Test the embryo before the transfer.

Preimplantation Genetic Testing examines an embryo's chromosomes before it is ever transferred. On day five or six of culture, a few cells are removed from the trophectoderm — the outer layer that goes on to form the placenta, not the baby — and analysed while the embryo is safely vitrified. Only embryos with the expected chromosome count are chosen for transfer.

This matters because chromosomal abnormality is the single largest cause of embryos failing to implant and of early miscarriage. An embryo can look flawless under the microscope, grade beautifully, and still carry an extra or missing chromosome that makes ongoing pregnancy impossible. Appearance alone cannot detect this; only genetic analysis can.

The proportion of embryos affected rises steadily with maternal age, which is why PGT is discussed most often for women in their late thirties and forties, for couples with recurrent pregnancy loss, and for those whose good-looking embryos have repeatedly failed to implant. It is also used where a specific inherited condition runs in the family.

It is important to be clear about what PGT does and does not do. It does not improve an embryo — it only tells us which existing embryos are worth transferring. It cannot create a normal embryo where none exists, and if every embryo in a cycle tests abnormal, no transfer takes place. What it reliably does is reduce wasted transfers, shorten the time to a viable pregnancy, and lower the risk of a miscarriage you would otherwise have had to live through.