Risk factors for unexplained antepartum fetal death in Norway 1967–1998

Abstract

Objective: To relate unexplained antepartum fetal death with maternal and fetal characteristics in order to identify risk factors. Design: Population-based study based on records of 1,676,160 singleton births with gestational age ≥28 weeks. Unexplained antepartum fetal death was defined as fetal death before labour without known fetal, placental, or maternal pathology. Results: Although unexplained fetal mortality in general declined from 2.4 per 1000 births in 1967–1976 to 1.6 in 1977–1998, the proportion among all fetal deaths increased from 30% to 43% during the same period of observation. Unexplained fetal death occurred later in gestation than explained. From 39 weeks of gestation, the risk increased progressively to 50/10,000 in women aged ≥35 years and <10/10,000 in women <25 years. In birth order ≥5, the risk was particularly high after 39 weeks of gestation. For birth weight percentile 2.5–9.9 and ≥97.5, unexplained fetal death was four and three times more likely to occur, respectively. We found an additive effect of maternal age and birth weight percentile 2.5–9.9. Women with less than 10 years education had higher risk than women with 13 years or more (OR=1.6). Weaker associations were observed with female gender, unmarried mothers, and winter season. Conclusions: Unexplained antepartum fetal death occurred later in gestation than explained and was associated with high maternal age, multiparity, low education, and moderately low and high birth weight percentile. The increased risk in post-term pregnancies and the additive effect of maternal age and birth weight percentile 2.5–9.9 suggests that older women would benefit from monitoring of fetal growth.

Introduction

During the last decades, the perinatal mortality rate in Norway has declined, although the occurrence of stillbirths has remained relatively stable [1]. Many of these stillbirths remain unexplained [2]. Information on risk factors for such unexplained fetal death is scarce.

Second and third trimester stillbirths are considered to comprise two distinct subgroups; one with a gestational age peak at 24 weeks and another at term [3]. The earlier subgroup includes late spontaneous abortions caused by malformations, chromosomal abnormalities, and congenital infections [4]. The late subgroup includes many cases with an apparently normal pregnancy in which no specific problems were documented but which resulted in an antepartum fetal death [5], [6], [7]. In many ways, these cases suggest a clinical entity, and the term ‘SIDS (sudden infant death syndrome) in utero’ or sudden intrauterine unexplained death has been introduced [7]. The present study focuses on third trimester stillbirths.

Established in 1967, the Medical Birth Registry of Norway holds information of more than 1,800,000 pregnancies and births including information about more than 10,000 stillbirths [1]. Registry data on unexplained fetal death have been validated (Rasmussen S et al. Unexplained antepartum fetal death in Norway 1985–1997: diagnostic validation and some epidemiological aspects. In press) with the conclusion that the Registry may serve as a basis for large-scale epidemiological studies of unexplained fetal death. The objective of the present article was, on this basis, to study maternal and fetal characteristics in order to establish risk factors for unexplained antepartum fetal death.