During an IVF cycle, the fertilized eggs go through several developmental stages. Your IVF report may reference these major stages of embryo development at various days following the fertilization:
• Zygote: When a sperm and egg meet, fertilization occurs. At this point, a zygote is formed.
• Morula: A grouping of cells resembling a blackberry. This forms around four days post-fertilization. The morula phase is the final phase prior to blastocyst development.
• Blastocyst: A blastocyst is a fertilized egg that has developed for five to six days and contains 3 distinct features. These features include a fluid-filled cavity and two distinct types of cell masses.
• Hatching Blastocyst: Around 5 or 6 days after fertilization, a healthy blastocyst will hatch from its protective outer shell. It is around 24 hours after this hatching process that embryo will “latch” onto the wall of the uterus to create a pregnancy.
Most ART cycles will transfer embryos on Day 5 when they reach Blastocyst or Hatching Blastocyst stage. (Some clinics prefer to transfer embryos during the Zygote stage, with the strategy that the embryos develop more normally in the uterus than in the laboratory.)
Typical IVF programs will allow your fertilized embryos to develop to Day 5 (and sometimes Day 6), and then apply a embryo quality grading system to clearly identify which embryos are likely to reach pregnancy and which others may safely be discarded. The most common embryo grading system is the Gardner grading system.
Human embryos fertilized in an IVF lab (or even developing naturally in the body) usually reach Blastocyst stage by Day 5 after fertilization. Blastocyst stage is reached when the embryo has developed to the point of having 2 different cell components and a fluid cavity. As development of Blastocysts progresses, cells in the 2 components divide and the fluid cavity enlarges, and eventually the Blastoscyst will “hatch” out of its shell. Blastocyst is the stage just prior to “hatching” within the uterus, attaching to the uterus wall, and becoming a fetus.
Blastocyst transfer can have high pregnancy success rates with very low risk of multiple births. Many IVF doctors will do embryo transfer only once the embryo reaches Blastocyst stage, although some others prefer to transfer sooner and let the embryo reach Blastocyst stage within the uterus.
The Gardner Embryo Grading System
According to the Gardner grading system, each blastocyst embryo is assigned 3 separate quality scores:
1. Blastocyst development stage – a range from 1 – 5 (with 5 being the most developed)
2. Inner cell mass (ICM) score, or quality – range A – C (A being the best)
3. Trophectoderm (TE) score, or quality – range A – C (A being the best)
So the least advanced blastocyst would be 1CC, and the most would be 5AA. However it’s generally preferred to do embryo transfer before reaching level 5 (which means it’s already hatching). So we look for Blastocysts that are level 4 with letter grades of A or B.
Not all embryos reach the Blastocyst stage within 5 or 6 days. Some are slow to develop and take longer to become Blastocysts (while others simply stagnate and don’t develop further). Slow developing embryos may be preserved by the clinic depending on their apparent health.
For those embryos that do not make it to Blastocyst stage by Day 5 or 6, if they reach the stage immediately before becoming Blasts they may be considered Early Blastocyst stage. These are less developed than Blastoscysts, but still have the possibility to develop into blasts within the uterus (although the chances are less likely). And the Blastocyst grading scale does not yet apply because the embryo doesn’t have the structures in place that define the grade of a Blastocyst. For these, the grade is based on the number of cells and the a few other criteria (fragmentation and nucleation for example). Clinicians give a grade from 0 to 3 (3 being the best formed).
The stage immediately before Early Blastocyst is called the Compact Stage (or even the Early Compact stage). Embryos that don’t make it past this point after 5 days are less likely to develop further and many clinics will discard these. Even though they may be eventually discarded, clinicians may still give a grade similar to Early Blasts, with the number of cells and a grade from 0 to 3.
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