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Embryo freezing is used widely by IVF clinics all across the world and is a common practice now. Historically, ‘slow freeze’ technology was used to freeze embryos though computerized freezers which were relatively inefficient and when compared to a ‘fresh transfer’, the success rate of a cycle involving frozen embryo transfer was lower. The explanation might be the low optimal survival rate of embryo after slow freezing.

The use of vitrification, which essentially means rapid cooling without ice crystal formation., as well as the technique of culturing embryos in the blastocyst stage for 5 to 6 days before freezing has led to dramatically positive results. In old days, the option of embryo freezing was availed only by patients that had extra embryos. Multiple embryos were transferred and ‘leftovers’ were frozen.

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Current Indications for Embryo Freezing:

1. Prevention of ovarian hyperstimulation syndrome (OHSS). Some women have an exaggerated response to the injectable hormone medications used during IVF. In pregnant patients, it could become a serious complication. Freezing all the embryos and transferring them the next month giving time to uterus to settle down, is one way of preventing it. Freezing embryos is highly recommended in patients with high estradiol levels and other high-risk factors for severe OHSS.

2. Research shows that there are less chances of pregnancy if the progesterone levels rise prematurely on the day of hCG trigger. These higher levels advance the uterine lining and reduces embryo receptivity during embryo transfer. Success rates are normalized when embryos are frozen and transferred in the subsequent cycle.

3. If the embryos need to be biopsied for family balancing or for preimplantation genetic screening: If there is a chance of the mother passing a genetic disorder to the baby, it is recommended to undertake genetic screening of the embryos before freezing them. The healthy embryos without the genetic disorder are then transferred to ensure a healthy outcome. Couples opting for PGT-A for balancing their families also utilize this technology.

4. Slow growing embryos: Unlike the embryos that divide normally after egg retrieval and take five days to reach the blastocyst, the slow growing embryos take six days. There is evidence that even though both embryos have similar success rates for frozen embryo transfer, the disadvantage with slow growing embryos is exhibited in the reduced pregnancy rates with fresh transfer of day 6 blastocysts.

5. Studies have given enough evidence to prove that frozen embryo transfer exhibit higher pregnancy rate in comparison to fresh embryo transfer cycles. The reason simply being that in fresh cycles, the lining is not very receptive as it has been exposed to plenty of estrogen. On the other hand, after a rest period and after ensuring that endometrial thickness is adequate, a frozen embryo transfer finds greater receptivity in a more physiological uterine lining.

6. Another contraindication for fresh embryo transfer is the presence of fluid or endometrial polyp or endometriosis in uterus on the day of transfer. These factors reduce the receptivity of uterine lining. A plan of action is put in place by most IVF centers before commencing with an IVF cycle.

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