Single vs. Double Embryo Transfer: A Comprehensive Guide

The embryo transfer is an exciting and pivotal time in the in vitro fertilization (IVF) process. It’s when the embryo is transferred to the uterus of the intended mother or a gestational carrier, in hopes that it snuggles in for nine months and becomes the child you’ve always dreamt of.

You may be wondering if you should transfer one (single embryo transfer, or SET) or two (double embryo transfer, or DET) embryos, to increase your chances of a happy, healthy baby.  

This article will dissect the complexities of this decision-making process, taking into consideration both personal choices, and also help answer this question to intended parents using donor eggs.

First off, what is an embryo transfer?

Understanding what an embryo transfer entails is important to appreciating the decision-making process between single or double embryo transfer.

After an egg, or eggs, are fertilized with sperm, an embryo develops in a controlled laboratory environment for a few days under the careful watch of a trained embryologist. If and when the embryo reaches a particular developmental stage (ie. blastocyst stage), it is suitable for transfer.

Now, at this point, some families opt to do PGT testing, which looks to see if the embryo contains the correct amount of chromosomes. Embryos with the right number of chromosomes — 46 — are considered “euploid,” and those with extra chromosomes or chromosome deletions are considered “aneuploid.” PGT is a way to screen for genetic disorders in embryos created through IVF before they are transferred to the uterus. Read more about PGT testing.

All embryos are then “graded” by the embryologist to evaluate their quality and chances of developmental success into a healthy pregnancy. Based on this assessment, your fertility doctor will recommend the order and number of embryos to be transferred. This is the stage where the choice between a single embryo transfer or a double embryo transfer is usually made.

Finally, the selected embryo(s) is transferred to the uterus via a thin catheter, typically under ultrasound guidance. If the embryo implants in the uterine lining, it may lead to pregnancy.

Embryo transfers and the history of IVF

When IVF was in its infancy, following the birth of the first IVF baby, Louise Brown, in 1978, the process was distinctly different from what it is today. In the early days, IVF involved the transfer of fresh embryos, without the technological advancements available now. This is because we didn’t have the technology to freeze and preserve embryos for genetic testing or future transfer.

At the time, this approach was taken to increase the likelihood of pregnancy, compensating for the uncertainties around embryo viability. However, this practice led to a higher incidence of multiple pregnancies (pregnancies of twins, triplets, or more), with associated health risks for both mothers and children.

With the development of cryopreservation techniques, it became possible to freeze embryos for later use, allowing for more flexibility and planning in the transfer process. This technology also enabled better timing for embryo transfer, and gave patients a break between the egg retrieval and embryo transfer. 

Simultaneously, genetic testing of embryos significantly increased the success rates of IVF, even with the transfer of a single embryo.

Guided by these advancements, the American Society for Reproductive Medicine (ASRM) now recommends elective single embryo transfer (eSET) in most cases, especially for women under the age of 35 with a favorable prognosis. This shift towards eSET reflects a growing emphasis on minimizing the risks associated with multiple pregnancies without compromising success rates.

So how many embryos should I transfer?

The decision regarding the number of embryos to transfer should weight the potential success rate against the risks associated with multiple pregnancies. Factors such as age, the quality of the embryos, previous IVF attempts, and individual patient preferences play an important role in this decision.

Let’s go through the pros and cons of each option:

Elective single embryo transfer (eSET)

Pros:

• Reduced risk of multiple births: eSET almost eliminates the chance of a multiple pregnancy (twins, triplets, etc.), thus reducing the associated health risks for both the mother and the babies.

Cons:

• Possibly lower success rates: Some studies have shown that eSET may yield a slightly lower live birth rate compared to double embryo transfer.

Double embryo transfer (DET)

Pros:

• Potentially higher success rates: By transferring two embryos, DET may improve the chances of pregnancy, especially when there are concerns about embryo quality.
• Chance to have twins: If you always wanted twins, transferring two embryos gives you a better chance of that dream.

Cons:

• Riskier pregnancy and birth: DET significantly increases the risk of twins or higher-order multiples, leading to more complicated pregnancies and a higher chance of preterm births and low birth weight.

It’s important to discuss this with your fertility doctor and get their input. Because ASRM usually recommends a single embryo be transferred, in most cases fertility doctors will recommend just that.  

How many donor egg embryos should I transfer?

When donor eggs are utilized, a couple other factors need to be considered. Donor eggs are from younger women who have gone through extensive medical screening, and thus, embryo quality tends to be higher. Because of this, it increases the chances of success, even if the mother is older, or if she experienced failed IVF in the past. In donor egg cases, eSET is generally preferred to minimize the risk of multiple pregnancies while maintaining a satisfactory success rate.

Summing it up

If you are wondering if eSET or DET is right for you, talk to your fertility doctor. They can help talk you through the pros and cons, and help select an approach that best fits your needs.

For those using donor eggs, the generally higher quality might lean the decision towards eSET, while personal preferences and specific medical situations might suggest otherwise. By carefully considering the nuanced pros and cons, you can make a well-informed choice in your pursuit of parenthood. We wish you all the best!