New Technologies Screen for all 23 Pairs of Human Chromosomes
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 and then the chromosomes can’t line up properly. In that case, the resulting embryo has a chromosomal abnormality and the pregnancy usually results in a miscarriage. According to the American College of Obstetricians and Gynecologists, mismatched chromosomes account for at least 60 percent of miscarriages.
Houston IVF is moving beyond the boundaries of traditional Preimplantation Genetic Screening (PGS) research by conducting a clinical trial of a new process called Comprehensive Chromosome Screening (CCS), where all 23 pairs of human chromosomes are screened prior to embryo transfer. This is a radical departure from traditional PGS, where only 5 to 9 pairs of the chromosomes were screened. This comprehensive chromosome screening 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 losses or who have had multiple failed IVFs. The goal of CCS is to reduce the likelihood of implantation failure, miscarriage and pregnancies affected with chromosomal abnormalities by preferentially transferring only embryos that have the correct number of chromosomes.
Some couples are at a higher risk than others for producing offspring with genetic disorders, such as carriers of cystic fibrosis or Tay-Sach’s disease. These couples may wish to prevent the birth of a child with a genetic disorder. An estimated 60 percent of all early miscarriages are associated with a chromosomal abnormality in the fetus. The purpose of CCS is to select and transfer to the uterus only embryos that do not have numerical abnormalities for the chromosomes tested in order to achieve higher implantation rates and fewer pregnancy losses. Research has shown chromosomal abnormalities in embryos may increase the risk of spontaneous miscarriage or the development of a genetically abnormal fetus in IVF pregnancies from the following indications: women 35 or older, couples with multiple-failed IVF cycles or implantation failure, and couples with repeated miscarriages.
Only embryos, identified after screening as euploid are transferred. This should theoretically reduce the likelihood of implantation failure, miscarriage and/or aneuploid offspring. Preimplantation genetic screening does have inherent false positives and negatives due to the limited number of cells available for testing. An inherent 10% error rate means that there is no guarantee of a healthy baby . If a pregnancy is established, traditional prenatal diagnosis like chorionic villus sampling or amniocentesis is highly recommended.
Today, it is common to transfer multiple embryos, which increases the likelihood of implantation, but also the chance of multiple births. A single, healthy baby is the optimal goal for both the woman’s health and that of her baby. With an implantation rate (per embryo transferred) of 65%, the CCS study gives potential to make single embryo transfer a successful reality.
Potential CCS Advantages
The more traditional technique of performing PGS employs fluorescent in situ hybridization (FISH). FISH analysis on a single cell is limited to analysis of 5-10 chromosomes out of the total 23 pairs of chromosomes. Since aneuploidy can affect any chromosome, it may be beneficial to expand PGS to include screening of all 23 pairs of chromosomes. This analysis of the entire oocyte or embryo chromosome complement and further improvements in technology could potentially further increase the likelihood of implantation and decrease the chance of miscarriage for your patients. The day 5 blastocyst stage embryo also allows us to obtain several cells, as opposed to current practice, which allows removal of only a single cell from the less advanced day 3 cleavage stage embryos.
Houston IVF believes Comprehensive Chromosome Screening (CCS) could be a promising and beneficial PGS procedure for patients seeking to increase their chances of implantation and a live birth. We are pleased to continue to be on the cutting edge by providing patients with the latest scientific techniques to ensure healthy and successful pregnancies.
Results Tell the Story
Thus far, a total of 154 patients in the CCS study have had egg retrieval and subsequent frozen blastocyst transfer. Indications for the testing include advanced maternal age, previous IVF failure(s), and a history of multiple miscarriage(s). After the blastocyst embryos are biopsied, they are cryopreserved using a new technique called vitrification, which has resulted in a 97% blastocyst survival rate. The uterus is then prepared for a frozen embryo transfer after the woman’s body is allowed to naturally rid itself of the ovarian stimulation drugs required during an IVF cycle. One hundred and eight of the 154 patients in the study (70.1%) have ongoing clinical pregnancies, including 76 healthy deliveries to date. The first baby conceived with this procedure was born in June 2008. To date, over 100 babies have been born using CCS.