Placental weight in singleton pregnancies with and without assisted reproductive technology: a population study of 536,567 pregnancies.

BACKGROUND Pregnancies conceived by assisted reproductive technology (ART) are at increased risk of adverse outcomes. Previous studies have suggested increased placental weight and increased placental weight/birthweight ratio in pregnancies associated with adverse outcomes. We therefore studied the association of ART with placental weight and placental weight/birthweight ratio. METHODS We included all singleton births in the Medical Birth Registry of Norway during the period 1999-2008 (n = 536 567, including 8259 after ART). We divided placental weight and placental weight/birthweight ratio into quartiles, and calculated the proportions of ART and spontaneous pregnancies in the lowest and the highest quartile by length of gestation. Thereafter, we estimated crude and adjusted odds ratios (ORs) for being in each quartile of placental weight for ART pregnancies with spontaneous pregnancies as the reference. The analyses were repeated with ART pregnancies subgrouped into IVF or ICSI. RESULTS Mean placental weight was 678.9 g in pregnancies conceived by ART, and 673.0 g in pregnancies after spontaneous conception. ART pregnancies were overrepresented in the highest quartile of placental weight and underrepresented in the highest quartile of birthweight, independent of length of gestation at delivery. Thus, placental weight/birthweight ratio was higher in ART pregnancies. For ART pregnancies, the OR for being in the highest quartile of placental weight was 1.37 (95% confidence interval 1.30-1.45) after adjustment for length of gestation, offspring birthweight, parity, fetal sex, maternal age, pre-eclampsia and diabetes. There was no difference in placental weight/birthweight ratio between IVF and ICSI pregnancies. CONCLUSIONS We found larger placentas and a higher placental weight/birthweight ratio among pregnancies conceived by ART compared with spontaneous pregnancies, and the difference was independent of length of gestation at delivery and ART method.