Home monitoring of uterine contractility. Summary of a workshop sponsored by the National Institute of Child Health and Human Development and the Bureau of Maternal and Child Health and Resources Development, Bethesda, Maryland, March 29 and 30, 1989.

At an interdisciplinary workshop on home monitoring of uterine activity, participants reviewed the basis of this technology and its use in the identification of women at high risk of preterm delivery and in the prevention of preterm birth. Although the guard-ring devices in current use appear capable of detecting uterine activity, they do not clearly distinguish between Braxton Hicks contractions and the contractions of early labor. There was agreement that women destined for preterm delivery have more uterine activity on average than do other women of comparable gestational age but that there is substantial overlap between the two groups. Thus it is uncertain whether this difference can be used effectively for screening purposes. Conferees agreed that there is considerable evidence that twice-daily monitoring of very-high-risk women, in conjunction with daily nursing support and high-quality obstetric care, may prevent preterm births. Available evidence does not clearly distinguish the contribution of tocodynamometry from that of nursing support. A number of areas were identified in which further research is needed.

Double jeopardy: twin infant mortality in the United States, 1983 and 1984.

The United States Linked Birth/Infant Death Data Sets: 1983 and 1984 Birth Cohorts from the National Center for Health Statistics were used to identify maternal and infant characteristics related to twin infant mortality; 41,554 white and 10,062 black live-born matched twin pairs were evaluated. Twin birth weight distribution was skewed with 48% of white and 63% of black twins born weighing less than 2500 gm. Overall infant mortality rates were 47.1 and 79.3 deaths per 1000 live births for white and black twins, respectively (five times the rates for singletons). Three fourths of deaths were among twins weighing less than 1500 gm. White like-gender twins had about twice the risk of both twins dying compared with unlike-gender twins. Likewise, white twin pairs with greater than 25% birth weight disparity had a 40% to 80% increased risk of both twins dying, compared with twins whose weights were within 10% of each other. Twins born to high-risk women (on the basis of demographic factors) were twice as likely to die as twins born to low-risk women. Thus strategies to decrease twin infant mortality must address both maternal and infant risk factors.

Comparison of infant mortality among twins and singletons: United States 1960 and 1983.

Infant mortality among US black and white twins and singletons was compared for 1960 and 1983 using the Linked Birth/Infant Death Data Sets from the National Center for Health Statistics. Both twin and singleton infant mortality rates showed impressive declines since 1960 but almost all of the improvement in survival for both twins and singletons was related to increased birth weight-specific survival rather than improved birth weight distribution. One-half of white twins and two-thirds of black twins weighed less than 2,500 g at birth, and 9% of white twin births and 16% of black twin births were in the very low (less than 1,500g) birth weight category. In 1983, twin infant mortality rates were still four to five times that of singletons. However, twins had a survival advantage in the 1,250-3,000 g range, which persisted after adjustment for gestational age. Cause-specific mortality among twins was considerably higher for every major cause of death: twin mortality risks due to newborn respiratory disease, maternal causes, neonatal hemorrhage, and short gestation/low birth weight were six to 15 times that of singletons. The lowest twin-to-singleton mortality ratios observed were for congenital anomalies and sudden infant death syndrome with relative risks twice that of singletons. The data underscore the need to develop effective strategies to decrease infant mortality among twins.

Racial differences in pregnancy outcomes.

Black mothers in the United States are twice as likely as their white counterparts to experience a wide variety of adverse pregnancy outcomes, including prematurity, low birth weight, and infant and fetal death. Although blacks have higher proportions of births with maternal risk factors such as young age, high birth order, low education, and unmarried mothers, these differences do not account for their higher rates of adverse outcomes; the reasons for the excess remain largely unknown. To develop effective interventions to reduce the racial disparities in pregnancy outcome, we must further our understanding of the mechanisms underlying premature onset of labor and intrauterine growth.

Racial differences in low birth weight. Trends and risk factors.

To identify the risk factors responsible for differences in birth weight between blacks and whites, we investigated the effects of four maternal characteristics (age, parity, marital status, and education) on rates of very low birth weight (less than 1500 g) and moderately low birth weight (between 1500 and 2500 g). Using 1983 national data, we found that the black:white rate ratio was 3.0 for very low birth weight and 2.3 for moderately low birth weight. The four maternal factors had directionally similar but quantitatively different effects on very low and moderately low birth weight among blacks and whites. Furthermore, the racial differences in infants' birth weights were greater among low-risk than among high-risk mothers, especially for very low birth weight (black:white ratios of 3.4 and 1.7, respectively). We also examined secular trends in the rates of low birth weight among blacks and whites. Between 1973 and 1983, births of infants with moderately low birth weights decreased more among whites than among blacks, whereas births of infants with very low birth weights increased among blacks and decreased among whites. Fifteen percent of the decline in the rate of moderately low birth weight among whites could be attributed to favorable changes in maternal characteristics (primarily an increase in educational level). Among blacks, adverse changes in maternal characteristics (primarily an increase in births to unmarried women) accounted for 35 percent of the increase in the rate of very low birth weight. The adverse effects of childbearing by teenagers on the outcome of pregnancy among blacks appear to have been overemphasized. The persistence of large racial differences in birth weight, even among mothers at low risk, emphasizes the need for specific targeted interventions to achieve further improvements in infant health.

Perceptions of risks of smoking and heavy drinking during pregnancy: 1985 NHIS findings.

As part of the Health Promotion and Disease Prevention Questionnaire administered in the 1985 National Health Interview Survey, nearly 20,000 respondents ages 18-44 answered questions about their awareness of the risks of smoking and heavy drinking during pregnancy. In reference to smoking, interviewers asked about miscarriage, stillbirth, prematurity, and low birth weight; in reference to heavy drinking, they asked about miscarriage, mental retardation, low birth weight, and birth defects, as well as fetal alcohol syndrome. For each of these adverse outcomes, a majority of subjects acknowledged increased risk because of smoking or heavy drinking during pregnancy. The range was 66-80 percent of respondents for the four questions on smoking, with the perceived association to smoking strongest for low birth weight. Approximately 84 percent of respondents associated heavy drinking with increased risk for each of the suggested pregnancy outcomes. Smoking seemed to be perceived to pose a lesser risk to pregnancy than heavy drinking. This relative lack of awareness of the pregnancy risks of smoking was more apparent among respondents with less education and more pronounced among blacks than whites. Women were more likely than men to express some opinion on these pregnancy-related questions and were more cognizant than men of the risks. On this limited survey, Americans ages 18-44 were not very knowledgeable about fetal alcohol syndrome. Among the 55 percent who had heard of fetal alcohol syndrome, fewer than one in four correctly identified it as a set of birth defects when offered three possible definitions. It will be interesting to correlate responses to these "knowledge" questions with NHIS data still forthcoming on reported actual smoking and drinking behavior among women respondents who were recently pregnant.

Uses of the 1980 National Natality and Fetal Mortality Surveys for the assessment of obstetric technologies.

Because of the multipurpose utility of followback surveys, six Public Health Service (PHS) agencies including the Bureau of Health Care and Delivery Assistance (BHCDA) financially assisted the National Center for Health Statistics (NCHS), which conducted the 1980 National Natality Survey (NNS) and 1980 National Fetal Mortality Survey (NFMS). These are called "followback" surveys because NCHS uses mail questionnaires and telephone to follow back informants (mothers, hospitals, and attendants at delivery) and medical sources of radiation named by informants, identified on Certificates of Live Birth and Reports of Fetal Death from all 52 state and independent registration areas of the United States. Ten basic data applications have emerged from the 1980 NNS/NFMS, and this paper addresses these 10 purposes simultaneously, in relation to NNS/NFMS uses in assessing obstetric technologies. The five obstetric technologies that are the focus of this paper are: cesarean delivery, electronic fetal monitoring, ultrasound, X-ray, and amniocentesis--and are often examined in relation to maternal complications and conditions. Data on these technologies are highlighted with reference to each of the following 10 data applications of followback surveys: produce national estimates of unique information; establish trends; facilitate analytic and epidemiologic research; use followback surveys as anchor data bases; undertake parallel studies; link followback surveys together to calculate new vital rates; conduct comparative studies vis-à-vis other data sets; assess volume of medical care; evaluate quality of information on vital records; and generate synthetic estimates and need assessments. Finally, this paper discusses NCHS plans to measure obstetric technologies in the 1988 National Maternal and Infant Health Survey, which will include natality, fetal mortality, and infant mortality components.

Low birth weight across generations.

The relationship between maternal birth weight and infant birth weight was studied in the Buffalo cohort of the Collaborative Perinatal Project (n = 1,348). Regression techniques were used to control for confounders. Compared with infants of mothers who had weighed 8 lb or more at birth, infants of mothers who had weighed 6 to 7.9 lb, 4 to 5.9 lb, and less than 4 lb at birth were 99 g, 244 g, and 170 g lighter, respectively. The adjusted odds ratios of giving birth to an infant of birth weight less than 2,500 g, with mothers of birth weight 8 lb or more as the reference group, were 3.46 and 1.66 for mothers of birth weight 4 to 5.9 lb and 6 to 7.9 lb, respectively. There were no infants less than 2,500 g among the 24 women who had weighed less than 4 lb at birth. These data suggest a possible role of genetic and familial factors in determining birth weight.

The changing pattern of low birth weight in the United States--1970 to 1980.

Analysis of US Natality Statistics using a bivariate (birth weight-gestational age) approach shows that trends among term-low-birth-weight infants (less than or equal to 2,500 g; greater than or equal to 37 weeks, term low birth weight) and preterm-low-birth-weight infants (less than or equal to 2,500 g; less than 37 weeks, preterm low birth weight) have different patterns over time and by race. Between 1970 and 1980 the incidence of preterm low birth weight for all races declined 7.1%, while the term-low-birth-weight incidence declined almost three times as much (20.9%). The incidence of preterm low birth weight among white infants, during the same period, declined 9.0% with the black preterm-low-birth-weight rate declining by 5.8%. The term-low-birth-weight incidence, however, was 24.6% lower among whites and 14.9% lower among black term-low-birth-weight infants for the same period. The reduction in the overall low-birth-weight incidence for both populations is principally caused by reduction in the incidence of term-low-birth-weight infants. Term- and preterm-low-birth-weight infants reflect different etiologic insults and require different intervention strategies for prevention.

Trends in maternal and infant health factors associated with low infant birth weight, United States, 1972 and 1980.

Today, low birth weight--weight less than 2,500 grams (5 pounds 8 ounces)--is recognized as the single most significant characteristic of the newborn associated with a greater risk of mortality and morbidity. This paper highlights maternal and infant health factors associated with low birth weight, using data from the 1972 and 1980 National Natality Surveys. Both surveys provide important information related to low birth weight that is not typically available from standard birth certificates and therefore not part of the usual natality vital statistics. In addition, induction of labor and cesarean section deliveries (first and repeat cesareans) were examined for 1980 to assess whether their use has exerted an influence on low birth weight.