Estradiol assumes the role of lead indicator because its level in blood reflects secretory activity of the dominant ovarian follicle coupled with that of numerous smaller follicles that have developed directly in response to the injected gonadotropins. For this reason pre-ovulatory estradiol is normally higher in stimulated cycles than in the spontaneous cycle. In the natural cycle, the dominant follicle releases estradiol in regularly increasing amounts from the sixth day CD until the pre-ovulatory peak, usually in the range of 125–250 pg/ml.

Measuring progesterone concomitantly with estradiol is a valuable safeguard because it represents another marker of the stimulation quality. However, levels should remain under 1 ng/ml throughout the stimulation process and readings that exceed 1 ng/ml would likely suggest one of the following:

During a normal follicular phase, progesterone originates in the theca interna and is converted to androgens for transport to granulosa cells where the production of estradiol is completed. In case of a relative lack of LH, more of the progesterone production may enter the blood instead of being moved along the steroidogenic pathways. An inappropriate increase of blood progesterone results in a thickened cervical mucus and an abnormally matured uterine endometrium that becomes visible on ultrasound. The stimulation cycle should be stopped when this occurs, although the treatment protocol should not necessarily be modified upon the initial occurrence. However, a repeated rise of plasma progesterone during a second consecutive treatment cycle should lead to consideration of protocol modification, or possibly to changing the gonadotropin itself. A progesterone level in excess of 5 ng/ml in the first monitoring sample might be a sign of an early pregnancy. Even if the patient believes she had a normal menses, a pregnancy test would be indicated.

A serum LH assay is mandatory when an intra-uterine insemination is planned, but LH measures are also useful for a simple stimulation cycle intended for a single intercourse, in order to know if a spontaneous ovulatory process might have begun prior to hCG administration, a sign of optimal follicular maturity, or if it is the hCG administration that will trigger the ovulation:

An LH rise together with a concomitant progesterone rise suggests that a spontaneous ovulatory process has started within the previous 12–24 h, and that an ovulation will occur on the following day. It will no longer be possible to block this spontaneous event by means of a GnRH antagonist.

Ovarian follicles appear as transonic structures within the ovary. The mean diameter of a follicle is estimated by the average of two perpendicular diameters, with the calipers placed in contact with the internal follicular wall [1].