title: Human sexual size dimorphism in early pregnancy.
abstract: 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.
authors.affiliation: Department of Obstetrics and Gynecology, University of Texas Medical Branch, Galveston, TX 77555-1062, USA. rkbukows@utmb.edu
authors.forename: Radek
authors.initials: R
authors.lastname: Bukowski
authors.forename: Gordon C S
authors.initials: GC
authors.lastname: Smith
authors.forename: Fergal D
authors.initials: FD
authors.lastname: Malone
authors.forename: Robert H
authors.initials: RH
authors.lastname: Ball
authors.forename: David A
authors.initials: DA
authors.lastname: Nyberg
authors.forename: Christine H
authors.initials: CH
authors.lastname: Comstock
authors.forename: Gary D V
authors.initials: GD
authors.lastname: Hankins
authors.forename: Richard L
authors.initials: RL
authors.lastname: Berkowitz
authors.forename: Susan J
authors.initials: SJ
authors.lastname: Gross
authors.forename: Lorraine
authors.initials: L
authors.lastname: Dugoff
authors.forename: Sabrina D
authors.initials: SD
authors.lastname: Craigo
authors.forename: Ilan E
authors.initials: IE
authors.lastname: Timor-Tritsch
authors.forename: Stephen R
authors.initials: SR
authors.lastname: Carr
authors.forename: Honor M
authors.initials: HM
authors.lastname: Wolfe
authors.forename: Mary E
authors.initials: ME
authors.lastname: D'Alton
mesh_terms: D001724:Birth Weight; D049628:Body Size; D005260:Female; D005333:Fetus / Q000033:anatomy & histology*; D006801:Humans; D008297:Male; D011247:Pregnancy; D011261:Pregnancy Trimester, First; D011446:Prospective Studies; D012727:Sex Characteristics