Published: FEBRUARY 3, 2020

A blastocyst is an egg fertilized in a laboratory during in vitro fertilization. It may be harvested from the reproductive tract of a mother or egg donor then joined with the father’s or donor’s sperm.

A blastocyst is made up of three parts: a fluid-filled cavity; trophectoderm cells, which become the placenta and embryonic sac; and a collection of cells called an inner cell mass, which hopefully becomes a baby.

A blastocyst is sometimes called a day 5 embryo or a day 6 embryo, depending on when it was fertilized prior to implantation.

More About Blastocyst

Once blastocysts form, they hatch from a protective coating called the zona pullucidia. Twenty-four hours after hatching, they can be transferred to their mother’s or surrogate’s uterus. Embryologists analyze the cell structure of the blastocysts and only transfer the ones that are most likely to result in a viable pregnancy. There, they will hopefully implant themselves into the uterine wall and develop into healthy fetuses.

While blastocysts are sometimes called day 5 or day 6 embryos, they are not really embryos at all. Their cells develop in a more advanced way to increase the chance of successful implantation. They also live in a lab for longer than embryos, which are transferred in just two to three days. Two to three days after fertilization, most embryos would still be traveling down the fallopian tubes. Some experts feel introducing embryos at this point could disrupt the biological process. Instead, they prefer blastocyst transfer, which more closely mimics the body’s natural timing.

The extra time blastocysts have in the lab also helps embryologists make more informed decisions about their viability. This doesn’t just lead to more successful pregnancies. It also means embryologists can implant fewer blastocysts, because they are more confident in their quality. This reduces the risk of multiple pregnancies.

While transferring blastocysts rather than embryos during IVF can be more successful, this procedure also costs more. Prolonged exposure to a lab, rather than a safe uterus, along with other factors may also show there are no viable blastocysts to transfer.


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