After an egg retrieval for a fresh cycle, or when contemplating a frozen embryo transfer, women often wonder how many embryos should be placed into the uterus. Usually, single embryo transfer (SET) is promoted, especially if an embryo is genetically tested (undergoes pre-implantation genetic testing, PGT). However, there are cases where a patient may want to transfer two or more embryos. Examining the pros and cons of multiple embryo transfer can help women make an informed decision.

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Preimplantation genetic testing (PGT) is the general term for genetic testing performed on embryos produced by in vitro fertilization (IVF) before the embryos are transferred to the uterus. The purpose of PGT is to improve the chances of having a successful embryo transfer.

Genetic testing does not impact or change the genetics of an embryo, but it does give doctors and prospective parents more information and may help them select the most viable embryos for transfer.  

There are three types of PGT: PGT for monogenic disorders (PGT-M), PGT for structural chromosomal rearrangements (PGT-SR), and PGT for aneuploidy (PGT-A). PGT-A is the most routine type performed and as such should be understood by prospective parents.

Embryo freezing, also known as cryopreservation, is an assisted reproductive technology which involves storing preimplantation-stage embryos created through in vitro fertilization (IVF). Cryopreservation is done at extremely low temperatures (-196°C or -321°F) to halt embryo development and preserve their vitality, in a process called vitrification. Following vitrification, these embryos can be safely cryopreserved (frozen) for extended periods until they are thawed for transfer back to the uterus (frozen embryo transfer).  

Cryopreservation is necessary when preimplantation genetic testing (PGT) is chosen. Typically, fresh embryos undergo a biopsy before freezing to allow time for the genetic tests to be conducted. However, there are cases where patients decide that they want to perform genetic testing of previously frozen embryos. In these circumstances, frozen embryos must be thawed for biopsy, then refrozen until further use.  This means that the embryos will undergo an additional freeze-thaw round compared to the typical protocol.

Diminished ovarian reserve is a condition in which the amount (quantity) of oocytes left in a women’s ovaries is low, as determined by ovarian reserve testing. This might result in infertility for some women. In addition, for females going through fertility treatments, DOR may put them at risk of having a poor response to fertility drugs.