Criteria for Extended Culture There are no established guidelines or criteria that determine when to utilize extended culture. Varying suggestions include maternal age 42 or younger with five or more two pronuclear stage (2PN) embryos on postretrieval day 1; maternal age of 40 or younger and three or more good-quality day 3 embryos having 4 to 10 cells with less than 15% fragmentation; maternal age of 41 to 42 or younger and four or more good-quality day 3 embryos having 4 to 10 cells with less than 15% fragmentation; and age less than 37 with four or more morphologically good embryos on day 3, or four or more embryos with 6 cells and less than 10% fragmentation (362,364,370).
Embryo Morphology Embryo morphology guides the choice of embryo for transfer.
Pronuclear embryos are assessed by their distribution and number of nucleoli, the position of the second polar body relative to the first, and cleavage rates (abnormal rates are too fast,
Cryopreservation of Embryos too slow, or arrested) (365). Preferred cleavage stage embryos have a normal developmental pattern characterized by early cleavage on day 1, four cells on day 2, and eight cells on day 3. Embryo fragmentation should be 10% or less, the blastomere size should be regular, and there should be no multinucleation (362). The Gardner and Schoolcraft system for scoring blastocysts uses a scale from 1 (worst) to 6 (best), with grades 1 to 3 indicating growth of the blastocele until it completely fills the embryo. Grade 4 blastocysts are expanded with a larger blastocele volume and a thinning zona pellucida. The trophectoderm in a grade 5 blastocyst is starting to hatch though the zona, and the grade 6 blastocyst has completely escaped or hatched from the zona. The inner cell mass is graded A to C based on tightness and cellularity (A is best), and the trophectoderm is assessed from A to C based on cohesiveness and cellularity (A is best) (361).
Number of Embryos to Transfer High-order multiple pregnancy (three or more fetuses) increases complications for mothers and fetuses, so guidelines have been developed to minimize this adverse outcome (371). Single embryo transfer should be considered for patients younger than age 35, particularly those undergoing their first ART cycle who have a large quantity of good quality embryos or patients who have conceived in a prior cycle. Otherwise, transfer should be limited to two embryos in women under 35 years of age. For older women, the maximum number of transferred cleavage-stage embryos should be three in women aged 35 to 37, four in women aged 38 to 40, and five in women older than 40 years of age. Because of their high implantation potential, no more than three blastocysts should be transferred to any woman regardless of her age. Limits on the number of embryos transferred when the embryos were created from donor oocytes should be based on the age of the donor, rather than the recipient (371).