We will exclude women with multiple gestation, because they are at risk of preterm labor. We will also exclude anyone else who had preterm delivery. In addition, we will exclude women with a diagnosis of liver disease, including intrahepatic cholestasis and hepatitis, chronic hypertension, intraamniotic infection, preeclampsia or its variant, HELLP syndrome, cholelithiasis, inadequate prenatal care, preterm labor, or induction of labor (other than due to premature rupture of membranes).