As this test is built around one family’s specific genetic change, it cannot simply be ordered off a shelf. A custom test has to be designed first, and this groundwork typically takes a few weeks to a few months before in vitro fertilization even begins. The laboratory usually needs a blood or saliva sample not only from both partners but often from other relatives as well, such as an affected child or parent. This is because the most reliable approach, called linkage or haplotype analysis, does not just look for the variant itself but it tracks a pattern of genetic markers running alongside the variant on the same chromosome, inherited together as a set. Tracking this surrounding pattern, alongside direct testing for the variant, helps the laboratory avoid a technical error called allele dropout, in which a single cell’s DNA fails to amplify properly and produces a misleading result.

Once the custom test is ready, the couple proceeds through a standard in vitro fertilization cycle:
• Ovarian stimulation and egg retrieval: hormone medications are used to help the ovaries produce multiple eggs, which are then collected in a minor procedure.
• Fertilization: eggs are fertilized in the laboratory, usually using a technique called intracytoplasmic sperm injection, in which a single sperm is injected directly into an egg, to reduce the chance of stray genetic material contaminating the sample.
• Growth to the blastocyst stage: embryos are cultured for five to six days until they reach a stage with a distinct outer layer, called the trophectoderm, and an inner cluster of cells that will go on to form the baby.
• Biopsy: a small number of cells are gently removed from the outer trophectoderm layer only, not from the inner cluster that becomes the fetus, and the embryo is then frozen while those cells are sent for genetic analysis.
• Result and selection: once results are available, an embryo found not to carry the familial variant, and generally forming normally, is prioritized for transfer in a later cycle.