The Auckland Stillbirth study, a case-control study exploring modifiable risk factors for third trimester stillbirth: methods and rationale

Background: In high-income countries, stillbirth rates have been static in recent decades. Unexplained stillbirths account for up to 50% of these deaths. Methods: A case-control study was conducted in Auckland, New Zealand, from July 2006 to June 2009 to explore modifiable risk factors for late stillbirth (>= 28 weeks of gestation). Eligible participants were women who had a singleton late stillbirth without a congenital abnormality. Two controls with ongoing pregnancies were randomly selected at the same gestation as each case. Data were collected through face-to-face interviews and from clinical records. Results: A total of 155/215 (72%) cases and 310/429 (72%) controls consented to take part in the study. Women who had a late stillbirth were more likely to be of Pacific ethnicity and of parity >= 4 (OR = 1.7, 95% CI: 1.1-2.6 and 2.7, 95% CI: 1.4-5.3, respectively). The median gestational age at diagnosis of fetal death was 261 days (IQR 239-279), and the median gestation at which the controls were interviewed was 264.5 days (IQR 240-274) P = 0.48. 'Unexplained antepartum death' (n = 61, 39.4%) and 'fetal growth restriction' (n = 29, 18.7%) accounted for almost 60% of stillbirths. The post-mortem rate for all cases was 47% (73/155) and 43% (26/61) for those classified as 'unexplained antepartum death'. Conclusion: This study of risk factors for stillbirth is novel in that it used gestation-matched controls with ongoing pregnancies. Its detailed investigation into maternal health and behaviour during pregnancy has the potential to lead to a better understanding of modifiable risk factors for late stillbirth.