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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.

Methods

We used the Danish National Birth Registry to identify 62 073 singleton and twin births from the 24th gestational week occurring between 1998 and 2001 to . . . [Full Text of this Article]

Olga Basso, MD, PhD
ob{at}soci.au.dk

Ellen Aagaard Nohr, MHS
Danish Epidemiology Science Centre
University of Århus
Århus, Denmark

Kaare Christensen, MD, PhD
Epidemiology, Institute of Public Health, and the Danish Twin Registry
University of Southern Denmark
Odense

Jørn Olsen, MD, PhD
Danish Epidemiology Science Centre
University of Århus

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