How does CARE choose which embryo to transfer?

IVF treatment generally results in the creation of several embryos. We aim to transfer the best one and are often able to freeze some of the remaining embryos. The best embryos are the one which will give you the highest chance of having a baby.

Embryos tend to look very similar to each other so how do CARE scientists choose which embryo is “the best”?

What tools do we use to help us choose?

CARE embryologists use microscopes to look at your embryos to count the number of cells and assess how they are developing. As we must try to minimise the amount of time embryos spend outside of the controlled incubators in which they grow and they are checked only once each day. When the time comes for us to choose the best one, we consider the information from the daily observations, as well as how the embryo looks at this time and make our choice based on all of this.

However, we can now use more sophisticated tools to help make this crucial decision. We have time lapse incubators which take images of the embryos every 10 minutes. This gives two major advantages over the standard microscope methods:

·       We don’t need to disturb the embryos by removing them from the incubator to check them.

·       We collect much more information on the appearance and development of the embryos. This helps us make a more informed decision.

Time lapse is a very precise embryo assessment technique. It allows us to record the time it takes an embryo to reach different stages (such as 4 cell stage or blastocyst stage) and observe just how it behaved during that journey (such as whether it stopped developing and tried to catch up).

Imagine a marathon race – if we had a timed snap shot image of the runners when they crossed the one-mile mark, and we had to pick the winner from this – that would be difficult. Probably, neither the fastest nor the slowest would be the final winner.

But, if we had a news feed of each runner’s progress and times that they reached each additional mile; plus information on when they took a rest or had cramp, with lots photos of how they looked in order to deduce how relaxed or energetic they were, we would presumably make a better judgement. Especially if we had all of this information on 5000 runners.

Embryo development, of course, is not a race, but having important information on timing and appearance is really helpful. CARE has time lapse records of over 5000 embryos which have been chosen and transferred and we know whether they resulted in a baby.

As a result of our work, we have identified important differences in the timings and patterns of embryo development (called ‘morphokinetics’) by comparing this information from transferred embryos resulting in a baby and those failing to succeed.

CAREmaps (morphokinetic algorithms to predict success) was developed from these findings. For example, we found that embryos which start to change form and become a blastocyst faster have a higher chance of being successful, when transferred. We therefore always choose these embryos for transfer and have seen improvements in birth rates. Another example relates to the way the cells in the embryo divide. We know if an embryo has a cell which divides into three, rather than two, that it has a much lower chance of success, despite it reaching the desired stage in the desired time frame.

CARE embryologists have recently presented these exciting findings at the UK annual embryology conference and continue to study embryo development using time lapse, in order to further improve the chances of IVF success for our patients.