Breast cancer risk in mothers of twins.

The risk of breast cancer associated with delivering a twin birth was examined in a population-based nested case-control study of nearly 4800 Swedish women with breast cancer and 47000 age-matched control subjects. All were aged less than 50 years and parous. After adjustment for age at first birth and parity, a 29% reduction in breast cancer risk was observed in mothers of twins relative to those who were not (odds ratio = 0.71, 95% confidence interval 0.55-0.91). These results provide evidence that women who bear twins are at reduced risk of breast cancer, one explanation for which may be their unusual levels of hormonal exposure.

Preterm birth and low birthweight among children of Swedish and immigrant women between 1978 and 1990.

The aim of the study was to estimate the occurrence of low birthweight (LBW) and preterm birth among immigrant and Swedish women in Sweden. Eligible for analysis were all 1,270,407 singleton births in Sweden between 1978 and 1990 to mothers aged between 15 and 44 years, whose own country of birth was known. The mothers of the children were born in Sweden (88.2%), or had immigrated from Finland (4.4%), other Scandinavian countries (1.2%), Western Europe or North America (1.3%), Eastern Europe (1.8%), the Middle East and North Africa (1.7%), Central and South America (0.6%), Asia and the Pacific Islands (0.6%), or Sub-Saharan Africa (0.2%). Multiple logistic regression was used to model LBW and preterm birth categorical outcomes. Each immigrant group was compared with the Swedish group. Odds ratios (ORs) for LBW were 1.13 (95% CI 1.04, 1.22) for Asia and the Pacific Islands, 1.21 (1.05, 1.38) for Sub-Saharan Africa and 0.89 (0.86-0.93) for Finland. Odds ratios for preterm birth were 1.15 (1.08-1.23) for immigrants from Asia and the Pacific Islands and 1.08 (1.04, 1.13) for immigrants from Eastern Europe. Remarkably small differences were found between immigrant women and native Swedish women.

Prenatal and neonatal risk factors for childhood myeloid leukemia.

Information about the etiology of childhood myeloid leukemia is limited. A population-based nested case-control study of prenatal and neonatal risk factors for childhood myeloid leukemia was performed with the use of the Swedish National Cancer Register and the Swedish Birth Register. A total of 98 cases of myeloid leukemia were identified in successive birth cohorts from 1973 through 1989. From the Birth Register, five controls were matched to each case. Fourteen of the 98 cases with myeloid leukemia and none of the controls had Down syndrome [odds ratio (OR) = infinity; 95% confidence interval (CI) = 21.0-infinity]. The risk for myeloid leukemia also increased among children who had physiological jaundice (OR = 2.5; 95% CI = 1.2-5.0; children who had been treated with phototherapy (OR = 7.5; 95% CI = 1.8-31.9); or who had been treated in an incubator (OR = 3.5; 95% CI = 1.2-10.2). Excluding cases with Down syndrome, however, decreased these risks, so that their 95% lower confidence interval included the no-effect value. Maternal age < 20 years old (OR = 2.5; 95% CI = 1.1-6.0), hypertension (OR = 2.4; 95% CI = 1.2-5.0), Cesarean section (OR = 2.5; 95% CI = 1.3-4.9), maternal smoking (OR = 2.4; 95% CI = 0.9-6.5), and being one of a multiple birth (OR = 3.6; 95% CI = 1.1-11.3) increased the risk for myeloid leukemia among those without Down syndrome. When the analyses were repeated, by restricting the cases to those with acute myeloid leukemia, the risk associated with young maternal age declined and became nonsignificant.(ABSTRACT TRUNCATED AT 250 WORDS)

Breast cancer in Swedish women before age 50: evidence of a dual effect of completed pregnancy.

We set out to detect a transient increase in risk of breast cancer following childbirth, the existence of which has been postulated, but for which empirical evidence is contradictory. Breast cancers and births occurring among the cohort of Swedish women born after 1939 were linked, yielding 3,439 cases and 25,140 age-matched controls with at least two children. Within three years of their last childbirth, women had an estimated rate of breast cancer of 1.21 (95 percent confidence interval [CI] = 1.02-1.44) times that of women whose last birth was 10 or more years earlier, after adjustment for parity and age at first birth. Further analyses suggested that this effect reflected, in part, a small transient increase in breast cancer risk that lasts for about three years following completed pregnancy. The effect of age at first birth on breast cancer risk appears to be confounded by time since last birth; the parity-adjusted rate ratio for having a first birth at age 35 years or more compared with under 20 years is reduced from 1.72 (CI = 1.14-2.58) to 1.36 (CI = 0.88-2.09) on additional adjustment for time since last birth. A transient increase in breast cancer risk after childbirth appears thus appears to account for part of the effect of age at first birth on breast cancer risk.

Prenatal and neonatal risk factors for childhood lymphatic leukemia.

BACKGROUND: Because the incidence of childhood acute lymphatic leukemia peaks between 2 and 4 years of age, the risk factors may exert their influence during the prenatal and/or the neonatal periods. Results of previous studies of perinatal risk factors have been contradictory, perhaps because most studies either have been hospital based or have been restricted to limited geographical areas. PURPOSE: A nationwide case-control study was carried out to identify maternal and perinatal risk factors for this disease. METHODS: The case-control study was nested in cohorts defined by all live births in Sweden recorded in the nationwide Medical Birth Register. Since 1973, this register has routinely collected information on all hospital births in regard to maternal demographic data, reproductive history, pregnancy, delivery, and the neonatal period. From the Swedish National Cancer Register, 613 case subjects were identified in successive birth cohorts from 1973 through 1989. Five control subjects per case subject were randomly selected from the pool of children matched by sex and month and year of birth. Conditional logistic regression was used to calculate the odds ratios (ORs) and 95% confidence intervals (CIs) for potential risk factors and to estimate their effects after adjustment for possible confounders. RESULTS: Risk of childhood lymphatic leukemia at all ages increased with Down's syndrome (OR = 20.0; 95% CI = 4.2-94.2), maternal renal disease (OR = 4.4; 95% CI = 1.6-12.1), use of supplementary oxygen (OR = 2.3; 95% CI = 1.5-3.6), postpartum asphyxia (OR = 1.8; 95% CI = 1.2-2.6), birth weight of more than 4500 g (OR = 1.7; 95% CI = 1.1-2.7), and hypertensive disease during pregnancy (OR = 1.4; 95% CI = 1.0-1.9). Down's syndrome affected risk mostly in children younger than 5 years, whereas other factors affected those children 5 years old or older. Being one of a multiple birth also increased risk among older children (OR = 2.5; 95% CI = 1.0-6.0). Use of supplementary oxygen may act as a causal intermediate (surrogate) for postpartum asphyxia and its causes, as would high birth weight for its causes. CONCLUSIONS: Several maternal and perinatal risk factors were found to be associated with childhood lymphatic leukemia, but they showed age-specific differences. Overall, only a few risk factors were identified, and these accounted for a small proportion of cases. We concluded that most risk factors for childhood lymphatic leukemia remain unidentified in very young children.

Administration of vitamin K to newborn infants and childhood cancer.

OBJECTIVES: To investigate whether childhood cancer is associated with intramuscular administration of vitamin K to newborn infants. DESIGN: Routines for administration of vitamin K to infants born after normal deliveries during 1973-89 were obtained from maternity hospitals. Occurrence of cancer up to the end of 1991 was identified by comparing these records with the national cancer registry. Adherence to the routine method of administering vitamin K was checked with the medical records of a sample of 396 infants (196 who had developed childhood cancer and 200 controls). SETTING: All maternity hospitals in Sweden. SUBJECTS: 1,384,424 full term infants born after non-instrumental deliveries, 1,085,654 of whom were born in units where vitamin K was routinely given by intramuscular injection and 272,080 of whom were born where it was given orally. MAIN OUTCOME MEASURES: Odds ratios for cancer after intramuscular administration of vitamin K versus oral administration after stratification for year of birth. RESULTS: Adherence to routine method of administering vitamin K was 92% in the 235 cases where individual information could be found. The risk of cancer after intramuscular administration of vitamin K was not elevated compared with that after oral administration: odds ratios of 1.01 (95% confidence interval 0.88 to 1.17) for all childhood cancers and 0.90 (0.70 to 1.16) for childhood leukaemia. CONCLUSIONS: The alleged association between intramuscular vitamin K prophylaxis to newborn infants and childhood cancer could not be verified in the present study of full term infants born after non-instrumental delivery.

Cancer risk in asthmatic subjects selected from hospital discharge registry.

We wanted to study cancer risk in asthmatic subjects. With the use of central health registries, a cohort of 64,346 patients, treated in hospital with an asthma diagnosis, was followed with respect to cancer development. The observed numbers of different types of cancer were compared with the expected numbers, estimated from population data, with consideration taken to patient age, sex, survival, and the year of diagnosis. In general, a marked reduction of cancer incidence (2 out of 3 of the expected numbers) was found, with the exception of two cancer types: cancer of the respiratory tract and cancer of endocrine glands. A more noticeable reduction in cancer risk was seen for multiple myeloma, malignant melanoma, mammary cancer, uterine body cancer, and stomach cancer. The causes of this "protective effect" are not indicated by the present analysis, and need further study.

Psoralen photochemotherapy (PUVA) and pregnancy.

BACKGROUND AND DESIGN: Psoralen photochemotherapy (PUVA) may carry a risk of mutagenesis and teratogenesis. Using a medical birth registry and other health registries, infants born to women who had been treated with PUVA were identified. Exposure information was obtained from treating hospitals and from the license-granting agency. A total of 504 infants were born of pregnancies occurring after PUVA treatment, and 689 infants were born of pregnancies occurring before such treatment. In another 14 cases, treatment occurred during pregnancy. RESULTS: No increase in infant or child mortality or in the presence of congenital malformations could be seen after PUVA treatment. There was a marked increase in low-birth-weight infants when pregnancy occurred after treatment, and this is probably not explained by maternal smoking, but could be an effect of the underlying disease. CONCLUSIONS: The theoretical mutagenic and teratogenic effect of PUVA treatment apparently does not carry any significant risk for abnormal delivery outcome.

Drug intoxication during pregnancy: a study with central registries.

By linking two health registries, one on all infants born and one on hospital discharges, 424 infants were identified born after pregnancies when the mothers had been hospitalized for an intoxication, mainly with drugs. In 126 infants, exposure had occurred during the organ forming period, 99 of them because of suicidal or accidental overdosage of drugs; 70 of these were psychoactive drugs. None of the 70 infants had a congenital malformation, but the upper 95% confidence limit includes a 10% risk increase. An effect on low birth weight rate was seen only after drug complications following medical treatments during weeks 12 to 30. The study supports an earlier study from Hungary that reported that drug intoxication during pregnancy does not carry a substantial teratogenic risk.

A registry study of very low birthweight liveborn infants in Sweden, 1973-1988.

Very low birthweight liveborn infants (less than 1,500 g, VLBW) born in Sweden 1973-88 were identified from the Medical Birth Registry and efforts were made to remove wrongly recorded birthweights--9% of infants with a registered birth weight below 1,500 g were removed. Some VLBW infants were not recorded in the register and the estimate of the prevalence at birth of VLBW infants is therefore slightly underestimated. It increased from about 5.5 per 1,000 during the period 1973-84 to 6.7 per 1,000 during 1987-88. 18% of VLBW infants were involved in multiple births. Median Apgar score at 5 min increased for each 100 g birthweight class. Even at a birthweight between 1,400 and 1,499 g, a low Apgar score at 5 min was seen in 20%. The rate of cesarean section increased between 1973 and 1983 from about 10% to 60%. One-year survival for infants with a birthweight less than 1,000 g increased from less than 20% in 1973-75 to 50% in the 1986-88 cohorts. Corresponding figures for infants with a birthweight between 1,000 and 1,499 g were 60% and 90%. A markedly better one-year survival is already evident in the 600-699 g class. On stratifying for 100 g birthweight class, perinatal death risk was higher in boys than in girls and higher in multiple births than in singletons. An increased rate of congenital malformations was seen in the 1,000-1,499 g class but not in the less than 1,000 g class.

Surveillance of smoking during pregnancy in Sweden, 1983-1987.

Since 1982, prospective information on smoking during early pregnancy is reported to the Swedish Medical Birth Registry for nearly all (93%) women who give birth. The present paper studies the validity of this information: effects on birthweight and perinatal mortality are very similar to those described previously in the literature. For each parity class, smoking decreases in inverse proportion to increasing maternal age; for each maternal age class, smoking increases with parity. A slight reduction in smoking rate is observed between 1983 and 1987, most pronounced for young women. There are marked geographic and social differences in the rate of smoking during pregnancy. This dataset can be used in the future to monitor the prevalence of smoking, and to study various factors associated with smoking and the impact of countermeasures taken against smoking during pregnancy. It can also be used to study possible associations between maternal smoking and rare events like congenital malformations and child cancer.

A quality study of a medical birth registry.

A quality control study was made of the Swedish Medical Birth Registry. This registry used one mode of data collection during 1973-1981 and another from 1982 onwards. The number of errors in the register was checked by comparing register information with a sample of the original medical records, and the variability in the use of diagnoses between hospitals was studied. Different types of errors were identified and quantified and the efficiency of the two methods of data collection evaluated.