The holy grail of assisted reproductive technology (ART) is the delivery of a healthy child. From the world’s first successful ART cycle of in vitro fertilization in 1978 (3 years later in the United States), the goal of every cycle is to provide the woman with an embryo that has the highest potential for implantation and, ultimately, a single live birth.
Dr. Mark P. Trolice
Embryo aneuploidy is a major factor in the success of human reproduction. As women age, aneuploidy is reported in less than 30% of women aged younger than 35 years but rises to 90% for those in their mid-40s. Intuitively and through randomized, controlled trials, chromosome testing of embryos is a reasonable approach toward improved cycle outcomes and allows for the transfer of a single euploid embryo.
Recently, the phrase “add-ons” has entered the vernacular of editorials on IVF. These additional procedures are offered to patients with the expectation of improving results, yet many have not been supported by rigorous scientifically controlled research trials, e.g., endometrial scratch, embryo glue, and time-lapse imaging of embryos. Where does preimplantation genetic testing (PGT) belong in the IVF armamentarium and why, after 30 years, are there two diametrically opposed views on its benefit? (We will not address testing for single gene defects or chromosome structural rearrangements.)
How did we get here?
The first iteration of PGT used fluorescence in situ hybridization to not only identify X-linked recessive diseases (Hum Genet. 1992;89:18-22) but also the most common chromosome disorders (13, 18, 21, X, Y) by removing one to two blastomere cells from a day 3 embryo (six- to eight-cell stage). Despite wide enthusiasm, the technique was eventually determined to reduce implantation by nearly 40% and was abandoned; presumably impairing the embryo by removing up to one-third of its make-up.
Because of extended embryo culture to the blastocyst stage along with the improved cryopreservation process of vitrification, the next generation of embryo analysis surfaced, what we now refer to as PGT 2.0. Currently, approximately five to six cells from the outer embryo trophectoderm are removed and sent to a specialized laboratory for 24-chromosome screening while the biopsied embryos are cryopreserved. Outcome data (aneuploidy rates, mosaicism) have been influenced by the evolution of genetic platforms – from array comparative genome hybridization to single-nucleotide polymorphism array, to quantitative polymerase chain reaction, to next-generation sequencing (NGS). The newest platform, NGS with high resolution, provides the most extensive degree of analysis by detecting unbalanced translocations and a low cut-off percentage for mosaicism (20%). The clinical error rate is approximately 1%-2%, improved from the 2%-4% of earlier techniques.
The phenomenon of mosaicism describes two distinct cell lines in one embryo (typically one normal and one abnormal) and is defined based on the percentage of mosaicism – currently, the lower limit is 20%. Embryos with less than 20%-30% mosaicism are considered euploid and those greater than 70%-80% are aneuploid. Of note, clinics that do not request the reporting of mosaicism can result in the potential discarding of embryos labeled as aneuploid that would otherwise have potentially resulted in a live birth. The higher the cut-off value for designating mosaicism, the lower the false-positive rate (declaring an embryo aneuploid when euploid). While there is no safe degree of mosaicism, most transfers have resulted in chromosomally normal infants despite a lower implantation rate and higher miscarriage rate.