Comprehensive chromosome screening is done to analyze, select and transfer only the embryos with an appropriate number of chromosomes – a euploid, or normal, blastocyst. Conversely, aneuploidy describes an embryo with too many or too few chromosomes.
Embryologists have been creating and studying embryos for decades. However, embryo selection using traditional methods, like conventional imaging techniques, is not as selective as you might think. Early cell division often looks similar regardless of the number of chromosomes, so it can be difficult to tell if an embryo is abnormal. Since it’s unclear exactly what they’re working with, some clinics may choose to transfer multiple embryos, which also has multiple risks – it can result in a high-risk pregnancy with twins, triplets or more, or a miscarriage with an abnormal embryo.
The testing is performed on a few cells from a day 5 embryo, called a blastocyst, which has hatched and started to develop an early placenta. Several cells are sampled from the placenta and sent to a genetics lab so that the chromosomes can be counted. The blastocysts are cryopreserved, or frozen, for about a week until testing is complete. Embryo survival after freezing is about 98%.
Only the embryos with the correct number of chromosomes are selected for transfer – typically a single embryo at one time - or will remain frozen until the intended parents decide to use them. By refining the embryo selection process, embryologists can truly narrow down the healthiest embryos that are most likely to result in an ongoing pregnancy. Preimplantation genetic testing can be done on the embryos to identify further issues, but CCS is often sufficient.
The latest technological breakthrough in preimplantation genetic testing, next-generation sequencing (NGS) is a DNA sequencing process that allows the study of all 23 pairs of chromosomes at a more comprehensive level and even deeper resolution. With NGS, an entire human genome can be sequenced within a day. For perspective, the previous sequencing technology – called Sanger sequencing - took several years to complete.
While anyone can opt for CCS, there are several factors that make it especially beneficial:
It’s suspected that 70% of early miscarriage is related to a chromosomal abnormality in the fetus. Screening the embryos in advance can help achieve a higher rate of pregnancy with fewer losses.
A 2018 research study from the Colorado Center for Reproductive Medicine showed that, after identifying a normal, single euploid blastocyst through CCS, “live birth rates were almost identical in both age groups” being studied: 60 percent in women 38-42, and 64.6 percent in women 37 or younger.
According to Dr. William Schoolcraft, “I’ve been taught and I tell my patients – or at least I have for the last 20 years – that age is everything. It’s a big if, but if you can get a chromosomally normal embryo suddenly that decline goes away. That to me has been the most remarkable thing about CCS.”
On average, 1 out of 10 eggs will typically produce a chromosomally normal embryo. If, for example, 10 retrieved eggs result in 7 average embryos, it’s reasonable to expect 1 to 3 embryos with normal chromosomes after CCS testing.
It should be noted that research shows implantation rates actually improve for a biological mother when a cryopreserved embryo is transferred; the thought is that a later, natural cycle – no longer under the effects of ovarian stimulation - allows for a more normal endometrial lining. If a surrogate is carrying the baby, she can be ready for transfer as soon as testing is complete.
CCS can also identify gender, which, depending on your stance, could be viewed as a pro or a con.
Due to the success of preimplantation genetic testing, experts are confident that CCS is an equally accurate and safe procedure.
When paired with gestational surrogacy, which already has high success rates due to extensive testing and excellent uterine quality, CCS should only improve your chances of a successful pregnancy. To learn more about building your family through egg donation or surrogacy, contact our team today!
All Things Conceivable is a blog dedicated to sharing the knowledge and expert opinions of the dedicated team at ConceiveAbilities, a Chicago-based egg donation and surrogacy agency.