Placental weight in pregnancies with high or low hemoglobin concentrations

To study the associations of maternal hemoglobin concentrations with placental weight and placental to birthweight ratio.

In this retrospective cohort study, we included all singleton pregnancies during the years 1998-2013 at a large public hospital in Norway (n=57062). We compared mean placental weight and placental to birthweight ratio according to maternal hemoglobin concentrations: <9g/dl, 9-13.5g/dl or >13.5g/dl. The associations of maternal hemoglobin concentrations with placental weight and placental to birthweight ratio were estimated by linear regression analyses, and adjustments were made for gestational age at birth, preeclampsia, parity, maternal age, diabetes, body mass index, smoking, offspring sex and year of birth.

In pregnancies with maternal hemoglobin concentrations <9g/dl, mean placental weight was 701.2g (SD 160.6g), followed by 678.1g (SD 150.2g) for hemoglobin concentrations 9-13.5g/dl and 655.5g (SD 147.7g) for hemoglobin concentrations >13.5g/dl (ANOVA, p<0.001). Mean placental to birthweight ratio was highest in pregnancies with maternal hemoglobin concentrations <9g/dl (0.203 (SD 0.036)). We found no difference in mean placental to birthweight ratio for maternal hemoglobin concentrations 9-13.5g/dl (0.193 (SD 0.040)) and >13.5g/dl (0.193 (SD 0.043)). Adjustments for our study factors did not alter the estimates notably.

Placental weight decreased with increasing maternal hemoglobin concentrations. The high placental to birthweight ratio with low maternal hemoglobin concentrations suggests differences in placental growth relative to fetal growth across maternal hemoglobin concentrations.