The practice of freezing embryos as part of in vitro fertilization (IVF) was thrown into chaos in Alabama this year, when the state supreme court ruled that such embryos should be considered children, exposing clinics to wrongful death claims in the event they are destroyed in the thawing process. In 2021, more than 80 percent of US IVF procedures involved the transfer of frozen embryos, according to a recent report by the US Department of Health and Human Services. IVF uses high doses of hormones to stimulate ovaries to produce as many eggs as possible. Once the eggs are extracted, the most mature are selected for attempts at fertilization with sperm. Over the next 5 to 6 days, healthy fertilized eggs grow into blastocysts—the earliest stage of embryo—containing roughly 100-200 cells. Blastocysts can be transferred into the uterus or frozen to be thawed for later use. After transfer into the uterus, if all goes well, the blastocyst implants itself in the wall of the womb and continues to grow. For some patients, freezing embryos and then waiting several weeks before a transfer into the uterus is more likely to lead to successful implantation, depending on their age, underlying health or hormone levels. The pause allows the body's hormone levels to normalize following ovarian stimulation. It also reduces the risk of ovarian hyperstimulation syndrome (OHSS), a potentially life-threatening effect of intense hormone use. More broadly, freezing embryos often means that only one course of painful, expensive ovarian stimulation and egg retrieval is needed. If an embryo transfer fails, additional embryos can be thawed and used. Freezing of embryos also allows patients to preserve fertility in advance of chemotherapy or other treatments that can damage the reproductive organs. Genetic screening of embryos before a transfer is only possible with freezing because it can take several weeks to get results. It is often employed when there is a history of recurrent miscarriages, previous IVF failures and maternal age over 35 or a family history of genetic diseases. The loss of the option to freeze embryos for later usage "would represent a significant setback for the field," said Dr. Steven Spandorfer of Weill Cornell Medicine in New York City, president of the Society for Assisted Reproductive Technology. A single pregnancy is the best way to promote the birth of a healthy child after IVF, so clinics are unlikely to return to the practice of transferring multiple embryos into the womb, Spandorfer said. Clinics could freeze eggs instead of embryos, but that approach has many limitations that would lower the overall success of IVF. The viability of those eggs would not be clear until they are individually thawed and IVF is attempted, creating the potential for delayed embryo transfers and the need for additional hormone use and retrievals.