A Blastocyst is an embryo that has developed two different cell types and also contains a central fluid-filled cavity. The outer cells, called the trophectoderm, will become the placenta, and the inner cells will become the fetus. Blastocyst formation in the human usually occurs on the 5th day after fertilization. By the end of the sixth day, healthy blastocyst should hatch from its outer shell (the zona pellucida), and within another 24 hours the hatched blastocyst begins to implant in the lining of the mother’s uterus.
Over the first two decades of experience with in vitro fertilization (IVF), embryos were routinely cultured for two to three days in the laboratory, and then transferred to the uterus. This is quite different than when embryos normally enter the uterus (day 5 or 6). After two or three days of growth inside the body embryos are found in the fallopian tubes and may not be ready to enter the uterus. Recently, laboratory culture conditions have been improved so that embryos can develop to the blastocyst stage in the laboratory, and therefore be replaced into the uterus at the more “natural” time, Day 5 or 6 after fertilization.
The additional benefit of waiting longer to transfer embryos is reducing the number of embryos needed to be transferred to result in a viable pregnancy. By culturing embryos to the blastocyst stage we have more opportunity to choose the healthiest ones for transfer. So, optimal pregnancies rates may be obtained by transferring fewer embryos and reducing multiple pregnancy rates.
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