Embryo grading and transfer
Embryo quality can be assessed when the fertilised egg has divided from a single cell into multiple cells. Embryo grading helps us select which embryo(s) have the best potential to implant and establish a pregnancy.
How are embryos graded?
Embryos are given a grade that relates to their development at the time of assessment. Early on (2 or 3 days after egg collection), embryos are assessed by the number and sizes of cells and whether there is any cell fragmentation.
If embryos are cultured to day 5, good-quality embryos should be at the blastocyst stage. A blastocyst looks like a fluid-filled sphere and has 2 cell types, the cells around the surface (‘trophectoderm‘) which form the placenta, and an inner ball of cells, the ‘inner cell mass‘ which forms the fetus. Blastocysts are graded according to the degree of their expansion and the appearance of the two cell types.
Blastocyst culture is suitable only for some fertility patients, and depends on the number and quality of embryos. Many embryos may not continue developing for five days and our experience has shown that overall only about half of embryos are capable of development into blastocysts in the laboratory.
When will embryo transfer occur?
The exact day of embryo transfer will depend on the number and quality of embryos available for selection (dictated by embryo development). Embryo transfer can take place from 2 to 5 days after egg collection. We culture embryos to day 5 if we are not able to select the best embryo(s) for transfer at an earlier stage, for example if there are several embryos of similar quality on day 2.
How many embryos will be transferred?
Patients usually have one or two embryos transferred in a single cycle. Three embryos may be transferred in patients over 40 years of age in exceptional circumstances.
The number of embryos we recommend for transfer will depend on both your embryo quality and your medical history, and is decided in partnership with you. Where the chances of conception are high the team may recommend that a single embryo is transferred – this will virtually eliminate the risk of a twin pregnancy without substantially decreasing the chances of becoming pregnant.
The recommendation for single embryo transfer should be viewed as a positive step as it means we consider the chances of success are high. Opting for a single embryo will not result in the other embryos being ‘wasted’ – if they are good quality they will simply be frozen for later use.