Embryo Stages...
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The two gametes - one egg (aka ovum) and one sperm - come together during fertilization to create a zygote, which is the single-celled beginning of the embryo. The number of cells grows as the embryo undergoes cleavage. Approximately 3 days after fertilization, the embryo reaches the morula stage, and approximately 5 days after fertilization, it reaches the blastocyst stage. The morula stage starts when the embryo has 16 cells. The blastocyst stage is marked by a division of the embryo into two parts, the inner cell mass (ICM) - which goes on to form the baby - and the trophoblast - which goes on to form the placenta.
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An embryo can be transferred (or frozen) at anytime between fertilization and day 6 (although they've happened, 7 day embryo transfers are basically unheard of, at least from what I've seen in the US). However, the most common days for transfer are 3, 5, and 6.
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Day 3 transfers don't seem to be as common as they were in the past (from what I've seen), and seem to be less common in surrogacy than when couples do their own IVF, possibly because egg donors are more prevalent in surrogacy. Often, 3 day transfers will be done if there is a concern that the embryo(s) will not reach the blastocyst stage in the lab. In these cases, is it often said that the embryo has a better chance to survive and progress inside the carrier's womb.
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Day 5 transfers are the most common, at least with surrogacy in the US. For one thing, it is believed that embryos which are strong enough to reach the blastocyst stage in the lab are the most likely to result in a viable pregnancy. Another reason that it is desirable to grow the embryos to blasts is that they can then be biopsied for genetic testing.
Day 6 transfers are generally done when the embryo is a bit slow to develop, and reaches blast on day 6 instead of day 5. Another scenario in which a 6 day old embryo might be transferred is if it reached blast on day 5 and you choose to do expedited PGS testing for a fresh transfer on day 6 (some, but not all, clinics are able to do this).
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The number of days that you are on progesterone is usually equal to the age of the embryo to be transferred, but not always. Sometimes it is found that a woman's uterus is more receptive on a different day. If a woman has experiences repeated implantation failure without another explanation, sometimes an endometrial receptivity array (ERA) test will be done, where a piece of the emdometrium (lining of the uterus) is taken and examined in order to find the ideal window of implantation.
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Rarely, embryos will be frozen right after fertilization at the 2pn, or 2 pronuclear, stage. This refers to the pronuclei of both the female and male gametes being visible in the embryo. This occurs within the first day of insemination (after approximately 18 hours).
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Sometimes these embryos will be implanted as is after thawing, and sometimes they will be left to develop in the lab until they are (generally) 3 or 5 days old.
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The two nuclei are clearly visible in the center of the 2pn embryo to the left.
Image source: Wikimedia Commons
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