Daily and hourly temporal association between delta-4-androstenedione-induced preterm myometrial contractions and maternal plasma estradiol and oxytocin concentrations in the 0.8 gestation rhesus monkey

Mild uterine contractions occurring before labor may become stronger labor contractions because of an increase in estrogen levels. Researchers used eight rhesus monkeys in late pregnancy to study the relationship between estrogen and oxytocin levels and the strength of uterine contractions. Estrogen and oxytocin levels were measured before and after labor induction, and before and after nighttime, which is when uterine contractions in the pregnant monkey normally increase. Estrogen levels increased after labor induction, and these increases preceded increases in oxytocin levels by 24 to 48 hours. Increases in oxytocin levels correlated with stronger uterine contractions. Increases in oxytocin levels precipitated by elevations of estrogen may cause preterm labor as well as term labor.