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Nitric oxide appears to play a role in the relaxation properties of umbilical blood vessels midway through pregnancy. Researchers studied the contraction and relaxation properties of chemically stimulated muscle strips, some with intact lining cells, from umbilical blood vessel samples obtained from premature newborns. L-NG-nitro arginine, a chemical that blocks nitric oxide, enhanced the contractions as well as blocking the relaxation of muscle strips with intact lining cells. Prostacyclin produced a similar but not as powerful a response.

Vasorelaxation in the human umbilical artery appears to decrease as gestation progresses. This may be caused both by reduced production of endothelium-derived relaxing factor (EDRF) and by decreased arterial sensitivity to EDRF. EDRF is released by histamine, as is nitric oxide, another compound involved in smooth muscle relaxation. Researchers obtained umbilical artery tissue from women who delivered at term, between 30 and 32 weeks and between 18 and 22 weeks. Specially prepared arterial muscle strips were analyzed for contractile properties. Contractile responses increased as gestation progressed. Tests designed to evaluate the effects of histamine, EDRF and nitric oxide indicated that relaxation was induced by histamine release of EDRF. The amount of EDRF released appeared to decline as gestation progressed. The tests also demonstrated that arterial sensitivity to EDRF decreased in later pregnancy.