Fertility Blog

Impact of Low-Level Mosaicism

Preimplantation genetic screening (PGS) has improved pregnancy and birth outcomes for women undergoing in vitro fertilization (IVF), especially in women at risk for an abnormal number of chromosomes (aneuploidy). Advances in PGS have even made it possible to spot a condition called mosaicism. Varying in severity, mosaicism is an early error in cell division that results in two or more cell lines with different genetic makeup.

Parsing mosaicism. An observational study examined the rate of low-level mosaicism in blastocysts with a normal complement of chromosomes (euploid) to blastocysts where no mosaicism was detected.1

  • Embryologists removed (biopsied) and “froze” (cryopreserved) 155 blastocysts from 22 patients.
  • Biopsied cells underwent PGS using next generation sequencing, with results classified as either euploid or aneuploid.
  • Among embryos diagnosed as euploid, embryologists subsequently transferred those with the best morphology—size, shape, and structure.

Results. Physicians transferred 36 euploid embryos to 22 patients over 26 frozen embryo transfers (FETs) with a total of 73 percent experiencing ongoing clinical pregnancies. Of these, 36 percent were classified as having mosaicism ranging from 15 to 45 percent.

Although implantation and clinical pregnancy rates were lower in embryos with mosaicism, they were not considered statistically lower. Embryos with low-level mosaicism had a 50 percent implantation rate and a 54 percent clinical pregnancy rate, compared with 64 percent and 65 percent, respectively, for embryos displaying no mosaicism.

Based on the study, the authors suggest that separating euploid embryos into two groups—those with and without mosaicism—might further enhance reproductive outcomes.

1. Kelk DA et al. “Low Level Mosaicism: Incidence and Implications on Clinical Pregnancies.” Presented October 18, 2016, at ASRM 2016 Scientific Congress & Expo.

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