The 24-hour rhythm of oxytocin and beta-endorphin secretion in human pregnancy

Animal work suggests that maternal oxytocin secretion is influenced by the secretion of endogenous opioids in pregnancy. Spontaneous labour and pre-labour uterine activity follow a 24-hour rhythm the origin of which has not been explained but may be related to diurnal changes in oxytocin secretion. This study was performed to document the changes over a 24-hour period in maternal oxytocin and beta-endorphin secretion.

A 4-hourly blood profile was undertaken for a 24-hour period.

Sixteen women with singleton pregnancies of more than 36 weeks gestation and 10 women with pregnancies in the mid trimester were studied.

Blood was sampled 4-hourly for 24 hours beginning at 1200 h. Oxytocin was measured in all patients and beta-endorphin-like immunoreactivity was measured in 15 patients.

A simple index was defined for comparing night-time levels to daytime levels for both oxytocin and beta-endorphin. In all cases more than 36 weeks gestation the index was positive for oxytocin (night-time levels were higher) and in all cases the index was negative for beta-endorphin (night-time levels were lower). In the mid trimester women all values of the index for oxytocin were positive but in the beta-endorphin group equal numbers demonstrated a positive or a negative index.

Reciprocal 24-hour rhythms were demonstrated between oxytocin and beta-endorphin; however, it is not clear whether this relationship is causal.