Mosaicism and IVF Success

Peter Kovacs, MD, PhD


April 13, 2018

Outcomes of Mosaic Embryo Transfer

Successful implantation requires a healthy embryo, receptive endometrium, and proper synchronization. In vitro fertilization (IVF) offers the best chance to conceive for infertile couples, but it still only offers a 25%-30% chance to give birth.[1]

There are several predictors of IVF success.[2] Female age is probably the most important among them. With age, ovarian reserve declines, as does oocyte quality. As a result, fewer embryos are obtained during IVF, and the proportion of genetically unhealthy embryos increases. This results in lower implantation rates, higher miscarriage rates, and longer time to achieve a successful pregnancy.

If we knew the genetic composition of the embryo before the transfer, we could avoid the transfer of unhealthy embryos and could expect improved clinical outcomes. This concept was the basis for the development of preimplantation genetic screening (PGS) of embryos. The technology has improved significantly over the years, but still a large proportion of embryos diagnosed as euploid do not implant. On the other hand, some of the embryos that tested abnormal do implant and result in healthy deliveries. A recent study[3] evaluated IVF outcomes after the transfer of mosaic embryos.

The study evaluated 77 women who had no euploid embryos and accepted the transfer of mosaic embryos. Blastocyst-stage trophectoderm biopsies were performed, and array comparative genomic hybridization and next-generation sequencing technologies were used for genetic screening. The results were classified as euploid, aneuploid, or mosaic. A total of 78 embryos were transferred. In 37 of 78 cycles, a positive pregnancy test was obtained, and among these were eight biochemical pregnancies and six miscarriages. The clinical pregnancy rate was 30%, and 24 children were born.

Mosaic embryos were divided according to the degree of mosaicism: either low (<50% abnormal cells) or high (>50% abnormal cells). Implantation (48.9% vs 24.2%), clinical pregnancy rate per transfer (40.9% vs 15.2%) and live birth rates (42.2% vs 15.2%) were significantly higher in the low mosaic group. Embryos with more chromosomes involved in the mosaicism were less likely to result in a positive outcome compared with embryos with single or double chromosome trisomy or monosomy.

Embryos with low mosaicism (<50%) resulted in similar clinical outcomes to those of euploid embryos. Implantation, pregnancy, and live birth rates, however, were lower for embryos with high degrees of mosaicism compared with euploid embryos.


The proportion of aneuploid embryos increases with age.[4] This is paralleled by a reduction in both spontaneous and IVF pregnancy rates.[2] PGS enables us to assess the entire chromosome content of the embryos created in vitro. Not counting those cases in which, for technical reasons, a result cannot be provided, in at least one third of the cases embryonic mosaicism is detected.[5] In these cases, two or more different cell lines can be detected in the embryo. When the patient has euploid embryos, those are transferred. Previously successful deliveries after the transfer of mosaic embryos have been reported.[6]

This study has also shown that good implantation and pregnancy rates can be achieved with the transfer of mosaic embryos, and the miscarriage rate is not increased. All of these pregnancies underwent prenatal genetic testing, and normal chromosome content was confirmed. The result was 24 healthy babies. The degree of mosaicism, the number of chromosomes involved, and the chromosome itself may differ. Embryos with high degrees of mosaicism or multiple involved chromosomes are less likely to implant. Spinella and colleagues[3] hypothesized that aneuploid cells may proliferate more slowly, may have a growth disadvantage, or may be eliminated by another mechanism (eg, apoptosis) during embryonic development.

They also stressed the need for prenatal genetic testing. Embryos may implant and progress normally with certain trisomies (chromosome 13, 18, 21); and, therefore, the transfer of embryos with such mosaicism should preferably be avoided. Only after proper counseling should patients be offered the transfer of mosaic embryos and only if euploid embryos are not available.


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