In vitro release of progesterone and estrone by the porcine placenta throughout gestation

In vitro release of progesterone (P₄) and estrone (E₁) by the porcine placenta was characterized at 12 days of gestation between d 20 and 110. Placental P₄ rose linearly from d 25 to 40, plateaued between d 40 and 50, continued to increase to a peak concentration at d 100, then decreased sharply to d 110. Placental E₁ decreased abruptly from d 30 to a nadir at d 40, then increased continuously from d 70 to peak values at d 110. This biphasic pattern for E₁ mimicked the pattern observed in allantoic fluid and maternal plasma pools. Only trace amounts of testosterone (T) were measured at d 20, 50, 60, and 100, suggesting rapid aromatization of C₁₉ steroids to estrogens. The results of this in vitro study indicate that the porcine placenta collected throughout gestation can release large quantities of P₄, although the lack of an increase in systemic P₄ suggests that under in vivo conditions P₄ is utilized and/or metabolized within the uterus.