PITUITARY GONADOTROPHIN SECRETION DURING THE FIRST WEEKS OF PREGNANCY

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Abstract
A longitudinal study of basal plasma LH [luteinizing hormone] and FSH [follicle stimulating hormone] and their responses to 25 .mu.g LRH [luteinizing hormone-releasing hormone] i.v. and basal levels of estradiol, progesterone, prolactin and HCG [human chorionic gonadotropin] was performed every week in 3 women, pregnant after heterologous insemination, from conception until the 6th wk of gestation. A comparative study was carried out in 7 women in cycles in which no conception occurred after insemination. All hormones were assayed with radioimmunoassay. LH was measured with a specific assay for native LH, which did not cross-react with HCG. A decrease in basal levels of LH and FSH and decreasing responses to LRH was found during the first 2 wk of gestation. These changes did not differ from what was observed during the luteal phase in the non-conception cycles. One week later the basal FSH levels and the FSH response in the pregnant women showed a further decrease, while in the non-pregnant women, now reaching the early follicular phase, a rise in FSH basal levels occurred. The basal levels of LH and the LH response did not differ from that found in the nonpregnant women at this time. FSH basal levels remained below the lower normal limit in eumenorrhoic women from the 3rd wk of gestation. By this time the FSH response was almost completely inhibited. LH basal levels remained above the lower normal limit in eumenorrhoic women, but the LH response to LRH progressively decreased and was completely inhibited by the 5th wk of gestation. In non-conception cycles the LH response varied with the levels of estradiol in plasma. This was not found in the pregnant women, as the decrease in gonadotropin response occurred while estradiol remained at mid-cycle levels during the first 4 wk of gestation. Rather, the increasing and continuously elevated level of progesterone, in the presence of appropriate levels of estradiol, might be the main gonadal steroid responsible for the diminishing pituitary secretion. The contribution of HCG to the further decrease in gonadotropin secretion after the 2nd wk of pregnancy cannot be answered by the present studies. Prolactin remained at non-pregnant levels until the 6th wk of gestation, and appeared to have no influence on the secretion of gonadotropins during early pregnancy.