The purpose of this study is to evaluate the effect of norepinephrine and phenylephrine on maternal CO in cesarean section by transesophageal echocardiography.Fetal umbilical cord blood, neonatal 1min Apgar score, 5 min Apgar scores and neonatal plasma catecholamine concentrations will be collected to understand the effects of norepinephrine and phenylephrine on fetal.