THE REGULATION OF PLASMA RELAXIN LEVELS DURING HUMAN-PREGNANCY

The factors that determine the circulating levels of relaxin during pr egnancy have been investigated by comparing the plasma levels of relax in throughout pregnancy in women who became pregnant spontaneously (si ngleton, n=240) or following superovulation (singleton and multifetal pregnancies (two to ten conceptuses), n=83). Some of the women with mu ltifetal pregnancies underwent selective fetal reduction to twin pregn ancies. Relaxin levels were higher at 7-34 weeks of gestation in singl eton pregnancies achieved following superovulation when compared with levels in spontaneously conceived singleton pregnancies (P<0.05-0.001) . In samples obtained between 10 and 12 weeks of gestation (before fet al reduction for the multifetal pregnancies), plasma relaxin levels co rrelated with fetal number (r=0526, P=0.0001). Reduction in fetal numb er to a twin pregnancy did not alter relaxin levels. These data sugges t that the circulating levels of relaxin throughout pregnancy are dete rmined during the cycle of conception by gonadotrophin stimulation, an d within the first 10 weeks of pregnancy by the luteotrophic stimulus from the conceptus. Furthermore, once corpus luteum synthesis of relax in is established, then reduction in the luteotrophic stimulus does no t appear to affect it.