Table I. Characteristics of the IVF study population (n = 4,224) and the naturally conceived control population (n = 314,605)

IVF population Control population P-value Characteristics of the mother Mean age (years) 33.3 29.7 < 0.001a Parity (%)     Primiparae 69.2 44.2 < 0.001b     Multiparae 30.8 55.8 Ethnicity (%)      Dutch 78.2 78.6 NS     Non-Dutch 21.8 21.4 Characteristics of the children Multiplicity (%)     Singleton 56.5 97.1 < 0.001b     Multiple 43.5 2.9 Gestational age (%)     <37 weeks 33.4 8.2 < 0.001b     37 weeks 66.6 91.8 at-test. bPearson 2-test. NS = not significant.