Standard practice in IVF used to be to replace embryos into the uterus after only two or three days of development, when the embryos are at the ‘cleavage stages’ and usually composed of somewhere between two and eight cells.
Extending the embryo culture to five or six days gives the doctor a longer period over which to observe embryo development. During this extra couple of days it is completely normal for only some embryos to progress and develop to form a blastocyst. Blastocysts are therefore a more select bunch, so have a greater potential to form a pregnancy than embryos at earlier cleavage stages.
It is also thought that transferring an embryo into the uterus at the blastocyst stage may improve its chance of implanting because we are getting the timing right. The blastocyst is the stage of development that would normally be in the uterus ready to implant. Cleavage stage embryos on the other hand would normally be in the fallopian tube, so when we put them back into the uterus they have to hang around for a couple of days before they can implant.
In the early days of IVF, doctors often transferred a number of embryos into the uterus to allow for the ones that wouldn’t develop successfully and this resulted in multiple births for some women. Most importantly, with blastocyst culture, as we are being more selective in our choice of embryo, we can confidently transfer fewer, often single embryos and avoid multiple pregnancies.
