Transfer Procedure The goal of transcervical embryo transfer is to atraumatically deliver the embryos to an optimal intrauterine location for implantation. Implantation is more likely after an easy transfer using a soft catheter and when fundal contact is avoided (353). Trial transfer, although not required, allows advance preparation such as cervical dilation or placement of a traction stitch, although uterine position and depth can be different at the time of the actual procedure. When performed at the time of embryo transfer, trial transfer should not go past the internal os. Trial transfer can be combined with an afterloading technique in which the outer sheath of the transfer catheter is left in place and the transfer catheter is threaded through the trial transfer sheath and into the uterus, although there is no advantage when compared to routine transfer (353,372). Soft catheters such as those made by the Cook or Wallace companies are preferred to rigid catheters to minimize prostaglandin release after cervical and/or endometrial trauma (353). The utility of cervical mucus removal prior to embryo transfer in improving embryo delivery remains controversial (353,372). Although intrauterine infections decrease pregnancy rates, the efficacy of antibiotic administration at the time of transfer is not clear. During a conventional embryo transfer, embryos are suspended in 20 μL of media at the tip of a syringe with air on either side of the ﬂuid. This creates an air–ﬂuid interface easily seen with ultrasound.
Abdominal ultrasound visualization during embryo transfer is useful to ensure deposit of the embryos 1.5 to 2 cm from the uterine fundus. Once the embryos are deposited, the inner and outer sheath should be removed as a unit to avoid suction within the device (353). No changes in patient position are necessary (372). Following transfer, the catheter is checked for retained embryos. If present, retained embryos should be transferred because there is no detriment in pregnancy rates (353).
Embryo cryopreservation at the pronuclear, cleavage, and blastocyst stages has allowed for multiple transfer cycles from a single oocyte retrieval. Because transfer of cryopreserved embryos is less expensive than a second fresh cycle, overall fertility treatment costs can be optimized. Embryo cryopreservation can be considered as a means to prevent ovarian hyperstimulation syndrome. Techniques for embryo cryopreservation include slow freezing and rapid freezing or vitrification. Slow freezing protocols use lower concentrations of cryoprotectants but are more time-consuming when compared to vitrification, which uses high-concentration cryoprotectants for rapid cooling and is less expensive. Embryo thawing is accomplished by brief exposure to air and warm water followed by rehydration (373). Although pregnancy rates for freeze/thaw transfer (FET) cycles using the two cryopreservation methods are similar, vitrification is associated with higher postthaw embryo survival (93% vs. 76% with slow freezing) (374). Infant outcomes are reassuring for slow freezing but are more limited for the newer technique of vitrification (375). Overall, use of frozen embryos results in lower pregnancy rates when compared to fresh transfer cycles, but this may be a result of embryo selection (the best embryos are typically