Currently fewer than three percent of mosaic embryos, which have a mixture of normal and aneuploid cells, are transferred into the uterus during IVF due to a belief they are not viable. This practice limits IVF where few alternative embryos are available to a patient, although there is limited evidence to suggest that mosaic embryos are not viable as a few abnormal cells might not affect fetal development.
'It was already known that putative mosaic embryos can develop to term and make healthy babies, but many of the previous studies that looked at this issue were affected by selection bias toward a population of patients that had a poor prognosis because they had previously failed implantations with euploid embryos,' said first author of the study Dr Antonio Capalbo, laboratory director at Igenomix, Marostica, Italy, which specialises in genomic testing services.
In the clinical trial, published in the journal American Journal of Human Genetics, embryos were tested with pre-implantation genetic testing that looked specifically for mosaic aneuploidy. In addition to euploid embryos (those with no aneuploidy), embryos with low-grade mosaicism (20 to 30 percent aneuploid cells) and medium-grade mosaicism (30 to 50 percent aneuploid cells) were blindly reported as euploid and implanted. This strategy avoided bias as any embryo that fitted the defined criteria could be used for implantation.
The investigators found that across 484 euploid, 282 low-grade mosaic, and 131 medium-grade mosaic embryos, the mosaic embryos were as likely to implant in the uterus as the normal embryos, and had the same likelihood of becoming a healthy baby carried to term. The obstetric and neonatal outcomes were also similar across the three groups.
Miscarriage rates were similar across the three groups and of the pregnancies which resulted in miscarriage which were investigated, none were due to aneuploidy.
'This is an area that has created a lot of controversy over the last decade, with some who believe it blindly and some who believe it's wrong. It has divided the field' Dr Ippokratis Sarris, director of King's Fertility, who was not involved in the study, commented in the Telegraph. 'Clinicians and doctors are asking: "what should I do with mosaic embryos?" We're becoming more confident in putting back mosaic embryos and seeing what happens.'
The researchers expect the study to change current guidelines on the use and reporting of mosaic embryos prior to IVF treatment, thus allowing patients to make more informed decisions about the transfer of mosaic embryos, which can be deemed healthy and normal according to the trial.