Placental weight relative to birthweight in pregnancies with maternal diabetes mellitus.

OBJECTIVES: To study the association of maternal diabetes mellitus with placental weight, birthweight and placental weight-to-birthweight ratio. DESIGN: Population-based study. SETTING: Medical Birth Registry of Norway. POPULATION: All singleton births in Norway during 1999-2008 (n = 536,997). METHODS: We compared the distribution of placental weight z-scores and placental weight-to-birthweight ratio between pregnancies with and without diabetes. The associations of diabetes with placental weight z-scores were also estimated as odds ratios with and without adjustment for birthweight, maternal age, parity, preeclampsia, smoking and cesarean delivery. MAIN OUTCOME MEASURES: Placental weight, birthweight and placental weight-to-birthweight ratio. RESULTS: Mean placental weight was 736.6 g in diabetic pregnancies and 672.1 g in non-diabetic pregnancies. The corresponding birthweights were 3682.1 g and 3557.0 g. In diabetic pregnancies, 26.2% of the placentas were in the highest decile of placental weight z-score, as compared with 9.7% in non-diabetic pregnancies (p < 0.001). The corresponding figures for being in the highest decile of placental weight-to-birthweight ratio were 18.2 and 9.9% (p < 0.001). The crude odds ratio for having a placenta in the highest decile of placental weight z-score was 3.29 (95% confidence interval 3.14-3.45) in diabetic pregnancies with non-diabetic pregnancies as the reference. After adjustment for birthweight and other variables, the odds ratio was 2.42 (95% confidence interval 2.29-2.56). CONCLUSIONS: In diabetic pregnancies, placental weight as well as placental weight relative to birthweight were higher than in non-diabetic pregnancies.