Please cite this paper as: Haavaldsen C, Samuelsen S, Eskild A. The association of maternal age with placental weight: a population-based study of 536 954 pregnancies. BJOG 2011;118:1470–1476.
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.