Human chorionic gonadatropin (hCG) during third trimester pregnancy

Separate reference values were recently established for routine blood samples during last trimester pregnancy. Previously, these were based on blood samples from healthy men or non-pregnant women. Normal changes in variation in the levels of steroid hormones in the last weeks of pregnancy before delivery are also incompletely investigated. This study of the preterm hormone levels was carried out in the search for events leading to increased contractility that might occur in the predelivery weeks and potentially influence the initiation of delivery.

Blood samples during pregnancy weeks 33, 36 and 39 as well as 1-3 h postpartum were collected from pregnant women (19-39 years, mean age 30) with at least one previous pregnancy without hypertension or pre-eclampsia. All women (n = 135) had had a vaginal delivery and spontaneous start of labour. The blood samples were analysed for serum hCG, oestradiol and progesterone. Postpartum, the values were retrospectively rearranged to correspond with the actual week before the day of delivery.

During the last trimester of normal pregnancy, a gradual increase was found in oestradiol (median 45980 to 82410 pmol/L), progesterone (median 341 to 675 nmol/L) and a gradual decrease in hCG (median 31833 to 19494 IU/L). Furthermore, a significant (p<0.03) decrease in hCG was found from the third to the second week before delivery, while oestradiol and progesterone continued to increase.

Hormone levels during third-trimester pregnancy have not previously been systematically investigated. Recent data suggest that hCG may have a role as an endogenous tocolytic in normal pregnancy by directly promoting relaxation of uterine contractions. In the present study a significant decrease in serum hCG level was found 2-3 weeks before the spontaneous start of labour. This might contribute to increasing the contractility in the uterine muscle and gradually initiate the onset of labour.