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TESTING DNA that is naturally discarded by embryos could safely聽reveal those produced using IVF that are most likely to聽lead to pregnancy.

IVF doctors try to transfer only聽the healthiest embryos for聽pregnancy. DNA testing to聽identify such embryos is already聽available, but it has to be聽done by removing a sample of聽cells, which carries risks.

鈥淭rying to refine our mechanisms for choosing the embryo that鈥檚 most likely to lead to pregnancy is something that鈥檚 been eluding us for ever,鈥� says Virginia Bolton, an embryologist in London who wasn鈥檛 involved in the work. 鈥淭his doesn鈥檛 damage the embryo in any way.鈥�

In IVF, sperm and eggs are put聽together in a dish, and any resulting embryos develop for about six days before, usually, the best one is transferred to the uterus. But embryos often fail to implant or end up miscarrying: birth rates can be about 30 per cent, or lower still for older women.

The key problem is thought to聽be embryos with an abnormal number of chromosomes, the 46聽packages of DNA in nearly every cell in the body. This is a common cause of miscarriage in聽non-IVF pregnancies too.

One solution in IVF is to take a few cells from the embryos in the lab to check their chromosomes. But such biopsies can harm the embryo, raising the chance of miscarriage. So this tends to be used only for older women, whose eggs聽are more likely to have chromosome problems.

There may be an alternative: some of an embryo鈥檚 cells die during normal development and their . As genetic analysis has improved, people have been trying to turn this into a safer form of testing.

Now Catherine Racowsky at Brigham and Women鈥檚 Hospital in聽Boston and her colleagues have shown that analysing this tiny amount of genetic material compares well with a standard biopsy. They used 52 embryos from IVF clinics that were no longer needed, and which had already had a biopsy done. They kept the embryos in a dish for 24聽hours and tested 10 microlitres of culture fluid from the dish. Then they analysed the whole embryos to double check their number of chromosomes.

Analysing the fluid was at least聽as good as the biopsy, and聽actually led to fewer false positives聽鈥� in which the test indicates problems even though the embryo is healthy.

鈥淭his shows DNA in spent culture medium can be reliably amplified and sequenced,鈥� Racowsky said at the Fertility 2019 conference in Birmingham, UK, where she presented the work.

But the team couldn鈥檛 get usable DNA results from the fluid of four of the 52 embryos. And we won鈥檛 know if this kind of testing would improve pregnancy rates until it has been tested in a randomised trial, says Mousa Shamonki at the University of California, Los Angeles, who is also investigating the technique.

Other groups have been looking at whether the best embryos can be identified from levels of biochemicals they secrete. But Bolton thinks DNA testing is more promising because results won鈥檛 be skewed by how much these chemicals are diluted. 鈥淭he DNA is either there or it isn鈥檛. How much it鈥檚 diluted doesn鈥檛 matter.鈥�