American Journal of Epidemiology Advance Access originally published online on March 6, 2007
American Journal of Epidemiology 2007 165(10):1216-1218; doi:10.1093/aje/kwm024
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ORIGINAL CONTRIBUTIONS |
Human Sexual Size Dimorphism in Early Pregnancy
1 Department of Obstetrics and Gynecology, University of Texas Medical Branch, Galveston, TX
2 Department of Obstetrics and Gynecology, University of Cambridge, Cambridge, United Kingdom
3 Department of Obstetrics and Gynaecology, Royal College of Surgeons in Ireland, Dublin, Ireland
4 Department of Obstetrics and Gynecology, University of Utah and Intermountain HealthCare, Salt Lake City, UT
5 The Fetal & Women's Center of Arizona, Scottsdale, AZ
6 Department of Fetal Imaging, William Beaumont Hospital, Royal Oak, MI
7 Department of Obstetrics and Gynecology, Columbia University College of Physicians and Surgeons, New York, NY
8 Department of Obstetrics and Gynecology and Women's Health, Albert Einstein College of Medicine, Bronx, NY
9 Department of Obstetrics and Gynecology, University of Colorado Health Sciences Center, Denver, CO
10 Department of Obstetrics and Gynecology, Tufts University School of Medicine, Boston, MA
11 Department of Obstetrics and Gynecology, New York University School of Medicine, New York, NY
12 Department of Obstetrics and Gynecology, Brown University School of Medicine, Providence, RI
13 Department of Obstetrics and Gynecology, University of North Carolina Medical Center, Chapel Hill, NC
Correspondence to Dr. Radek Bukowski, Department of Obstetrics and Gynecology, University of Texas Medical Branch at Galveston, 301 University Boulevard, Galveston, TX 77555-1062 (e-mail: rkbukows{at}utmb.edu).
Received for publication August 21, 2006. Accepted for publication November 15, 2006.
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.
cohort studies; fetal development; pregnancy
Abbreviations: ASC, assisted conception (pregnancies resulting from conception assisted by in vitro fertilization or intrauterine insemination); CI, confidence interval;
GA, the difference in days of gestation; SPON, spontaneous conception
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