Ovarian stimulation for IVF profoundly alters the early luteal phase endometrial development. It has been hypothesized that this process has already started in the late follicular phase, as the endometrium has already been exposed to high steroid concentrations since that phase. The aim of the present study was to prospectively investigate the effect of multi-follicular ovarian stimulation for IVF on the late follicular phase endometrium histology and the expression of estrogen receptor (ER) and progesterone receptor (PR).
In a cross-over study, 11 infertile women with normal ovulatory function, participating in an IVF programme and treated with GnRH antagonist/recombinant FSH ovarian stimulation, were enrolled in the study. Endometrial biopsies were taken in a natural cycle on the day of the onset of the surge of the LH, and in a subsequent stimulation cycle on the day of hCG administration for final oocyte maturation. Endometrial histological dating was carried out according to Noyes' criteria. Immunohistochemistry was performed, using commercially available antibodies for ER and PR endometrial expression. The immunohistochemical signal was recorded in 1000 epithelial cells in each compartment (glands and stroma). Endometrial expression for each of the two receptors was graded on a scale of 0-3, based on the intensity of nuclear staining. Then a score range between 0 and 3000 was recorded, and expressed as a mean score per 1000 stroma or glandular cells per sample (range: 0-3).
Histological examination of biopsies both in natural and stimulated cycles showed no secretory changes. However, in stimulated cycles, PR expression was significantly up-regulated compared to natural cycles in both glands (1.67 versus 1.34, P < 0.05) and stroma (1.98 versus 1.62, P < 0.05), whereas ER was down-regulated in glands (1.15 versus 1.43, P < 0.05). In IVF cycles, the progesterone measurements, although within normal values (range 0.8-1.4 microg/l), were significantly higher than in natural cycles (0.99 vs 0.63 microg/l, respectively, P = 0.008). An ongoing pregnancy rate of 37.5% was achieved in the stimulated cycles.
Although the current study found no early secretory transformation in stimulated endometria before hCG administration, the ER and PR expression in these endometria is similar to the one described during the first days of the luteal phase in natural cycles. Supraphysiological concentrations of estradiol and subtle progesterone rises in the late follicular phase might be responsible for this modulated steroid receptor profile. This phenomenon indicates accentuated maturation of the endometrium in IVF cycles from the pre-ovulatory phase onwards.