become secondarily sterile or choose to be so to avoid further births. Biases such as truncation, censoring, and selectivity are overcome by using birth intervals and a stochastic model. Data were obtained from the 1980 Egyptian Fertility Survey to estimate fertility and secondary sterility of noncontracepting, currently married women by age, residence,and educational status. The model is specified, the data identified, and the process of application presented. The gestation period is assumed to be 9 months, and postpartum amenorrhea (PPA) constructed by age, parity, and duration of marriage. The constant period of nonsusceptibility is considered to be the gestation period of 9 months plus the average value of PPA by socioeconomic group. In the inflated model of OBI, a truncated model of OBI is constructed at different points designated as the risk of occurrences (lambda) and the probability of a birth after the first birth (alpha), which are obtained by the Gauss-Newton and Marquardt iterative methods. The results show the expected: that fecundity decreases with age. After 25 years of age, the fecundity parameter lambda shows a steady and diminishing decline as age increases. There is some evidence of declines in fecundity from rural to urban areas. No schooling to 3 years plus of schooling shows a similar pattern. The patterns of women aged 30 years reflects the opposite education effect, which might be interpreted as high coital frequency in modern marriages and among more educated women. The inflated model of OBI follows similar trends, but OBI data have higher estimates of fecundity for younger women and lower estimates for older women than LCBI data. Age patterns of secondary sterility increase until age 30; the proportion of sterile women increases thereafter.

Effects of breast feeding after resumption of menstruation on waiting time to next conception.

We investigate the association between breast feeding after resumption of menstruation and the duration from resumption of menses to the next conception. Data from a survey, "Breast-Feeding and Its Effect on Fertility," conducted in 1987 under the auspices of the Centre of Population Studies, Banaras Hindu University, India, were used. We used hazard models of conception rates after the return of menstruation with breast-feeding duration as a time-dependent covariate. The interaction of breast-feeding duration after resumption of menses and postpartum amenorrhea have a significant effect on the rate of conception after return of menses. After resumption of menstruation, when there is no breast feeding, the risk of conception increases with the increase in postpartum amenorrhea. However, breast feeding attenuates the effects of postpartum amenorrhea. These results suggest that breast feeding beyond the resumption of menstruation plays a significant role in the proximate determinants to reduce fertility.

Age and season of birth in sudden infant death syndrome in North Carolina, 1982-1987: no interaction.

Sudden infant death syndrome (SIDS) is a major cause of death in the postneonatal period. SIDS peaks in the winter and at age 3 months. The hypothesis that season and age interact to determine SIDS survival was tested in race-specific hazards models that included an interaction term for season of birth and survival time. The study population was the 1982-1984 and 1985-1987 North Carolina birth cohorts. The interaction term had null effect in all models, indicating that season and age are independent determinants of SIDS survival. These results may be confounded by exposure to cigarette smoke, for which no data were available.