We aimed to determine whether ACE A11860G genotype is associated with small for gestational age babies (SGA) and to determine whether the association is affected by environmental factors and fetal sex. Three thousand two hundred thirty four healthy nulliparous women with singleton pregnancies, their partners and babies were prospectively recruited to the Screening for Pregnancy Endpoints (SCOPE) study in Adelaide, Australia and Auckland, New Zealand. Data analyses were confined to 2121 Caucasian parent-infant trios, among which 216 were SGA pregnancies and 1185 were uncomplicated pregnancies. Women with ACE A11860G GG genotype in the combined and Australian cohorts had increased risk for SGA (OR 1.5, 95%CI 1.1-2.1; OR 2.0, 95%CI 1.3-3.3) and delivered lighter babies (p=0.02; p=0.007) compared with those with AA/AG genotypes. Maternal ACE A11860G GG genotype was associated with higher maternal plasma ACE concentration than AA/AG genotypes (p<0.001). When the Australian cohort was stratified by maternal socio-economic index (SEI) and pre-pregnancy green leafy vegetable intake (PPGLV), the association of maternal ACE A11860G GG genotype with increased risk for SGA was observed among women with SEI <34 (OR 4.9, 95%CI 1.8-13.4) or PPGLVI<1 serve/day (OR 3.3, 95%CI 1.6-7.0). Furthermore, the associations of maternal ACE A11860G with customized birthweight centile observed among Australian women with SEI<34 or PPGLV<1 serve/day were female-specific. The current study identified an association of maternal ACE A11860G with SGA. More interestingly, this association was modified by modifiable environmental factors and fetal sex, suggesting an ACE A11860G-environment-fetal sex interaction.