The association of maternal age with placental weight: a population-based study of 536,954 pregnancies

Objective: To study the association of maternal age with placental weight, birthweight and placental weight/birthweight ratio.

Design: Population-based study.

Setting: Medical Birth Registry of Norway.

Population: All singleton births in Norway in the period 1999-2008 (n = 536,954).

Methods: Z-scores of placental weight and birthweight were calculated and divided into deciles. The proportions with a small or a large placenta were calculated within each maternal age group. Also, the odds ratios of having a small (lowest decile) and a large (highest decile) placenta, according to maternal age, were estimated, with and without adjustment for birthweight in grams, parity, smoking, pre-eclampsia and diabetes.

Main outcome measures: Placental weight, birthweight and placental weight/birthweight ratio.

Results: The mean placental weight increased with maternal age: 647.1 g in women below the age of 20 years and 691.3 g in women aged 45 years or older. Among the oldest group of women (≥45 years) 15.8% of placentas were in the highest decile of placental weight z-score, whereas this was true for just 7.0% of women below the age of 20 years (Wald test, P < 0.001). Using women younger than 20 years of age as a reference, the odds ratio for having a placenta in the highest decile of placental weight z-score was 2.50 (95% CI 1.92-3.26) for women aged 45 years or older, after adjustment for offspring birthweight, parity, maternal smoking, pre-eclampsia and diabetes.

Conclusion: We found an association between increased placental weight and maternal age, and this finding may be important in understanding the causes of adverse events associated with high maternal age.