title: Gender mix in twins and fetal growth, length of gestation and adult cancer risk.
abstract: This study evaluated the effect of gender mix (the gender combinations of twin pairs) on fetal growth and length of gestation, and reviewed the literature on the long-term effects of this altered fetal milieu on cancer risk. In singletons, it is well established that females weigh less than males at all gestations, averaging 125-135 g less at full term. This gender difference is generally believed to be the result of the effect of androgens on fetal growth. The gender difference in fetal growth is greater before the third trimester and less towards term, with males growing not only more, but also earlier than females. Plurality is a known risk factor for reduced fetal growth and birthweight. Compared with singletons, the mean birthweight percentiles of twins fall substantially (by 10% or more) below the singleton 10th percentile by 28 weeks, below the singleton 50th percentile by 30 weeks, and below the singleton 90th percentile by 34 weeks. In unlike-gender twin pairs, it has been reported that the female prolongs gestation for her brother, resulting in a higher birthweight for the male twin than that of like-gender male twins. Other researchers have demonstrated that females in unlike-gender pairs had higher birthweights than females in like-gender pairs. Analyses from our consortium on 2491 twin pregnancies with known chorionicity showed longer gestations and faster rates of fetal growth in both males and females in unlike-gender pairs compared with like-gender male or female pairs, although these differences were not statistically significant. The post-natal effects for females growing in an androgenic-anabolic environment include increased sensation-seeking behaviour and aggression, lowered visual acuity, more masculine attitudes and masculinising effects of the auditory system and craniofacial growth. In contrast, there is no evidence to suggest that there might be a similar feminising effect on males from unlike-gender pairs. This hormonal exposure in utero may influence adult body size and susceptability to breast cancer.
authors.affiliation: Department of Epidemiology and Public Health, University of Miami School of Medicine, Miami, FL, USA. bluke@med.miami.edu
authors.forename: Barbara
authors.initials: B
authors.lastname: Luke
authors.forename: Mary
authors.initials: M
authors.lastname: Hediger
authors.forename: Sung-Joon
authors.initials: SJ
authors.lastname: Min
authors.forename: Morton B
authors.initials: MB
authors.lastname: Brown
authors.forename: Ruta B
authors.initials: RB
authors.lastname: Misiunas
authors.forename: Victor Hugo
authors.initials: VH
authors.lastname: Gonzalez-Quintero
authors.forename: Clark
authors.initials: C
authors.lastname: Nugent
authors.forename: Frank R
authors.initials: FR
authors.lastname: Witter
authors.forename: Roger B
authors.initials: RB
authors.lastname: Newman
authors.forename: Gary D V
authors.initials: GD
authors.lastname: Hankins
authors.forename: David A
authors.initials: DA
authors.lastname: Grainger
authors.forename: George A
authors.initials: GA
authors.lastname: Macones
mesh_terms: D000328:Adult; D001724:Birth Weight / Q000502:physiology; D005260:Female; D047109:Fetal Development / Q000502:physiology*; D005865:Gestational Age; D006728:Hormones / Q000502:physiology; D006801:Humans; D008297:Male; D009369:Neoplasms / Q000209:etiology* / Q000503:physiopathology; D012307:Risk Factors; D012737:Sex Factors; D018433:Twin Studies as Topic; D014427:Twins / Q000502:physiology*; D014429:Twins, Dizygotic / Q000502:physiology; D014430:Twins, Monozygotic / Q000502:physiology
chemical_list: D006728:Hormones