Prolactin synthesis and release during pregnancy and puerperium

Prolactin (PRL) synthesis and its release following thyrotropin-releasing hormone (TRH) administration during pregnancy and puerperium was studied in 45 women. Mean baseline E2 increased from 1,900 +/- 384 (SEM) pg/ml in the first trimester to 3,520 +/- 849 in the second trimester (P less than 0.05) and 43,057 +/- 5,765 pg/ml in the third trimester (P less than 0.001) of pregnancy. Mean baseline progesterone increased from 27.6 +/- 3.2 ng/ml in the first trimester to 41.9 +/- 6.6 in the second trimester (P less than 0.01) and 109.3 +/- 11.2 ng/ml in the third trimester (P less than 0.001) of pregnancy. Ten days after delivery, mean E2 dropped to 13 +/- 2.9 pg/ml and progesterone dropped to 0.56 +/- 0.07 ng/ml in the lactating women; in the nonlactating women, mean E2 level was 100 +/- 44 pg/ml and mean progesterone was 0.63 +/- 0.09 ng/ml. Baseline PRL increased from 27 +/- 15 ng/ml in the third trimester (P less than 0.002). The increased synthesis of PRL with increasing gestation was thought to be due to the stimulatory effects of E2 and progesterone, resulting in hyperplasia of the lactotrophs. In response to TRH, PRL demonstrated a significant increase from the first trimester to the second, with no further increase in the third. Therefore, it appears that the PRL reserve increases only during the first and second trimesters of pregnancy. Ten days after delivery, baseline PRL in response to TRH decreased to levels found in the first and second trimesters. However, the lactating women released less PRL than the nonlactating subjects (P less than 0.01), since PRL is released with each lactating episode which in turn probably reduces the PRL reserve.