Aneuploidy Screening for Abnormal Chromosomes
Euploid embryos have the correct number and amount of chromosomal material. Aneuploid embryos do not. Healthy cells have 23 sets of chromosomes, including one set of chromosomes called the sex chromosomes that determine the sex of the child. With aneuploidy screening, embryos are screened to determine if they have the correct number of chromosomes, or if there is a rearrangement of the chromosomes, known as a translocation.
A Trisomy 21, or Downs Syndrome Embryo has an aneuploid amount of chromosomes. It has an extra chromosome 21. In the past, Dallas IVF, and other IVF programs that offered PGD would typically screen for only 5 or 10 sets of chromosomes through a technique known as fluorescent in-situ hybridization (F.I.S.H.). Although this techniques is still available throughout the country, our practice now offers array CGH (comparative genomic hybridization) to our patients. Through the technique of array CGH, we can now screen all 23 sets of chromosomes to know which embryos are healthy and which are not. Array CGH has revolutionized PGD and has greatly benefited our patients.
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, our team at Dallas IVF will perform trophectoderm biopsies on all healthy appearing blastocysts on day 5. In addition, we reevaluate the embryos again on day 6 and if any additional blastocysts are present, we will perform additional biopsies. Array cGH, comparative genomic hybridization, is then performed on the biopsied cells. The laboratory results will then clearly show which embryos are healthy for not only the translocation, but also give us information on all 23 sets of chromosomes.