PLACENTAL NITRIC-OXIDE PRODUCTION AND UMBILICAL ARTERY VASCULAR IMPEDANCE IN EARLY-PREGNANCY

Objective: To evaluate whether the vasorelaxant molecule nitric oxide (NO) and its second messenger cyclic guanosine monophosphate (cGMP) mo dulate the reduction in resistance within the fetoplacental circulatio n that occurs during the first trimester of pregnancy. Methods: We stu died 27 women undergoing termination of pregnancy for psychosocial ind ications between 9 and 15 weeks' gestation. Each had ultrasound dating of the fetus and Doppler umbilical artery (UA) now investigation imme diately before the operation. Placental tissue was assayed for nitric oxide synthase (NOS) activity and cGMP content. Results: Both UA pulsa tility index (PI) and placental endothelial NOS activity decreased sig nificantly with advancing gestation (r = -.52 and -.41, respectively). Umbilical PI correlated positively with endothelial NOS activity and cGMP level. Multiple linear regression analysis indicated that cGMP co ntributed most strongly to UA PI (P < .001). Conclusion: Our findings suggest somewhat paradoxically that NOS activity and cGMP levels are h ighest in the early gestations, when umbilical PI is also high, and de crease as umbilical PI decreases. Further studies are required to dete rmine whether the predominant effect of NO is that of a vascular relax ant or a modulator of new villous vessel formation. (C) 1998 by The Am erican College of Obstetricians and Gynecologists.