Relaxin and the control of primate parturition.

In primate pregnancy, circulating relaxin, solely a product of the corpus luteum, peaks in the first trimester of pregnancy then declines and levels off for the remainder of pregnancy. Relaxin actions in pregnancy include increasing cervical pro-MMP-1 and pro-MMP-3 and decreasing TIMP-1, changes which soften the cervix. Relaxin, from early pregnancy, increases endometrial natural killer cells, macrophages and neutrophils, blood flow and arterial number. Hyperrelaxinemia correlates with preterm birth.