Epidemiological studies of stillbirth and early neonatal death

Abstract

The aim of this thesis was to investigate risk factors for stillbirth and early neonatal death. The associations between matemal weight, weight gain during pregnancy, hemoglobin concentration, socioeconomic status and stillbirth risk were assessed in case-control studies nested within the Swedish Medical Birth Register. 725 cases of stillbirth were identified among 220,712 single births to primiparous women within a geographically defined area in central Sweden between 1987 and 1996. For each case, two controls were randomly selected, matched by year and hospital of birth. Prospectively collected information from antenatal, obstetric and neonatal records was retrieved for 702 cases and 702 controls (retrieval rate, 97% and 96%, respectively).

Compared to lean women (body mass index (BMI) <19.9), overweight (BMI 25.0-29.9) and obese (BMI >=30.0) women were at increased risk of antepartum. stillbirth, especially term antepartum stillbirth. Among obese women, the risk of antepartum stillbirth was partly mediated by an increased prevalence of gestational diabetes, preeclampsia or eclampsia. Matemal weight gain during pregnancy was not associated with risk of antepartum. stillbirth.

The risk of stillbirth was increased among women with a high hemoglobin concentration (>=146 g/1) in early pregnancy. Restricting the analysis to non-malformed preterm and small-for-gestational-age (SGA) antepartum stillbirths, or excluding women with preeclampsia and eclampsia further increased these risks. Average weekly change in hemoglobin concentration during early or late pregnancy was not significantly associated with risk of stillbirth, although a larger decrease tended to be protective.

Compared to women with high socioeconomic status, women with low socioeconomic status were at more than a two-fold increased risk of stillbirth. The increase in risk was even higher for intrapartum and term antepartum stillbirths. Although an extensive number of matemal and pregnancy characteristics were investigated, the increased risk of stillbirth for women with low socioeconomic status could not be explained.

The relations between time of birth and risks of intrapartum stillbirth and early neonatal death were assessed in a population-based cohort study of 694,888 singleton births with vaginal onset of delivery in Sweden between 1991 and 1997. Infants of high-risk deliveries were more often delivered during daytime. Compared to infants born during daytime, infants born at night were at increased risk of early neonatal death, but not intrapartum death. There was no association between weekend or holiday births and risks of intrapartum. or early neonatal death.

The influences of interpregnancy interval and previous reproductive history on the subsequent risks of stillbirth and early neonatal death were investigated in a nation-wide study of 410,021 women's first and second singleton delivery between 1983 and 1997. Long, but not short interpregnancy intervals were associated with increased risks of stillbirth and early neonatal death. A previous adverse outcome such as stillbirth, early neonatal death, and preterm or SGA delivery were generally strong and independent risk factors for stillbirth and early neonatal death in the subsequent pregnancy.