Embryos go through many stages after fertilisation. When they have developed for five to six days, they are called a blastocyst. It’s at this stage that an embryo would normally move out of the fallopian tube into the uterus to implant itself into the lining of the womb.
Many clinics offer women blastocyst transfer for IVF treatment. This means that embryos are allowed to develop to the blastocyst stage in the lab before they are transferred to the womb. The advantage of doing this is that the embryo should have a stronger chance of implanting itself because it’s a more natural time for the embryo to be in the uterus.
In some women this can lead to a higher pregnancy rate and live birth rate. Conventional IVF tends to transfer the embryo after two or three days (known as the ‘cleavage stage’). Because it’s difficult to predict on day two or three which embryos are more likely to produce a pregnancy, four or more embryos are often transferred in the hope that at least one will result in a live birth.
The risk is that this can lead to multiple births and greater medical risks, higher costs and emotional worries.
Blastocyst transfer aims to avoid this – fewer embryos are transferred yet the pregnancy success rates are similar (limits on how man can be transferred vary country by country – see below).
Beware that some clinics only show success rates for blastocyst transfers. Ask your clinic for their cleavage stage success rates so you can compare.
There are pros and cons of waiting until day five and the decision requires careful thought.
Pros of blastocyst transfer
Higher chance of becoming pregnant than transfer on day two or three for certain groups of women.
These groups include younger women (typically under 35) who doctors feel have a good chance of pregnancy from IVF and women who produced good quality embryos in a previous IVF cycle but they failed to implant in the womb.
Waiting until day five allows doctors to see which embryos have developed into blastocysts and choose the best for transfer – it’s not possible to see which embryos will grow into blastocysts on day two or three (embryos that do not become blastocysts do not lead to pregnancy).
Cons of blastocyst transfer
You may not get any embryos that develop to the blastocyst stage (all the embryos fail to develop)..
Fewer embryos will be available for you to freeze.
There’s still a small chance of giving birth to twins (a blastocyst can split and produce identical twins).
If you produce fewer than normal healthy eggs, this option may not be suitable as it reduces your chances of developing enough good embryos.
Not all clinics offer it and it may cost more.
Strict limits to no limits at all
In the UK, women under the age of 40 are legally allowed to have a maximum of 2 embryos transferred and women over 40 can have up to three. Younger women with high quality blastocyst embryos are usually advised by clinics to transfer a single embryo as the chances of pregnancy and a live birth are high. Extra embryos can be frozen.
Some countries have a strict policy of single embryo transfer (SET), such as Sweden, Finland and Norway. Eastern Europe is more liberal and allows multiple embryo transfers. In Italy, the Catholic laws insist that all fertilised embryos must be implanted. In the USA, Spain and Portugal there are no legal limits.
Will a single blastocyst transfer (SET) reduce my chances of getting pregnant?
Actually no. Studies show that this isn’t true. In Sweden, two-thirds of all IVF cycles are with one embryo. The rate of birth of twins has fallen from 20% to 5% but the pregnancy success rate is about the same.
It’s only better quality embryos that survive for five days, so they have a better chance of implanting themselves in the womb.
Learn more about blastocysts and assisted hatching: