Dallas IVF
Infertility
Dallas TX
(214) 297-0020

IVF

Preimplantation Genetic Diagnosis

Preimplantation Genetic Diagnosis (PGD) or Preimplantation Genetic Screening (PGS) is the diagnosis of a genetic disease prior to a woman conceiving. Individual oocytes or embryos can be screened for specific genetic or chromosomal abnormalities and unaffected embryos are then transferred back into the woman. Currently, Dallas IVF performs PGD for a variety of genetic disorders. The most common indication for PGD at Dallas IVF has been for couples with chromosomal abnormalities, gene disorders, and chromosome translocations. The most common method of PGD performed by Dallas IVF is the technique of embryo biopsy. On the third day following the egg retrieval, most embryos are at a 6-10 cell stage. Our embryology staff will remove one of the cells and examine the chromosomes of that cell. This biopsy usually does not harm the dividing embryo as the embryo continues to divide. Currently, studies have not shown any adverse effects upon offspring born after PGD. The results of the genetic testing are then available two days later (day-5). These results will benefit the couple in choosing which blastocyst(s) to return to the uterus. The embryo transfer is performed on day-5 when the embryo is at the blastocyst stage.

Screening for Abnormal Chromosomes

Aneuploidy

With aneuploidy screening, embryos are screened to determine if they have the correct number of chromosomes. Through the technique of fluorescent in-situ hybridization (F.I.S.H.) the laboratory can determine if an embryo has chromosomal abnormalities associated with Down Syndrome (chromosomes 13, 18, or 21), or those that are commonly associated with miscarriages or failed implantation. Fluorescent markers bind to the individual chromosomes that can then be seen with a special microscope. For example, healthy embryos should have only two chromosome number 21s. However, a Down Syndrome embryo contains 3 chromosome 21’s (Trisomy 21). With F.I.S.H., the embryo displays three markers instead of the normal two.

Translocations

Approximately one in 600 individuals carries a balanced translocation. With a balanced translocation, an individual has the correct amount of chromosomal material and is healthy. However, a piece of one chromosome has switched places with another chromosome. Since the human embryo contains half its genetic material from each parent, there is a high probability that an embryo formed from an individual with a balanced translocation will either have too little or too much genetic material. The abnormal amount of genetic material causes the embryo to be ‘unbalanced’ and poses an extremely high rate of miscarriage. For couples who suffer from repetitive miscarriages, also known as recurrent abortion, our office will obtain parental chromosomal studies to determine if either the man or woman has a balanced translocation that could be the contributing cause for their pregnancy losses. With PGD performed for translocations, Dallas IVF will test each dividing 8-cell embryo to determine if it is ‘balanced’ or ‘unbalanced.’ Dallas IVF will only place ‘balanced’ or unaffected embryos back in to the woman at the time of her embryo transfer.

Single Gene Disorders

Many genetic diseases are caused by mutations or changes in the individual genes that comprise chromosomes. The three most common types of single gene disorders are:

  • Recessive Disorders - Every person has recessive genes. If an individual has a recessive gene for a disease, such as Cystic Fibrosis, they are a carrier for the disease. Although individuals carrying a recessive trait may not express the disease, they can pass it on to their offspring. If the man and the woman are both carriers for the same disease, there is a 25% chance that their offspring will have the full disease, and a 50% chance that they will just be a carrier. Different recessive disorders are more common in different ethnic groups. For instance, Cystic Fibrosis (CF) is more common in Caucasians and Sickle Cell disease is more common in African Americans.
  • Dominant Disorders - If either parent has a dominant gene disorder, each of his or her children have a 50% chance of having the disease. One of the more common dominant disorders is polycystic kidney disease (PKD).
  • X-linked disorders –These disorders depend upon the X chromosome. A woman normally has two X chromosomes (XX). An X-linked disorder can occur when one or both X chromosomes carry a disorder. If the woman only has one affected X chromosome, she may not express the disease. However, she has 50% chance of passing on that X-linked trait to her offspring. A male child will have a 50% chance of inheriting and expressing the disease whereas a female child will have a 50% chance of being a carrier and not necessarily expressing the disease.