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.