—We agree with Dr Luke that our proposal has limited applicability in the United States. The mean duration of pregnancies may be influenced by racial factors.1 Maternal height is positively correlated with duration of pregnancy,2 and maternal body mass index may also be positively correlated with gestational length.3 Japanese women are shorter and slimmer compared with white American women. Thus, the mean duration of pregnancy among white American women may be longer than that among Japanese women. However, as pointed out by Luke, pregnancy with twins is different from that involving triplets or higher-order pregnancy. Twin pregnancy is apparently different biologically from singleton pregnancy, regardless of race, with respect to the duration of pregnancy, as suggested by our study. The problem is that the definition of term delivery, which may have been derived from populations in which singleton pregnancies accounted for more than 95% of all pregnancies,
Minakami H, Sato I. Date of Delivery in Multifetal Pregnancies-Reply. JAMA. 1996;276(6):452. doi:10.1001/jama.1996.03540060028029
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