Do pregnancies with pre-eclampsia have smaller placentas? A population study of 317 688 pregnancies with and without growth restriction in the offspring

Objective: To study whether placental weight is related to pre-eclampsia risk, independent of offspring birthweight.

Design: Registry study.

Setting: Medical Birth Registry of Norway.

Population: All singleton pregnancies in Norway from 1999 to 2004, 317 688 births.

Methods: Placental weight was grouped into deciles of placental weight z-scores. The proportion of pregnancies in each placental weight decile was calculated by maternal pre-eclampsia status for pregnancies with and without small-for-gestational-age (SGA) offspring.

Main outcome measures: Pre-eclampsia risk (proportions and odds ratios) according to placental weight.

Results: In pregnancies with SGA offspring, approximately 60% of pregnancies were in the lowest decile of placental weight, 59.9% in pregnancies with and 61.4% in pregnancies without pre-eclampsia. Pregnancies without SGA offspring were evenly distributed across placental weight deciles, but were slightly higher in the lowest (9.5% versus 8.5%) and highest (11.9% versus 10.2%) deciles in pre-eclamptic pregnancies compared with pregnancies without pre-eclampsia. A weak U-shaped association of placental weight with pre-eclampsia was also estimated as odds ratios in pregnancies with SGA, but not without SGA, in the offspring.

Conclusions: Placental weight is linked to the offspring's birthweight, but is not clearly associated with pre-eclampsia risk, suggesting that placental weight is not a useful indicator for the placental dysfunction in pre-eclampsia.