Family Balancing & Gender Selection
What are chromosomes and genes?
Chromosomes are structures found in the nucleus or center of a cell that contain all of our genes. The genes are composed of DNA and are carried on the chromosomes. Genes direct the production of all of the molecules that form the structures of a cell, and determine the inherited characteristics that distinguish one individual from another.
Human cells should have 23 chromosome pairs (46 total). Two of the 46 are the sex chromosomes, which are the X and Y chromosomes. Normally, females have two X chromosomes and males have one X and one Y chromosome. During reproduction, each parent contributes 23 chromosomes. The egg has 22 chromosomes plus an “X” chromosome. The sperm has 22 chromosomes plus an “X” or “Y” chromosome, and determines the sex of the baby.
Preimplantation Genetic Diagnosis
Preimplantation Genetic Diagnosis (PGD) is a technique that provides us with information about an embryo before it is transferred to the uterus (preimplantation). It is different from amniocentesis and chorionic villus sampling which are used to determine fetal abnormalities nine weeks or more after implantation (post-implantation). PGD technology allows us to identify single gene defects (cystic fibrosis, Huntington’s disease etc), chromosomal translocations (Robertsonian, Reciprocal) as well as numerical chromosome abnormalities (Down syndrome, Turner syndrome) in the early embryo.
How is PGD Performed?
PGD begins with embryo biopsy
Patients electing to have PGD will undergo an IVF cycle where the woman is given fertility medication to increase the number of eggs that will mature and be retrieved during the egg retrieval. Her male partner will provide a sperm sample, and the eggs and sperm will either be incubated together to achieve fertilization or each egg will be injected with a single sperm in a procedure called Intracytoplasmic sperm injection (ICSI). Normally, one-half or more of the eggs will become fertilized and develop into embryos. The embryos must grow in the laboratory for three days before the PGD process can be initiated. By this time, the embryos should have between 6 and 8 cells, called blastomeres, and a micromanipulation procedure called embryo biopsy is done. During embryo biopsy, a small opening is made in the protective coating or shell surrounding the embryo, called the zona pellucida. A tiny glass tube or “pipette” is used to enter the shell and gently remove one of the blastomeres. This blastomere will be handled in different ways depending on whether PGD will be done to determine chromosomal abnormalities (aneuploidy) or an inherited genetic disorder (single gene defects) in the embryo.
What are the risks of the PGD procedure?
While PGD is a relatively new procedure offered to the IVF patient, the embryo biopsy technique itself employs methods (i.e. micromanipulation) that are already commonly used in an IVF lab. The actual risk of damaging an embryo during a biopsy procedure is less than 1% in experienced PGD centers. In addition, removal of one cell does not eliminate any part of the future fetus. The embryo at this point is “totipotent” or has “all potential”. This means that any single cell in an embryo up to approximately four days in culture has the ability to produce a baby. In other words, the cells at this point are not differentiated. With PGD, the chances of having a misdiagnosis are 10%. However, a false negative result (the chance of classifying an abnormal embryo as normal) is only 3.5%.
What are the benefits of the PGD procedure?
By assessing your embryos for chromosomal abnormalities, we can avoid the risk of implanting an abnormal embryo into your uterus. Aneuploidy screening avoids the transfer of embryos that would never implant due to chromosome abnormalities; thus maximizing your chances of getting pregnant in a single cycle. Additionally, studies have shown that PGD for aneuploidy increases implantation rates, reduces the rate of pregnancy loss by half, and increases take-home baby rates.
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