This study was performed to establish the dynamics of human chorionic gonadotropin, prolactin, and growth hormone throughout pregnancy in serum and amniotic fluid. Two hundred fifty healthy women at 8 to 42 weeks' gestation were studied. The highest serum human chorionic gonadotropin level was measured between weeks 8 to 12 (53,715 +/- 3574 mIU/ml, mean +/- SEM), with a decline to a mean plateau of 11,806 +/- 1250 mIU/ml from week 18. Amniotic fluid human chorionic gonadotropin had a similar pattern with a mean of 68,100 +/- 8422 mIU/ml at weeks 8 to 10, declining from week 18 to a plateau of 2005 +/- 260 mIU/ml. Human chorionic gonadotropin showed a significant correlation (r = 0.85, p less than 0.001) between levels of both compartments demonstrating an even distribution. Prolactin levels showed a dichotomy of patterns and levels. Serum prolactin showed a continuous rise from 45.3 +/- 14 ng/ml at week 8 to 224 +/- 20 ng/ml at week 36. In contrast, amniotic fluid prolactin remained low until week 14 (33.1 +/- 0.8 ng/ml), followed by a sharp and significant (p less than 0.001) increase to a plateau of 3750 +/- 200 ng/ml between weeks 18 to 26, declining to a second plateau of 500 +/- 50 ng/ml at week 36. Serum growth hormone increased from a mean of 3.5 +/- 1.4 ng/ml seen at weeks 8 to 10 to a mean of 14 +/- 2.0 ng/ml at weeks 28 to 30, followed by a plateau of similar levels. The pattern of growth hormone secretion in amniotic fluid demonstrated a sharp increase during the 14-16 interval with a maximum mean level of 15.5 +/- 1.5 ng/ml and a slow steady decline thereafter. In conclusion, the similar pattern and concentration of human chorionic gonadotropin throughout pregnancy in both maternal and amniotic fluid are probably the result of direct human chorionic gonadotropin diffusion from the placenta. The dissimilar pattern and concentration of prolactin are the result of two different sources of prolactin secretion during pregnancy. Serum prolactin originates from the pituitary and amniotic fluid prolactin from the decidua. Since the pattern of growth hormone secretion resembles that of prolactin, it is possible that growth hormone, like prolactin, is secreted by the same sources.