Human sexual size dimorphism in early pregnancy

Radek Bukowski, Gordon C.S. Smith, Fergal D. Malone, Robert H. Ball, David A. Nyberg, Christine H. Comstock, Gary D.V. Hankins, Richard L. Berkowitz, Susan J. Gross, Lorraine Dugoff, Sabrina D. Craigo, Ilan E. Timor-Tritsch, Stephen R. Carr, Honor M. Wolfe, Mary E. D'Alton

Research output: Contribution to journalArticlepeer-review

44 Scopus citations


Sexual size dimorphism is thought to contribute to the greater mortality and morbidity of men compared with women. However, the timing of onset of sexual size dimorphism remains uncertain. The authors determined whether human fetuses exhibit sexual size dimorphism in the first trimester of pregnancy. Using a prospective cohort study, conducted in 1999-2002 in the United States, they identified 27,655 women who conceived spontaneously and 1,008 whose conception was assisted by in vitro fertilization or intrauterine insemination and for whom a first-trimester measurement of fetal crown-rump length was available. First-trimester size was expressed as the difference between the observed and expected size of the fetus, expressed as equivalence to days of gestational age. The authors evaluated the association between fetal sex, first-trimester size, and birth weight. Eight to 12 weeks after conception, males were larger than females (mean difference: assisted conception = 0.4 days, 95% confidence interval (CI): 0.1, 0.7, p = 0.008; spontaneous conception = 0.3 days, 95% CI: 0.2, 0.4, p < 0.00001). The size discrepancy remained significant at birth (mean birth weight difference: assisted conception = 90 g, 95% CI: 22, 159, p = 0.009; spontaneous conception = 120 g, 95% CI: 107, 132, p < 0.00001). These data demonstrate that human fetuses exhibit sexual size dimorphism in the first trimester of pregnancy.

Original languageEnglish (US)
Pages (from-to)1216-1218
Number of pages3
JournalAmerican journal of epidemiology
Issue number10
StatePublished - May 2007
Externally publishedYes


  • Cohort studies
  • Fetal development
  • Pregnancy

ASJC Scopus subject areas

  • General Medicine


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