Purpose: The importance of endometrial maturation at estimated time of implantation for the outcome of IVF treatment in regularly menstruating women with tubal infertility was evaluated.

Methods: FSH was measured on cycle day 3, on days 10-15 urine and blood were collected to estimate the day of the LH peak, and E2 and P4 were measured during the luteal phase, on cycle days 19-26. An endometrial biopsy was obtained on days LH + 3 to LH + 6.

Results: The number of subjects with delayed endometrial maturation was larger in the group of infertile women who did not become pregnant compared to pregnant women and controls. Those infertile women who did not become pregnant after IVF treatment also presented with a higher basal FSH on cycle day 3 and lower E2 and P4 AUC in the luteal phase. Six infertile women and two controls presented with mid- and late-proliferative endometrium in the luteal phase on cycle days LH + 3 to LH + 6, in the presence of adequate E2 and P4 secretion. Six morphological characteristics were compared in the three groups: (1) 17 infertile women who became pregnant, (2) 18 who did not become pregnant, and (3) 28 controls. The pregnant infertile women did not differ from the controls. The numbers of glandular and stromal mitoses were significantly higher in those women who did not become pregnant (P < 0.01) compared with those who became pregnant. Endometrial biopsies obtained on cycle days LH + 5 and LH + 6 showed significant differences in glandular epithelial height (P < 0.05) and number of vacuolated cells among the nonpregnant women (P < 0.01), the pregnant women (P < 0.05), and controls.

Conclusions: A higher frequency of retarded endometrial development in women who did not become pregnant following IVF treatment was found. In some cases, endometrial insensitivity could most likely cause retarded endometrial development and failure of implantation after IVF treatment, which could not be overcome by routine luteal-phase support. However, our results do not allow conclusions concerning its relative importance compared to preembryo quality; this has to be investigated further.