Twinning rates according to maternal birthweight.

Dizygotic twinning rates have changed over time, which has been seen as a sign of a decline in fecundity. Since a woman's birthweight has been shown to be a marker of her fecundity, maternal birthweight may correlate with subsequent twinning rates. In the Danish National Birth Cohort (1996-2002), we examined if maternal birthweight, and whether she was born at term or preterm, correlated with her probability of multiple birth. For 20,719 live born infants, we had self-reported information about maternal birthweight, collected during the first wave of the 7-year follow-up, and information on multiple births from record linkage. The association between maternal birthweight and multiple births was investigated by use of logistic regression and presented as odds ratios (ORs) with 95% confidence intervals (CIs). Compared to women born at term with a birthweight of 3001-4000 g, women with a birthweight > 4500 g appeared to have higher chance of multiple birth while women with a birthweight of 4001-4500 had a lower chance, especially if the analysis was restricted to women with a BMI < 25 (Adjusted ORs 2.3 [1.0-5.4] and 0.4 [0.2-0.9] respectively). Odds ratios for dizygotic twinning were of the same magnitude. In women with a BMI >or= 25, no obvious pattern was present. Our findings do not indicate that twinning is a fecundity indicator. Women with a birthweight that may indicate a pregnancy complicated with gestational diabetes had the highest rate of multiple birth. These findings are new and should be put to a critical test in other data sources.