The Risk of Twins with Double vs Single Embryo Transfers

We are having trouble deciding on between a double or single embryo transfer. We would like your thoughts. This will be our first transfer attempt, and the clinic gave us some recommendations but we don’t really know how to interpret them. It’s tempting to transfer two embryos to get a better chance of a pregnancy, but we’re worried about having twins. Mostly because we hear that twins have a high chance of premature birth (60% or more) and weeks in the NICU. –Jeremy

Hello Jeremy.

In the United States IVF clinics are required to report all IVF cycles and their outcomes. The results form a de facto ranking of IVF clinics by success rates, and can be viewed online. Among the data are stats on whether clinics perform single or double embryo transfers and the results.

The results of the data are not what you might expect. The clinics with the highest success rates are overwhelmingly those that transfer only one embryo at a time. After perusing the results of IVF clinics across the US, those that are more amenable to double embryo transfers have success rates well below their counterparts.
 

Pregnancy Rates with Double Embryo Transfers

Of the 10 top ranked IVF clinics nationwide, the median Live Birth Rate using donor eggs was 62.9% with an average of 1.2 embryos per transfer. Compare that with the clinics ranked from 20 to 30, which have a Live Birth Rate of 60.5% using 1.6 embryos per transfer. In the comparison, the top ranked IVF clinics use fewer embryos in each transfer.

Now, this result could be for a number of reasons… For example it could be that the most successful clinics are more likely to be working with healthy donors and surrogates. It could also be that high-ranked clinics more strictly follow the ASRM guidelines (which urge single embryo transfers).

Many couples fear that a single embryo transfer will reduce the chances of pregnancy. But many studies have been done over the years, and they conclusively show that (with a good quality surrogate and embryos) there is hardly any difference is pregnancy rates between single or double embryo transfers. One study of 12,000 IVF cycles showed the pregnancy rate after double embryo transfer was 68%, while pregnancy rate after a single embryo transfer was about 65%. At least one other study showed that single embryo transfers in younger women resulted in live birth rates rising from 51% to 55%.

But regardless of the specific reasons, the data clearly indicates that there is no sign that transferring more embryos will get you a higher likelihood of a pregnancy.

As always, I would defer to your doctor’s judgement when deciding how many embryos to transfer. There are always qualifying circumstances that will push you in one direction or another. Only your doctor is aware of those circumstances.
 

Embryo Transfers and Twins

But that said, my experience is that transferring two embryos does not drastically increase the likelihood of a pregnancy, but it does drastically increase the likelihood of a twins pregnancy.

A single embryo transfer may still result in twins because of embryo “splitting”. There is about a 1.6% chance of identical twins arising naturally after a single embryo transfer. (Twins pregnancies can arise because the embryo splits, or also because more than one egg is released during ovulation, resulting in two unrelated pregnancies.) Meanwhile, when two embryos are transferred, the chance of a twins pregnancy can range from 35% to 55% depending on the clinic.

Clearly, if you want to avoid a twins pregnancy, then you should transfer one embryo at at time.

Yes, twins pregnancies have a high occurrence of premature birth. (More on that at this article in the Guide.)

Singleton pregnancies have a 9% risk of low birth weight, a 2% risk of very low birth weight and a 14% risk of premature delivery according to a report by the CDC. Twins pregnancies have respective 57%, 9% and 65% risks. Triplet pregnancies, have 96%, 34% and 97% risks respectively.

Twins pregnancies are also hard on the surrogate and she’ll require additional support during the pregnancy, it’s very possible she’ll need extra medical care — You mentioned your surrogate has a history of kidney problems, and I suspect twins would exacerbate that.

Also, there are studies that show that if you transfer two embryos of unequal developmental quality, the lower quality embryo will lessen the likely success of the more healthy embryo. (There are medical reasons for this, which I won’t go into.) So if you transfer two, don’t transfer your best along with your worst. The studies suggest that two transferred embryos should be about the same quality.

I think if you are not determined to have twins, the likelihood of a healthy pregnancy is higher with 2 transfers of 1 embryo each, rather than 1 transfer of two embryos. If you do want to have twins, transfer your two highest quality embryos, and then prepare the surrogate for a harder than normal pregnancy.

I hope this make sense and is helpful.
— Bill