Physiology of Oocyte Maturation Prior to maturation, oocytes are arrested in the prophase stage of meiosis I, also known as the germinal vesicle (343,344). Meiosis I oocytes must reach at least the early antral follicle stage to respond to FSH and be competent to resume meiosis. In vivo, LH receptors on the follicle are induced by FSH during later stages of follicular development (345). Therefore, only fully grown oocytes respond to the LH surge in vivo to begin the cytoplasmic and nuclear maturation that are required for developmental progression toward the metaphase stage of meiosis II. At this point, the developmentally competent oocyte will extrude the first polar body, the oocyte-cumulus complex will detach from the ovarian wall, ovulation will occur, and fertilization is possible (343–346).
Oocyte Maturation during Assisted Reproductive Technology Cycles Because spontaneous LH surges occur inconsistently during non-ART gonadotropin cycles and are suppressed in ART cycles, hCG has been used to trigger ovulation. In combination with its long half-life, homology between hCG and LH (identical α subunits) allows for cross-reactivity with the LH receptor and induction of final ococyte maturation and ovulation (347). hCG is derived from urine (5,000 to 10,000 IU intramuscularly) or through recombinant technology (250 μg subcutaneously, equivalent to 5,000 to 6,000 IU of intramuscular urinary product) (156). The half-life of hCG is 2.32 days, compared to 1 to 5 hours for LH (347). Ovulation is typically triggered when at least two follicles are 17 to 18 mm or larger in average diameter (but <24 mm) and the endometrial thickness is 8 mm or more (111,302,303,306–308,348). Similar clinical outcomes have been noted when 5,000 IU or 10,000 IU of hCG (349) and urinary or recombinant preparations are used for triggering (348,350). If there is concern for ovarian hyperstimulation syndrome (OHSS, see below), GnRH agonists can be substituted for hCG to trigger ovulation in antagonist protocols or recombinant LH can be substituted for hCG in agonist protocols; however, both protocols are associated with decreased pregnancy rates in nondonor ART cycles (347,351).
Oocyte retrieval is performed via transvaginal ultrasound-guided needle puncture into each follicle followed by aspiration of follicular ﬂuid. Either general anesthesia or intravenous conscious sedation may be used (352). Prophylactic antibiotics such as ceftriaxone are recommended at the time of retrieval (353). The vaginal preparation can be performed either with sterile saline alone or with povidone iodine followed by vigorous saline ﬂushing (354,355). The highest oocyte yield is obtained when oocyte retrieval is performed 36 to 37 hours after the hCG injection.
Earlier retrieval (35 hours) is associated with a much lower oocyte yield, and later retrieval risks ovulation; spontaneous follicular rupture appears to occur at a mean of 38.3 hours following hCG administration (348).
The rationale and regimens for luteal phase support with progesterone are discussed in the “Uterine Factors” section. The timing for luteal progesterone support varies, but lower pregnancy rates have been noted with initiation prior to oocyte retrieval or later than 5 days following retrieval (356,357). Luteal phase estradiol supplementation is not necessary (358,359) .