Fertility Blog

More Males After Blastocyst-Stage Transfers


The study methods. In the study, 535 patients underwent IVF using their own eggs and intracytoplasmic sperm injection (ICSI), using sperm from males with no severe male factor infertility. Next:

  • Embryologists cultured embryos to the blastocyst stage—a stage where there is a central fluid-filled cavity and 2 distinct cell types, and cells have just begun to differentiate.
  • On Day 5 or 6, embryologists removed (biopsied) a few cells from the embryos.
  • These cells underwent comprehensive chromosome screening (CCS) to ensure a normal complement of chromosomes (euploidy).
  • Embryologists “froze” (vitrified) the embryos.
  • With the help of CCS, physicians subsequently transferred only euploid embryos —one at a time—and patients did not request sex selection.

Males favored. Despite gender equivalency at the blastocyst stage, female embryos appeared to end up with the short end of the stick. Of the live births following a frozen single euploid blastocyst transfer, 239 were females and 296 were males—an approximate 10 percent disparity.

In addition, this gender bias was consistent whether the blastocyst was biopsied on Day 5 or Day 6. Among those biopsied on Day 5, 149 infants were females and 188 were males. Among those biopsied on Day 6, 90 infants were females and 108 were males.

Why the difference? The study was not designed to answer this question. However, imprinted X chromosome inactivation (iXCI) is a possible explanation, say the researchers. This occurs when one copy of the X chromosome becomes inactivated through an epigenetic process—a genetic change not related to DNA. This inactivation might influence female mortality early after implantation, and alter gender ratios after birth. More research is needed to understand exactly what’s behind the significant gender bias in IVF.

1. Brown BK et al. “Altered Live Birth Gender Ratio Following Frozen Single Euploid Blastocyst Transfer.” Presented October 17, 2016, at ASRM 2016 Scientific Congress & Expo.

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