A blastocyst is a human embryo that is five, six or seven days old. Until fairly recently, day-3 embryos were routinely transferred into the womb during IVF treatment. Most clinicians now believe that transferring better developed embryos i.e. those that have reached the blastocyst stage makes pregnancy more likely.
Under the microscope, a blastocyst is markedly different to a day-3 embryo. There is a fluid-filled cavity in the middle called the blastocoel. Another area, housing a dense collection of cells, is called the inner cell mass and will become the fetus. A third assortment of cells, near the central cavity, will form the placenta.
There is no hard-and-fast rule: every patient is different. But the consensus is that an embryo that survives for five days or more, particularly a top-grade one, is probably more viable for implantation. Many younger embryos don't develop to five days and their future is harder to assess. So blastocyst transfers are, generally speaking, more likely to succeed than transfers using younger embryos.
Transferring blastocysts can make things more straightforward for implantation. In non-IVF reproduction, a day-2 embryo (usually comprising four cells) will still be rolling down the fallopian tube towards the uterus. Putting a blastocyst directly into the womb on day five or six is an easier prospect for the now highly receptive uterus. Also, genetic testing such as PGD is more accurately carried out on blastocysts.
The key point about blastocysts is that multiple births are less common than with younger transfers. The transfer of one good-quality blastocyst is often recommended. That said, the one-embryo-only drive has been watered down recently in the UK following a court ruling effectively supporting older IVF patients rights to transfer two.
In the past, clinics might transfer up to four day-3 embryos and hope for the best. Blastocysts allow for more informed choices and better control over outcomes.
In certain groups of women,yes. If you are under 35, the statistics say that live birth rates after blastocyst transfer are higher.
Blastocysts can split, usually after transfer. Identical twins are not uncommon. Many couples can live with that. The most dramatic scenario is if both blastocysts split and progress to live births.
You may not get there. Depending on factors like your medical history, age and the number and quality of embryos created in your IVF cycle, your consultant may feel it is safer to transfer a younger embryo. Better to put in a tiddly one than none at all.
One disadvantage is that you may have fewer or no surplus embryos to freeze. Freezing of embryos for a second attempt is often tactically overlooked by IVF patients. The fact is that frozen embryo transfers, particularly those using blastocysts, are getting more successful. If you have two blastocycts, freezing one and transferring the other may be sensible for certain patients.
Recent research on selective single blastocyst transfer says that the likelihood of a live birth from one transferred blastocyst is virtually the same as for two. But another study said this only applies to younger women and when the first cycle uses fresh embryos and the second uses frozen ones. However, 34 per cent of those younger women who had two blastocysts transferred ended up with twins. Just seven per cent had twins from a single blastocyst transfer. So multiple pregnancies are the continued concern.
If you choose to have your IVF abroad at our partner clinic, and you have several cultured blastocysts, you have the final say on how many to transfer (though two is generally the maximum).