What is blastocyst culture?

As fertility treatment continues to evolve, so too do the options available to patients. One area that has seen significant progress in recent years is blastocyst culture – an approach that allows embryos to develop further in the laboratory before transfer or freezing.

While many people starting IVF are familiar with the term ‘embryo,’ ‘blastocyst’ is often less understood. However, it plays an increasingly important role in how treatment is planned and can influence both timing and outcomes.

At Leicester Fertility Centre, we are increasingly discussing blastocyst culture with patients as part of their individualised treatment plans. Advances in laboratory techniques now mean that embryos can be safely cultured for longer, reaching the blastocyst stage before being transferred or frozen. This gives us more information about how an embryo is developing and can help guide the next steps in your care.

In this article, we explain what a blastocyst is, why blastocyst culture may be recommended, what the process involves and what it means for your treatment timeline.

What is the definition of a blastocyst?

Blastocyst(s) is the name given to the fertilised egg after the early stages of embryo development have passed. It usually forms around five days after sperm has fertilised the egg.

The blastocyst is the stage at which a fertilised egg would be entering the uterus in a natural pregnancy. It also shows greater competency – which can lead to a higher pregnancy rate.

On days 2-3, after fertilisation, the embryos are examined by our team and graded based on how many cells there are, how even the cells are in size and shape and if the cells have any fragmentation (bits that the embryo has excluded when splitting). They are given a number for how even the cells are and how much fragmentation they have. This grade is out of 4, with 4 being the best and 1 being the worst.

Not all embryos will develop into blastocysts though, whether that is inside the body or during culture. For this reason, only the best embryos will be allowed to continue in culture to the blastocyst stage.

Provided you have at least 1 top quality embryo, at least a 4 cell grade 3/3 or above, you can have a blastocyst transfer.

Unlike the earlier day 2-3 embryos, Blastocysts also have two types of cells called the outer trophectoderm (troph) and inner cell mass cells (ICM). The ICM forms the foetus and the troph will form the placenta.

When would a blastocyst culture or frozen blastocyst culture be done?

All patients undergoing fresh IVF – with or without ICSI – will have the opportunity to go to blastocyst culture.

After your eggs are collected, the embryology team will keep you informed about how many eggs have fertilised and how many embryos have developed.

How your embryos are developing will influence whether we can extend the culture to the blastocyst stage. We’ll also consider any previous cycles and any relevant medical history.

Because blastocysts have greater competency and higher pregnancy rates, we may recommend that only one be replaced at embryo transfer. Blastocysts can also be frozen in the same way that embryos can – but they also have the advantage of being available for freezing on either day 5 or 6.

Blastocyst culture timeline

One of the most common questions we’re asked is how long it takes for an embryo to reach the blastocyst stage, and when a transfer would typically take place.

Understanding the timeline can help you feel more prepared and confident as you move through treatment. To support this, we’ve outlined the key stages below, so you know what to expect at each step of the process.

• You will first undergo stimulation and egg collection as part of your normal IVF and/or ICIS cycle
• You will be taken to theatre 36 hours after your trigger injection, where your eggs will be collected and either inseminated with sperm (IVF) or injected with sperm individually (ICSI)
• Day 1: Our lab team check the egg for signs of fertilisation
• Day 2: The embryo will have started to divide. This ‘cleavage stage’ embryo is then given two grades, one for cell shape and evenness and the other for any fragmentation. The embryologist will use information about the embryo at this stage to determine if you are eligible for extended blastocyst culture.
• Day 5: This is the day of blastocyst transfer. The embryologist will choose which of your blastocysts is the most advanced, and therefore, most likely to cause a successful pregnancy. You will also be given a picture too, so you can see the blastocyst!
• Day 5/6: Blastocysts can be frozen on either of these days providing they are of sufficient quality to survive the freeze thaw process

What is the procedure for a blastocyst transfer?

The transfer for blastocysts is the same as for cleavage stage embryos. The procedure is typically painless and can be done within 20 minutes. You will need a full bladder for this procedure to help us see, via ultrasound, where the blastocyst is being replaced.

A transfer will involve using a flexible and thin catheter to gently place the blastocyst into the uterus whilst the consultant watches via an ultrasound scan.

What happens after a blastocyst transfer?

A blastocyst will expand, before ‘hatching’ and then hopefully, implanting into your uterine lining within seven days of it being replaced.

After having the blastocyst replaced, you will need to wait around two weeks before taking a pregnancy test to see if the procedure has been successful. This may be followed up with a blood test.

Is a blastocyst culture more successful at achieving pregnancy?

Some studies suggest blastocyst culture is more successful at achieving pregnancy (see here).

Only one or two of the best blastocysts are selected for transfer.

This approach more closely mirrors the natural process of conception, as the embryo is allowed to develop in line with what would typically happen in the body.

Blastocyst frozen transfer

Frozen blastocysts can be transferred in either a medicated or natural cycle. All blastocysts, either frozen on day 5 or 6, are treated as day 5 blastocyst to ensure your uterine lining is in sync with the blastocyst.