Risk of Twinning as a Function of Maternal Height and Body Mass Index

Abstract

To the Editor: Rates of twinning declined from the 1950s through the 1970s in many countries^1,2 even after controlling for age and parity² but began to increase again in the 1980s.¹ Monozygotic twinning is considered an essentially random event with constant rates around the world, although recent evidence suggests that it may be more likely among women undergoing infertility treatment.¹ Changes in twinning rates are thus largely attributable to dizygotic twinning, which is influenced by maternal age, race, family history,^1,3 and, possibly, height and weight.^3,4 Widespread use of infertility drugs is presumably the main cause of the recent increase in dizygotic twinning in most industrialized countries.¹ We examined the relationship between twinning and maternal height and body mass index (BMI) in a large national cohort.