Pre-implantation genetic diagnosis (PGD) uses embryo biopsy to evaluate the chromosomes of the embryos created through in vitro fertilization (IVF), and screens them for the most common abnormalities associated with infertility and miscarriage. Healthy, chromosomally normal embryos can then be distinguished from nonviable and diseased ones, and only those are used for transfer.
Traditionally, embryos conceived by in vitro fertilization (IVF) were only assessed microscopically to predict their viability; as suggested by their overall appearance. Pre-implantation genetic diagnosis (PGD) goes a step further, by actually screening the genetic information contained within the embryo prior to deciding which embryos to transfer. In other words, embryos free from disease can be selected and preferentially transferred back to the patient.
A normal embryo must have 46 chromosomes in the right combination to enable normal viability - 23 from the mother and 23 from the father. If the early dividing cells do not divide equally and distribute these chromosomes equally and in the right way, then genetic imbalances result. This occurs more often in older women, and is the most significant reason why fertility declines with increasing age. This also explains why chromosomal and congenital anomalies also increase with increasing age of the female partner.