['The purpose of this study was to determine (1). whether the inherited thrombophilias (the factor V Leiden and prothrombin gene mutations and the methylenetetrahydrofolate reductase [C677T] polymorphism) are increased in women with "idiopathic" (normotensive) small-for-gestational-age pregnancies and/or in their babies and (2). whether fetal carriage of a thrombophilia is associated with abnormal umbilical Doppler studies.', 'This was a case-controlled study of normotensive women who were delivered of a singleton small-for-gestational-age baby (birth weight, <10th percentile adjusted for sex) with no clinical evidence of chromosomal or congenital abnormality. Control subjects were healthy women who were delivered of appropriate-for-gestational-age babies.', 'One hundred forty-five women with small-for-gestational-age pregnancies and 290 control subjects were recruited. Small-for-gestational-age babies were born at an earlier gestational age (38 +/- 3.0 weeks) and with a lower birth weight (2373 +/- 521 g) than control babies (39.7 +/- 1.3 weeks and 3606 +/- 423 g, P <.01). There were no differences in the rates of factor V Leiden (2.8% vs 3.8%; relative risk, 0.79; 95% CI, 0.34-1.85), prothrombin gene mutation (2.8% vs 3.1%; relative risk, 0.92; 95% CI, 0.40-2.09), and methylenetetrahydrofolate reductase C677T polymorphism (13% vs 9%; relative risk, 1.27; 95% CI, 0.87-1.84) between mothers with small-for-gestational-age babies and control subjects, respectively. Inherited thrombophilias were not increased in small-for-gestational-age babies compared with control babies. Of small-for-gestational-age babies with abnormal umbilical artery Doppler studies (n = 25), 21% had a thrombophilia compared with 11% with normal umbilical artery Doppler studies (n = 68; relative risk, 1.75; 95% CI, 0.81-3.81).'] Are inherited thrombophilias increased in `` idiopathic '' small-for-gestational-age pregnancies?