PGT-A Screening: What It Actually Does (And What It Doesn't)

PGT-A is a genetic test available to parents going through IVF that checks whether each embryo has the right number of chromosomes before transfer. A few cells are biopsied from the outer layer of a Day 5 or Day 6 blastocyst, the lab counts chromosomes, and embryos with the correct number (euploid) get prioritized. It's the most common add-on in embryo screening, and nearly half of all IVF cycles in the US now include it.

But here's the thing most clinics don't mention: PGT-A doesn't improve your overall chance of taking a baby home. The largest randomized trial ever run on it showed that. What it does do is reduce miscarriage per transfer, meaningfully improve outcomes for women over 35, and rescue embryos that would otherwise be thrown away. That's a different kind of value than "better success rates," and it's the one worth paying attention to.

If you're exploring what PGT-A can tell you about your embryos, or how it connects to deeper screening options, you can reach out to us here.

Half of IVF patients use it, but the evidence is complicated

Between 2014 and 2020, PGT-A use in the US jumped from 11.5% to 49.0% of all IVF cycles. Two-thirds of those patients are 35 or older. The test costs $3,000 to $12,000 depending on how many embryos you're testing.

And yet the largest randomized controlled trial on PGT-A, published in the New England Journal of Medicine, found no difference in cumulative live birth rates between PGT-A and morphology-based screening: 77.2% versus 81.8% in women aged 20 to 37 with at least three good-quality blastocysts. The STAR trial, a multicenter RCT, reached a similar conclusion for good-prognosis patients. ASRM's 2024 committee opinion characterized universal PGT-A as "unproven" for improving pregnancy rates across all patients.

So why does adoption keep climbing? Because the people promoting PGT-A are measuring the wrong thing. The cumulative live birth rate isn't the whole story. What matters is what happens along the way.

Where PGT-A actually changes the math

The same NEJM trial that showed no cumulative benefit found something that matters a lot if you've lived through a miscarriage: PGT-A reduced miscarriage rates per transfer from 12.6% to 8.7%. Fewer failed transfers. Less time waiting. Potentially fewer cycles to reach the same endpoint.

Then there's the age question. The STAR trial's post-hoc analysis found that women aged 35 to 40 had a 51% ongoing pregnancy rate with euploid transfer versus 37% with morphology-based screening. That's a big difference. Aneuploidy rates climb with maternal age, and by 35 a meaningful fraction of your embryos will carry chromosomal errors that looking at them under a microscope can't detect.

And there's a finding that gets surprisingly little attention: what PGT-A does for slow-developing embryos. Day 7 blastocysts have historically been discarded because their outcomes are poor. Untested D7 embryos have a live birth rate around 7.3%. But euploid D7 embryos identified through PGT-A have a live birth rate of 33.3%. That's the difference between an embryo most clinics would throw away and one with a real chance. For patients with limited embryos, that matters.

Mosaics and what PGT-A can't tell you

PGT-A measures something real. A prospective, blinded non-selection study confirmed that euploid embryos genuinely implant and result in live births at higher rates than aneuploid ones. When we compare sequencing results to born-child DNA, the concordance exceeds 99%.

But about 5 to 15% of results come back as mosaic: a mix of normal and abnormal cells in the biopsy. And here's where it gets complicated. The biopsy samples the trophectoderm, the outer cell layer that becomes the placenta, not the inner cell mass that becomes the baby. A mosaic reading from the trophectoderm doesn't necessarily mean the embryo itself is mosaic. When researchers rebiopsied embryos diagnosed as whole-chromosome mosaics by NGS, only 35% remained mosaic on the second biopsy. A good chunk of mosaic calls are measurement noise.

The clinical data is actually reassuring. PGDIS reviewed over 1,000 mosaic embryo transfers and found reduced implantation potential but low risk of chromosomally abnormal births. An analysis of 1,000 mosaic transfers proposed a ranking system showing that low-grade mosaics can yield outcomes similar to euploid embryos. A systematic review of PGT diagnostic accuracy supports the concordance between biopsy results and whole-embryo status, though it highlights the inherent sampling limitation.

A mosaic result isn't a death sentence for an embryo. It's a data point that needs context.

The same biopsy data can tell you more than chromosome counts

Here's the part most people don't know about. The sequencing data from PGT-A can also provide polygenic risk information through a technology called ImputePGTA. Without any additional biopsy, the existing data can generate risk scores for conditions like heart disease, type 2 diabetes, and breast cancer. If you've already had PGT-A done elsewhere, we can work with that external data combined with parental sequencing to extract polygenic risk scores that standard PGT-A labs don't provide.

PGT-A tells you which embryos have the right chromosome count. It reduces miscarriage per transfer. And it provides a data foundation for understanding your embryos at a level most labs don't touch. Whether that's worth the cost and timeline depends on your age, your embryo count, and what questions you're trying to answer. Our team is here if you want to explore what your embryo data can tell you.