Embryo transfer is the final step of the in vitro fertilization process. It usually occurs two to three days after the fertilization of the egg when the embryos are at the two- to eight-cell stage. Some embryo transfers occur five days after fertilization; instead of transferring the embryo at the cell stage, the day-5 embryo is now a blastocyst. The cells in the blastocyst have just begun to differentiate, increasing the chances of viability. With blastocyst embryo transfer, fewer — but hopefully better-quality — embryos will need to be transferred. This would reduce the possibilities of multiple pregnancies without compromising pregnancy success.
Procedure & Medications
The timing of the transfer is important as the endometrium needs to be appropriately prepared to receive the embryos. The ideal window of receptivity is during the luteal phase, which is during the second half of a woman's menstrual cycle and when progesterone is significantly higher. If frozen embryos are being transferred, a woman may be given estrogen treatments for two weeks prior to transfer and then oestrogen and progesterone for endometrium preparation.
Just before transfer, the transfer recipient is usually required to have a full bladder to enable easier viewing of the uterus on the sonogram. A sonogram is used to help guide the physician during the transfer process. A predetermined number of embryos is loaded into a thin transfer catheter and inserted through the cervical canal. The sonogram helps guide the catheter into the uterine cavity and more specifically within 1-2 cm from the top of the uterus (fundus). The embryos are deposited and the catheter will be checked to ensure that all contents have been transferred.
Per the guidance of the physician, the recipient may receive an injection of human chorionic gonadotrophin (hCG) to stimulate the ovaries to produce more progesterone during the implantation phase (three to four days after transfer). Self-injections of hCG may also be recommended in the days following the embryo transfer, as well as progesterone suppositories. Estrogen medication may also be given to recipients after embryo transfer.
Following the procedure, the recipient will usually be advised to rest on her back in a recovery room for a couple of hours. Some physicians may recommend bed-rest for the first 24 hours after transfer while others may allow their patient to resume normal activities shortly after the procedure. There are no documented studies that suggest either bed-rest or continuing normal activities has a better pregnancy success rate.
A pregnancy test is typically administered two weeks after transfer.
Number of Embryos
The number of embryos transferred during an IVF cycle has been heavily debated over the years. While some physicians may recommend up to six embryos in one transfer, some professional societies and current medical literature state no more than four embryos will produce optimal results. Multiple pregnancies only increases the possibility of complications for both the mother and the fetuses.
Not all transfer recipients are the female who has provided the eggs. Embryo transfer may be performed on a woman who has an appropriately prepared uterus but doesn't have the ability to produce eggs. The donated eggs may be fertilized with a donor's sperm or with the recipient's partner's sperm. For women who cannot carry a pregnancy to term but can produce viable eggs, a transfer recipient, or surrogate, may be elected to carry the embryo(s) to term.
Embryo transfer, the last step in IVF, has opened the doors of possibility for many women who wouldn't be able to conceive otherwise.