Can one transferred embryo become twins?

Yes, definitely, yes. This is something that has nothing to do with the treatments. A single embryo can split in two and even three. I remember a case in which we had a single embryo transfer, and the patient got pregnant with identical triplets. That is very uncommon, and probably we see it twice or three times a year at the most. This is something that happens after the embryo has been transferred.

Depending on when this division takes place, you may have babies that are identical with the same placenta and the same sac. If the division takes place after these initial steps of the pregnancy, the embryos can be in different sacs in the same placenta, and in very strange circumstances, they may even have two sacs and two placentas, and in an extreme situation, it could be the Siamese twins in which the embryos are still linked by the belly, brain or something like that.

As I’ve said before, that’s something that can happen in any case. It is not related to the fact of having an IVF or any other ART treatments.

A straight answer is: yes. Obviously, the blastocyst can split: day 3 embryos evolving into day 5 embryos can split. However, if we transfer two embryos and then you get a twin pregnancy, this is dizygotic. So they are not sharing the same placenta because they are different – and we know that we have put two embryos. In the consultation regarding how many embryos we should put if you really have a major ‘no’ regarding having twins (because, for example, you already have 1 or 2 children) and insist on having only one embryo, that’s very well, it’s your decision. So yes, in 99% you can have a singleton pregnancy. However, we always discuss a remote possibility of this embryo splitting into two. So you can end up with twins and in that case, it’s going to be a monochorionic twin. If we are putting two separate embryos, it will be dichorionic (or dizygotic). However, if we put only one embryo and it splits, it will be monochorionic, which is a higher risk of twins and you will be under the guidance of a fetal medicine specialist.

Very low because if we’re talking about a single embryo transfer, we’re then specifically talking about what is the rate of monozygotic twinning. The rate is 2 to 3%, so it should not be more than 2 to 3% of single embryo transfers that result in twins.

Yes, we can transfer one embryo, and we can have twins. The chances for this to happen are very slim, though. Usually, the twins that we get from the transfer of one embryo are called identical twins and by the medical definition, they are called monochorionic twins. The chances for that are pretty low, though.

I don’t think that there is a difference in twins. There is no big difference between fresh and frozen embryos. If we transfer one blastocyst, the probability for each blastocyst to split into two is 2.5%. So if we transfer one embryo, it’s 2-3% of chance that it will become twins, monozygotic twins. We think that it’s due to manipulation with the embryo in the laboratory. Blastocysts are very fragile, and they can be split by manipulation, or sometimes when we have a difficult transfer. So only mechanical conditions: manipulation with embryos is the most common reason for twins. Also, an embryo can be split without any reason. But the most important thing is the manipulation of embryos.

It’s practically the same with the fresh embryo transfer. There are two causes of twins or multiple pregnancies. The first cause can be on the side of the doctor. If we transfer more than one embryo to the patient, it can cause a multiple pregnancy. This is from our side and the second one, even if I transfer only one embryo, you can have identical twins. This is mostly coming from the genetic heritage of the patients. So, in these two cases, they can have multiple pregnancies.

Any embryo can split into twins or even triplets. Normally, the chance of an embryo splitting and giving identical twins is around 8-10 percent. That is the same in a fresh and a frozen cycle. In the past, there’s been some evidence, but very low evidence, that if we did assisted hatching (we used to create a little hole in the shell of the embryo to help the embryo implant), there was an increased risk of twins.

But nowadays, I think assisted hatching is no longer performed, because it’s on the red list, at least in the United Kingdom from the Human Fertility and Embryology Authority, because it doesn’t really improve the chances, and it might in fact create damage. But overall, yes, an embryo can split, and that is why when we counsel the patients before starting an IVF or frozen cycle, we always say that there is still a risk of multiple pregnancy. That is one of the potential side effects of IVF, even if patients are happy to have twins. But we just need to be aware that yes, this can happen also with a frozen embryo.