Objective. Preterm and term pre-eclampsia may differ in etiology. This could be reflected in differences in placenta weight. Therefore, we compared placenta weight in pregnancies with preterm or term pre-eclampsia to placenta weight in pregnancies without pre-eclampsia. Design. Population study. Setting. Medical Birth Registry of Norway. Population. All singleton pregnancies in Norway from 1999 through 2004 delivered at or after 21 weeks’ gestation (n = 317,688). Methods. Placenta weight in pregnancies without pre-eclampsia (n = 304,875) was compared to placenta weight in pregnancies with preterm pre-eclampsia (delivery before week 37 of pregnancy, n = 3,070) and term pre-eclampsia (delivery on or after week 37 of pregnancy, n = 9,743). Placenta weight z-scores were calculated to adjust for offspring sex and length of gestation, and grouped in tenths. Placenta weight according to pre-eclampsia status is presented as proportions within each tenth. Main outcome measures. Pre-eclampsia status. Results. In preterm pre-eclampsia, placentas were over-represented in the two lowest (33.8%; 95% CI 32.1–35.5) and under-represented in the two highest (13.1%; 95% CI 11.9–14.3) tenths of placenta weight compared to pregnancies without pre-eclampsia (20%). In term pre-eclampsia, placentas were over-represented in the two lowest (22.0%; 95% CI 21.2–22.8) and the two highest (22.7%; 95% CI 21.9–23.6) placenta weight groups. Mean placenta weight z-score was higher in term pre-eclampsia compared to pregnancies without pre-eclampsia. Conclusions. Small placentas were associated with pre-eclampsia, and more strongly with preterm than term pre-eclampsia. In term pre-eclampsia, the association with placenta weight was u-shaped, yielding higher proportions of both low and high placenta weight compared to pregnancies without pre-eclampsia.
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