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A human egg is being injected with a syringe, and is surrounded by double helices.

Q: What is preimplantation genetic diagnosis (PGD)?

A: PGD is a procedure for testing the genetic makeup of a very early embryo created through in vitro fertilization (IVF), before it is implanted into the womb of the woman who will carry it to term. At this stage, the embryo has about eight cells, which are typically identical; they have started to multiply but have not yet begun to differentiate. One cell is extracted and analyzed; the remaining cells are capable of developing into a complete fetus.

Most IVF protocols involve hormonal treatments that hyper-stimulate women's ovaries to produce multiple eggs, which typically generate several embryos. PGD offers a tool for selecting which embryo(s) to implant in a woman's uterus.

Q: What is the purpose of PGD?

A: PGD was developed to allow couples at risk of passing on serious genetic diseases to have children not affected by it. Since its introduction in 1990, it has been most widely used to prevent the birth of children with conditions such as Down's syndrome, Tay-Sachs disease, cystic fibrosis, sickle cell, Huntington's chorea, and...