Preimplantation Genetic Testing (PGT-A)
Selecting Healthy Embryos: PGT-A & In Vitro Fertilization
PFC offers preimplantation genetic testing for aneuploidy (PGT-A) as an option for patients undergoing IVF. PGT-A is one of the technologies that sets apart fertility programs such as Pacific Fertility Center, and we are pleased to offer this testing to our patients.
- Improve IVF pregnancy rates per transfer
- Support elective Single Embryo Transfer (eSET), reducing the risk of multiple gestation
- Reduce the risk of miscarriage
- Reduce the risk of chromosomal abnormalities such as Down syndrome
Patients who can benefit from PGT-A include those who have:
- Experienced recurrent miscarriage for unknown reasons or due to prior aneuploidy
- Increased risk for aneuploidy (such as Down syndrome) due to maternal age
- Additional medical risks associated with the possibility of twin pregnancy
- Risk for sex-linked disease in offspring or other reasons for sex selection
- Plans to use a gestational carrier
If you choose to utilize PGT-A you will be required to meet with our in-house Genetic Counselors prior to your treatment cycle.
How does PGT-A work?
Providing high pregnancy rates with minimum risk after in vitro fertilization requires high quality embryos. But a problem remains in the IVF lab – which embryos are high quality? Embryos that look good under the microscope don't always have what it takes to produce a pregnancy. PGT-A allows us to screen embryos for aneuploidy and select chromosomally normal embryos that are eligible for transfer. However, PGT-A alone does not increase the number of chromosomally normal embryos available for transfer. Embryos are biopsied on days 5-7 of development (the blastocyst stage). A few cells from the trophectoderm (which would eventually become the placenta) are sent to a genetics lab for testing.
PGT-A testing shows us two possible results:
- Euploid: a euploid or “normal” results means that 23 pairs of chromosomes were detected in the embryo biopsy.
- Aneuploid: an aneuploid or “abnormal” result means that at least one chromosomal abnormality was detected in the embryo biopsy.
The biology behind PGT-A
An embryo that contains a normal number of chromosomes is a 'euploid' embryo. An embryo that carries an abnormal number of chromosomes is 'aneuploid'. Examples of aneuploid embryos include trisomy 18 and trisomy 21, otherwise known as Edward syndrome and Down Syndrome respectively. Embryos can also be affected by complex combinations of abnormal chromosomes, and even contain an extra entire set of chromosomes (triploidy).
Aneuploidy is quite common and, more often than not, PGT-A testing will show that an embryo is aneuploid. Half or more (sometimes all) embryos in a particular IVF treatment cycle are aneuploid, and while we can't prove this, we suspect that the majority of natural conceptions are affected by aneuploidy as well. The common clinical scenarios of age-related infertility, miscarriage, and Down syndrome are all expressions of aneuploidy.
In the early days of embryo development, most of these aneuploid embryos don't grow, and are never seen clinically. A few aneuploid embryos continue development and result in clinical problems. Miscarriage occurs in about a fourth of pregnancies – most of these are aneuploid. The rate of aneuploid embryos resulting in miscarriage increases dramatically with maternal age such that 50% or more of pregnancies after age 40 will miscarry. In milder forms of aneuploidy, embryos can survive longer, with a few continuing on into pregnancy, accounting for common problems like Down syndrome.
Most eggs and their resulting embryos do not have the ability to produce a viable pregnancy. On average, about 10 eggs are required to produce a euploid embryo. It works like this: approximately two-thirds of eggs fertilize and form an embryo. Roughly half of the embryos develop to the blastocyst stage at day 5-7, and about half or less of the day 5-7 embryos will be euploid. In other words, 10 eggs can produce 1-2 euploid embryos (created from eggs of a woman in her 30s). About 3 out of 4 of these euploid embryos will implant on average (75 percent implantation rate), with high pregnancy rates per transfer.
Age is the best predictor of aneuploidy and a major factor to consider when thinking about aneuploidy. In a woman under age 30, about half of embryos are aneuploid. At age 42, around 80% of embryos are aneuploid. On occasion, especially in older age groups, all embryos from a cycle are aneuploid.
PGT-A Pros and Cons
Pros of PGT-A include:
- Improved selection of embryos most likely to result in pregnancy and birth
- Reduction in aneuploidy
- Reduction in miscarriage rate
- Improved implantation rate
- Reduced multiple (twin or more) pregnancy risk
- Identification of sex of embryo
- Rare errors in test results, assumed to be mostly due to mosaicism (mixture of euploid and aneuploid cells in an embryo) that is not detected
- Few randomized prospective trials
- Data on safety incomplete, but reassuring
- Data on delivered child outcomes is under continued study
Despite the fact that there are few randomized trials and data on outcomes is limited, we have enough experience with pre-implantation genetic testing to feel confident that this is a safe and highly accurate procedure.