How IVF Gender Selection Works

Last reviewed 2026-10-27

The sex is read from a few cells taken off the outer layer of a day-5 embryo. Those cells go to a genetics laboratory, which counts the chromosomes — the point of the test is to find chromosomally normal embryos, and the sex chromosomes come with the answer. The embryos are frozen while you wait for the report. You then transfer one embryo that is both chromosomally normal and the sex you chose.

Nothing in this process is specific to sex selection. It is ordinary IVF with preimplantation genetic testing for aneuploidy — PGT-A — which clinics worldwide perform to improve the odds of a chromosomally normal transfer. Sex is a by-product of counting chromosomes, because two of the chromosomes being counted are X and Y. That is the whole mechanism, and understanding it tells you immediately why some claims about this treatment cannot be true.

The six steps

Ovarian stimulation, about 10 to 14 days
Injectable hormones so that many follicles mature in one cycle instead of one. Monitoring by ultrasound and blood tests. The number of eggs collected is the single biggest driver of whether you will have a choice at the end, which is why your egg yield matters more to this treatment than the genetics laboratory does.
Egg collection and fertilisation
Eggs are retrieved under sedation and fertilised, usually by ICSI — a single sperm injected into each egg — because ICSI avoids stray sperm DNA contaminating the later genetic test. Not every egg is mature, and not every mature egg fertilises.
Culture to day 5 or 6 (blastocyst)
The embryos grow in the laboratory for five to six days. This is where attrition concentrates: a substantial share of fertilised eggs stop developing before day 5 and never reach biopsy. Only blastocysts can be biopsied safely.
Trophectoderm biopsy
An embryologist removes roughly five cells from the trophectoderm — the outer layer that becomes placenta, not the inner cell mass that becomes the baby. The embryo is then vitrified (flash-frozen) immediately. Each biopsy is labelled to its embryo so the report can be matched back.
PGT-A in the genetics laboratory, usually 1 to 3 weeks
The cells are amplified and sequenced. The report gives, per embryo, whether the chromosome count is normal (euploid), abnormal (aneuploid) or mixed/inconclusive (mosaic or no result) — and the sex chromosomes. Ask to see this report itself rather than a summary. An embryo listed as mosaic or no-result is a real category with real decisions attached, and a verbal summary tends to lose it.
Frozen embryo transfer
In a later cycle, the lining is prepared and one embryo — euploid, and the sex you chose — is thawed and transferred. Because of the biopsy and the wait for results, the transfer is always a frozen one. Remaining embryos stay in storage.

PGT-A, PGT-M, PGD, PGS: which acronym is which

The names have changed over time and clinics use them loosely, which makes comparing two quotes harder than it should be.

Term	What it is	Relevance to sex selection
PGT-A	Preimplantation genetic testing for aneuploidy — counts whole chromosomes.	This is the test that is actually used. Sex comes out of it as a by-product.
PGS	The former name for PGT-A. Same thing, older label.	Same test. If a quote says PGS, it means PGT-A.
PGD	The former umbrella term, now split into PGT-M and PGT-SR.	Still the most-searched name for this treatment, which is why clinics keep using it in marketing.
PGT-M	Testing for a specific inherited single-gene condition.	Different purpose. This is the route used for medical sex selection, where the aim is to avoid a sex-linked disease.
PGT-SR	Testing for structural chromosome rearrangements.	Not related to sex selection.

Why the embryo grade tells you nothing about sex

Embryos are graded on appearance — a number for how expanded the blastocyst is, then two letters for the quality of the inner cell mass and the outer layer. 4AA, 5AB, 3BB and so on. People search for these grades alongside “gender” constantly, and the answer is the same every time: a grade is a photograph, not a karyotype. It carries no information about chromosomes, so it carries none about sex. The widely repeated idea that higher-grade embryos skew male is not something you should plan around.

Where this can go wrong

No embryos reach biopsy. The commonest failure, and it has nothing to do with the genetics lab. If nothing reaches day 5, there is nothing to test and nothing to choose.
All euploid embryos are the other sex. Not an error — just the coin flip. See the arithmetic.
Mosaic or inconclusive results. A real and reasonably common category. Ask in advance what the clinic's policy is on transferring mosaic embryos, because the answer varies between clinics and you do not want to discover the policy at the same moment you discover the result.
Embryo does not survive the thaw. Uncommon with modern vitrification, but not zero.
Sample or labelling error. Rare, and the reason a reputable laboratory's chain of custody and accreditation are worth asking about by name.

Questions worth asking before you pay a deposit

Which genetics laboratory performs the PGT-A, and is it accredited — by whom?
For a patient of my age, how many euploid blastocysts does a cycle typically produce in your programme?
Will I receive the laboratory's own report for every embryo, including mosaic and no-result embryos?
What is your policy on transferring mosaic embryos?
Is ICSI used for all eggs, and is that included in the quoted price?
What does the quote include if no embryo is suitable for transfer — is any part refundable or carried forward?
What does annual embryo storage cost after the first year, and what happens if I stop paying?
Is non-medical sex selection lawful where this clinic operates, and under which instrument?

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How is the sex of an embryo determined in IVF?

About five cells are biopsied from the outer layer of a day-5 embryo and sent to a genetics laboratory, which counts the chromosomes. The sex chromosomes, X and Y, are part of that count, so the sex is reported along with whether the embryo is chromosomally normal.

Is PGD the same as PGT-A?

Not exactly. PGD was the old umbrella term; the test used for sex selection today is PGT-A, formerly called PGS. PGD now generally refers to PGT-M, which tests for a specific inherited condition.

Does the embryo biopsy harm the embryo?

The cells are taken from the trophectoderm, the part that becomes placenta rather than the fetus. Large-scale practice has not shown harm to resulting children, but it is an invasive step on the embryo and a small number of embryos do not survive biopsy and freezing.

Can you tell the gender from the embryo grade, like 4AA or 5AB?

No. Embryo grading describes appearance under a microscope and carries no chromosomal information. Only a genetic test identifies sex.

How long does the whole process take?

Roughly two weeks of stimulation, then one to three weeks for the genetics report, then a separate frozen transfer cycle. From first injection to transfer is typically six to ten weeks.

Sources

Franasiak JM et al., The nature of aneuploidy with increasing age of the female partner: a review of 15,169 consecutive trophectoderm biopsies, Fertility and Sterility, 2014 — the reference dataset for how euploid embryo rates fall with maternal age
American Society for Reproductive Medicine / SART — practice guidance on preimplantation genetic testing and the limits of PGT-A — link
Human Fertilisation and Embryology Authority (UK) — patient-facing explanation of PGT and embryo testing — link

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