The acute release of maternal prolactin by instrumental cervical dilatation simulates the second stage of labor.

Objective: We hypothesize that artificial stimulation of the cervix causes an acute secretion of maternal prolactin (PRL) that mimics PRL secretion during the second stage of human labor.

Methods: Eighteen women scheduled for first-trimester therapeutic abortion had a blood sample drawn at the following times: before and after laminaria tent (LT) insertion for cervical ripening, 18-24 hours later upon entry in the procedure room, after LT removal, during instrumental cervical dilatation and uterine evacuation by suction curretage, soon after evacuation, and 1/2 hour and 1 hour after evacuation. Local cervical anesthesia, intravenous analgesics, and sedation were administered before the procedure. Blood serum was assayed for PRL, human chorionic gonadotropin (hCG), and estradiol.

Results: The initial PRL levels were 34 ng/mL +/- 4.2 standard error of the mean (SEM) and did not change after 18-24 hours of LT placement nor after LT removal. However, PRL increased significantly (t = 5.8; P = .00001) over basal levels by 204% +/- 15 at the time of instrumental dilatation and rose to 378% +/- 33 at 1/2 hour postevacuation. After 1 hour, PRL had declined to 173% +/- 10%. Serum levels of estradiol and hCG changed only after evacuation.

Conclusion: Artificial stimulation of the cervix causes an immediate surge in PRL levels. We suggest that the acute increase in pituitary PRL secretion that occurs during the second stage of labor results from a neural signal from the cervix and not from endocrine changes associated with labor.