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What is a mosaic embryo?


A recent Wall Street Journal article, IVF Testing Spurs a Debate Over “Mosaic” Embryos, discusses the categories of embryos, normal, abnormal and mosaic by genetic testing, known as Preimplantation Genetic Screening (PGS) when undergoing in vitro fertilization.

Classifying Mosaicism

Most fertility clinics and labs classify embryos either as “normal” (euploid) or “abnormal” (aneuploid). While embryos that are indicated as “normal” are approved to implant, “abnormal” embryos are not and mosaic embryos are unknown. Mosaic embryos have both normal and abnormal cells. Most fertility clinics and experts do not transfer or recommend this because of the risk of miscarriage, possible increased risk of birth disorders along with lower implantation rates.

Only recently have mosaic embryos been by advanced technology. According to a study from Fertility and Sterility in January, approximately one-third of mosaic embryos have developed into a healthy baby, although the study size was small and the genetic testing of the children was not performed.

Patient Awareness of Mosaic Embryos

Sumathi Reddy, the author of the WSJ article, cited that many of the genetic-testing companies across the country (from San Francisco to New Jersey) and internationally do not provide mosaic information to their fertility clinics. Some physicians report mosaicism, but most fertility clinics do not transfer mosaic, only if there are not any other embryos. A policy of using “abnormal” would need legal releases and waivers from the patients in order to implant them.

In the Wall Street Journal story, the family decided to transfer the two embryos that they had even though the PGS results showed one as undetermined and one as mosaic. The family’s reasoning was that one of the counselors had informed them that “some women had transferred mosaic embryos and had healthy babies.” The couple now has two healthy girls who, “have given [them] so much joy and a reason to keep going.”

The key observation is that there is a possibility of parenthood with mosaic embryos. Many women who had transferred embryos that were classified as mosaic can go on to have healthy babies. PGS and Preimplantation Genetic Diagnosis (PGD) techniques should be done by a highly qualified physician, like Dr. Lori Arnold from California Center for Reproductive Medicine (CACRM) in San Diego, CA, who can provide insights on the chances of a successful pregnancy and the possibilities of genetic disorders.

CACRM’s policy follow PGDIS guidelines in such that mosaics are considered to be transferred if there are no euploid (normal) embryos and couple has undergone genetic counseling to understand the potential risks of transferring a mosaic embryo.

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