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.