PGD can help couples not only select the gender of their child but the process can also assist couples who could potentially transmit a sex-linked genetic disease (disease carried on the sex chromosome) to their children. For example, hemophilia can only be transferred from an affected mother to her son.
With PGD, the first step is to gather mature eggs from the woman’s ovaries so that they can be fertilized in a lab (IVF). IVF generally requires the woman to take fertility hormones to prepare her for egg retrieval. IVF-PGD is fertilization of an egg by a sperm in a laboratory dish with either the parents’ egg and sperm or donor eggs and/or sperm — but with an important extra step. The embryo grows in the lab dish for 3 to 4 days until it reaches the 6- to 8-cell stage. Then one cell, called a blastomere, is removed from the embryo and its sex chromosomes are examined under a powerful microscope, to determine whether it’s a male or female embryo (and if the couple opts, to examine the cell for any chromosomal abnormalities).
Scientists remove the cell by making a small incision with a laser beam in the single cell’s mucus layer, called the zona pellucida, which surrounds the embryo. This cell is then analyzed using one of several techniques, each of which has advantages and disadvantages:
- fluorescent in situ hybridization (fluorescent in situ hybridization (FISH)),
- polymerase chain reaction (PCR),
- single nucleotide polymorphism (SNP) analysis, and
- comparative genomic hybridization (CGH).
fluorescent in situ hybridization (FISH) is the most common PGD technique for gender selection and for genetic diagnosis. Following the analysis, the embryo, minus this one cell, is transferred into the female’s uterus through IUI, via the vaginal canal. IUI, a minor, out-patient surgery requires local anesthesia.