Purpose: While the precise timing of the maternal recognition of pregnancy is not known, it is known that the prevention of return to ovarian cyclicity relies on a conceptus-derived signal.
Methods: In an attempt to identify the first luteotropic signals detectable in the maternal circulation, a sensitive Leydig cell luteotropin bioassay was employed, and data were compared for nine clinically pregnant and nine nonpregnant patients in an in vitro fertilization program. Blood samples were drawn every other day for 10 days after embryo transfer (ET).
Results: The first detectable rise in bioactive luteotropin levels occurred between 6 and 8 days post ET. Serum E2 levels increased on the same days. Differences in luteotropin levels between pregnant and nonpregnant patients were significant between days 6 and 8 (P < 0.0001) and between days 8 and 10 (P < 0.002).
Conclusions: Based on morphological studies reported by others, bioactive luteotropic signals identified in this study were detectable in the maternal circulation at about the time of trophoblast lacunae coalescing with maternal uterine blood vessels.