Birthweight to placental weight ratio and placental pathology in clinically unanticipated stillbirths: a retrospective cohort study

INTRODUCTION: Clinically unanticipated stillbirths may result from placental dysfunction that is not detected during pregnancy. We investigated the association between clinically unanticipated stillbirths and placental dysfunction assessed by the birthweight-to-placental-weight (BWPW) ratio and placental histopathology. METHODS: This retrospective cohort study included placentas from 295 stillbirths at ≥24 weeks' gestation (2000-2017) at a tertiary referral centre. Placental pathology reports were retrieved from a national registry and re-evaluated according to the Amsterdam consensus criteria. Stillbirths were classified as due to placental causes, other identified causes, or clinically unanticipated. Groups were compared for BWPW ratios and placental lesion distribution. The primary outcome was the proportion of abnormal BWPW ratios (<10th or >90th percentile). RESULTS: Of 295 stillbirths, 109 (36.9%) were due to placental causes, 57 (19.3%) to other causes, and 129 (43.7%) were clinically unanticipated. BWPW ratio distributions were similar between placental cause and clinically unanticipated stillbirths. Abnormal BWPW ratios occurred in 71.3% of clinically unanticipated stillbirths, comparable to placental causes (70.6%, p = 0.91) but higher than other causes (54.4%, p = 0.03). Histological review (n = 182) showed maternal vascular malperfusion predominated in placental cause stillbirths, while fetal vascular malperfusion was more frequent in clinically unanticipated stillbirths (16.9% vs 5.7%). CONCLUSIONS: Clinically unanticipated stillbirths show placental abnormalities comparable to placental cause stillbirths, despite later gestation and often normal birthweight. These findings suggest that many unanticipated stillbirths result from unrecognized placental dysfunction. Systematic macroscopic and histological placental examination may aid postmortem classification and risk assessment in subsequent pregnancies.