Testing Your Embryo's Chromosomes
In a normal embryo, there are 23 pairs of chromosomes, either 46XX or 46XY. Chromosomes are the tiny structures in each cell that carry our genes. All of us have 23 pairs of them, consisting of one set from our mother and one set from our father. In some instances, when the egg and sperm meet, one or the other is faulty. In that case, the resulting embryo has a chromosomal abnormality and the pregnancy often results in a miscarriage. Mismatched chromosomes account for more than 60 percent of miscarriages.
Today all of the 23 pairs of human chromosomes may be genetically tested prior to embryo transfer. Previously used technologies only allowed testing of 5 to 11 pairs of chromosomes. Screening all the chromosomes becomes particularly important to patients over 40, who are prone to chromosomal errors in their eggs, as well as those patients with a history of pregnancy loss and those with repeat failed cycles of IVF. The goal of PGD for these patients is to reduce the likelihood of implantation failure and miscarriage by transferring embryos with the correct number of chromosomes.
Women who undergo PGD significantly reduce their chance of having a baby with a genetic disease. However, it is recommended that additional testing be conducted once the pregnancy is established. A chorionic villus sampling can be performed at 10 to 12 weeks, or an amniocentesis can be performed after 15 weeks to help confirm a normal pregnancy.
Avoiding Sex-linked Disease with PGD
PGD can also be utilized to select embryos of a particular gender to avoid passing on one of the more than 500 sex-linked genetic diseases to their child, or for family balancing. Gender selection rates approach 100 percent with the use of PGD.
Men determine the gender of their offspring. When a sperm with a Y chromosome fertilizes an egg, it makes a boy. When an X chromosome bearing sperm fertilizes, it makes a girl. Any given sperm sample contains an approximately even (50/50) amount of X (female) and Y (male) bearing sperm.
Gender selection is becoming more common as many couples want to experience the joy of raising children of both genders. A couple may have several children of one gender already and would like to have another child, but would only consider doing so if the 50/50 odds could be shifted in favor of the other gender. Or, perhaps a couple is seeking infertility treatment, already has one child, and would prefer to employ gender selection for the next child for the purpose of family balancing.