Differential responses to a simulated implantation signal at various stages of the luteal phase in women.

Abstract

Human CG (hCG) was administered to three groups of four normally cycling women in the early luteal phase (LH +4 to +5, group I), the midluteal phase (LH +8 to +9, group II), and the late luteal phase (LH +11 to +12, group III). Two hundred and fifty IU hCG were given im followed in half of the subjects by 750 IU hCG 24 h later. Serial blood samples were then taken at 15- or 30-min intervals following either the first or second hCG injection and continued for 12 or 24 h. The samples were stored frozen at -20 C until assayed for LH, progesterone, estradiol, and hCG concentrations. Treatment with 250 IU hCG at each stage of the luteal phase did not result in any marked change in hormone concentrations. Further hCG administration (750 IU 24 h later) in both the early and the midluteal phases elicited a clear increase in progesterone concentrations. Further hCG treatment in the late luteal phase did not evoke any rise in progesterone levels. Further hCG administration in the midluteal phase resulted in a sharp decline in LH concentrations, brought about mainly by a decrease in LH pulse frequency, this response was not apparent at any other stage of the luteal phase. Despite the lack of any pulsatile steroidogenic stimulus at this time, progesterone was clearly secreted in a pulsatile manner. The decline in LH levels following hCG administration in the midluteal phase resembled that seen in spontaneous conception cycles following implantation. The restriction of this response to the time of normal implantation may suggest a role for the pituitary in the establishment of the "implantation window." The importance of this pituitary response and the mechanisms involved are currently unknown. Its absence in the early luteal phase would suggest that it cannot be directly attributed to either progesterone or hCG. It is possible that some other luteal factor may be responsible for the midluteal decline in LH concentrations.