This is due to a number of reasons and most commonly because couples ask for more than one embryo transfer to increase the likelihood of IVF success. However, when the uterus is receptive, all the embryos will successfully implant.
An embryo becomes a blastocyst after 5 days. This technique of keeping the embryo in the lab incubator for 5 to 6 days is becoming quite popular as it assures a higher probability of the IVF procedure success.
It has also been found that blastocyst has a higher rate of multiple pregnancies. This has been concluded in a study where it was found that the blastocyst had a tendency to divide. This caused the formation of a monozygotic twin, which are identical twins. Monozygotic twins are formed from a single embryo and are absolutely similar.
Embryos that get to the blastocyst stage have a higher survival rate and are chromosomally better balanced and have a better overall development.
However, there is also the risk that not all embryos survive to the blastocyst stage in the lab, and hence some doctors suggest that couple do a day 2 or 3 transfer to the uterus to give the embryo a better survival chance.
Some doctors feel that culturing the embryo for longer than 3 days allows them to study the embryos and monitor the ones that do not become a blastocyst. This enables the doctors to choose the best embryo to transfer for the IVF procedure.
Today good culture media is available, and this allows the embryos to develop successfully into a blastocyst. Earlier it was found that many embryos arrest by day 3 and do not develop into blastocyst due to poorer quality culture media.
The rate of pregnancy success has improved with Blastocyst transfers as compared to day 3 transfers. The blastocyst has a better chance of implantation as it is mature and already beginning to hatch.
With day 3 transfer, many couples preferred 2 or 3 embryo to be transferred simultaneously. However, due to the higher probability of the blastocyst implanting, doctor’s usually advice against the transfer of more than one blastocyst. There is a 50% to 60% probability of twin pregnancies with the transfer of two or more blastocyst.
The risk of blastocyst splitting and become monozygotic twins is 2%. Multiple pregnancies hold many risks for the mother as well as for the babies. Doctors do their best to avoid multiple births. Couples, on the other hand, are quite keen on multiple births and would prefer to have twins rather than one baby.
Many couples feel that it is a great advantage to have two babies at one time instead of going through the IVF treatment again. However, doctors are worried about the health risks this poses for both mother and babies. Multiple pregnancies have a high probability of preterm births, and this puts the health of the babies and the mother at high risk. Some couples seem willing to take this risk.
Blastocyst does have a possibility of multiple pregnancies due to the embryo dividing, but the percentage is very low, and hence couple do not need to worry about it. Sometimes even when the embryo does split one of the babies, the weaker one does not survive — thus leaving the mother with one healthier baby that will make it to the full term.