PGS, prenatal genetic screening, is a general term for testing the chromosomes of the embryo, or testing for a specific genetic carrier state in the embryos.
We accomplish this through a trophectoderm biopsy, a technique that entails taking a sample of the cells from the part of the embryo that is destined to become the placenta, or trophectoderm. However, the group of cells that will become the fetus is not touched. The biopsy is performed under high-power magnification using a micro-laser to separate some of the cells from the embryo. Those cells are then sent for genetic studies.
In experienced hands, this technique is safe and will not damage the embryo or affect its ability to implant in the uterus. In fact, the pregnancy rates resulting from chromosomally normal embryos is much higher than those of embryos selected based on their appearance alone. We have been performing this procedure since 2014, so our embryologists have extensive experience using the micro-laser for trophectoderm biopsy.
PGS and Elective Single Embryo Transfer
Many, if not most, patients will benefit from PGS under a variety of circumstances. Selecting a chromosomally normal embryo for transfer to the uterus allows for single embryo transfer, thereby minimizing the risk for multiple pregnancies that are a common result of transferring two or more embryos into the uterus. Multiple pregnancy is complicated by premature labor, premature delivery, cesarean delivery, complications in the newborn secondary to prematurity, and increased risk to the mother in developing gestational diabetes and pregnancy-induced hypertension.
Recurrent Miscarriage and Failed IVF Cycles
Most miscarriages and implantation failures are due to chromosomally abnormal embryos. Therefore patient’s with recurrent miscarriage and previously failed in vitro fertilization cycles may certainly benefit from PGS and trophectoderm biopsy.
Genetic abnormalities are very common in the human embryo; 50% of embryos may be chromosomally abnormal even in younger patients. As the patient ages, particularly over the age of 35, these genetic abnormalities increase, with a natural decrease in pregnancy rates and an increase in miscarriage rates. There is also an increased risk of genetic abnormalities in the newborn such as trisomy 21 or Down syndrome. One may view this technique as a way of avoiding transfer of chromosomally abnormal embryos.
Preventing Genetic Disorders
If both prospective parents are carriers of serious genetic traits, such as cystic fibrosis, that can have a detrimental affect on their future children, we can offer special screening techniques. A reproductive genetics laboratory can develop a probe to detect the particular trait in the embryo, thereby avoiding transfer of an embryo with two copies of the genetic trait that will cause the condition. Also standard genetic screening will also be performed so that both a chromosomally normal and non affected embryo may be selected for transfer.
Next Generation Sequencing (NGS)
At Westchester Fertility, we recommend the most up-to-date chromosome testing, called NGS, or next generation sequencing. This allows complete sequencing of the chromosomes after they are amplified in the reproductive genetics laboratory. This technique will detect mosaicism, a condition in which two or more cell lines are present in the trophectoderm tissue biopsy. This suggests that the fetal genetic status may also be similarly affected and is a cause for failure of implantation and miscarriage. An earlier genetic technique called array comparative genomic hybridization (aCGH)does not detect mosaicism, giving the impression of a chromosomally normal embryo, and in the presence of mosaicism, will most often not implant or result in a miscarriage.
Frozen Embryo Transfer
Because NGS requires several days for results, the embryos are immediately cryopreserved after the biopsy and the patient is prepared for a transfer in the following cycle. This does require addition of hormones to prepare the uterus but no stimulation of the ovaries takes place. While earlier techniques for cryopreservation resulted in comparatively lower pregnancy rates over fresh transfers, the technique of vitrification, which is a rapid freezing process in liquid nitrogen currently employed in our laboratory, results in excellent embryo survival in the hands of our experienced embryologists. The transfer therefore of a chromosomally normal embryo into a uterus prepared in a controlled hormonal environment, avoiding the high estrogen levels of a stimulated cycle, results in pregnancy rates of 70-80%, far surpassing that of fresh, untested embryo transfers in the stimulation cycle.