When a couple ends up with several embryos after IVF, that is good news. But before transfer, a new question comes up: should one embryo be transferred, or two at once, to improve the chances of pregnancy?
At first glance, transferring two embryos may look like a way to raise the odds of success. In practice, when choosing the number of embryos, it is important to weigh not only the probability of pregnancy, but also the safety of how it continues afterward.
In this article, we look at how transferring one embryo differs from transferring two, whether two embryos really raise the chances of success, why doctors try to reduce the likelihood of a multiple pregnancy, what the number of embryos actually depends on, and what happens to embryos that are not transferred right away.
One or two embryos: what is the difference?
When one embryo is transferred, the doctor is aiming for a singleton pregnancy. The possibility of twins cannot be ruled out entirely, though: in rare cases a single embryo can split into two and lead to an identical twin pregnancy.
When two embryos are transferred, each of them may implant. In that case, a twin pregnancy — a form of multiple pregnancy — can occur. Multiple pregnancy is linked to a higher risk of complications for both the woman and the babies, including preterm birth and low birth weight.
That is why, when choosing the number of embryos, the doctor considers not only the likelihood of pregnancy, but also the risks tied to multiple pregnancy. The goal of fertility treatment is, wherever medically possible, a healthy singleton pregnancy.
Why is single embryo transfer often preferred?
The main advantage of transferring one embryo is a lower risk of multiple pregnancy.
When selecting an embryo, the doctor assesses its characteristics and the likelihood of successful implantation. The aim is to support pregnancy while reducing the complications associated with carrying twins.
The American Society for Reproductive Medicine (ASRM) recommends transferring a single euploid embryo — an embryo with a normal set of chromosomes — regardless of the woman’s age.
Research also shows that in certain situations, transferring one chromosomally normal embryo can deliver results comparable to transferring two embryos without genetic testing. At the same time, the risk of a twin pregnancy is significantly lower.
What does the number of embryos depend on?
The decision about how many embryos to transfer is always individual. The doctor considers not only the woman’s age, but also other aspects of the IVF program:
- the woman’s age — affects the overall treatment prognosis;
- embryo quality — helps gauge their potential;
- the stage of embryo development — factored into the transfer strategy;
- the results of genetic testing, if it was performed;
- the results of previous transfers;
- the possibility of preserving other good-quality embryos;
- the woman’s health — especially if a multiple pregnancy would carry added risk.
In deciding how many embryos to transfer, the doctor and patient take into account individual circumstances, including age, health, embryo quality, the option of cryopreservation, and the overall treatment prognosis.
That is why there is no one-size-fits-all answer. The doctor shapes the approach around the specific medical situation of the couple.
When might transferring two embryos be considered?
Transferring two embryos is not a universal solution, but in some situations the doctor may consider it.
For example, if the treatment prognosis is less favorable, or if previous transfers of good-quality embryos did not lead to pregnancy, the doctor may discuss a different strategy with the patient. The additional risk of multiple pregnancy is always part of that conversation.
In other words, the decision to transfer two embryos is not made simply because of a wish to raise the chance of pregnancy. The doctor weighs the potential benefit of this approach against the possible risks.
Are two embryos twice the chance?
Not necessarily. If two embryos are transferred, each one has an opportunity to implant. But that does not mean the chances of pregnancy simply add up: none of the embryos may implant, only one of them may, or both may.
So transferring two embryos can raise the chance of pregnancy compared with transferring one — but not by a factor of two. And the chance of twins becomes higher.
Multiple pregnancy places more strain on the woman’s body and carries a higher risk of complications: babies are more often born preterm and with low birth weight, and pregnancy and delivery can be more challenging for the woman herself.
What happens to the other embryos?
A quick reminder of how IVF works: after ovarian stimulation, the doctor retrieves eggs, they are fertilized in the lab, and the embryos then develop for several days under embryologists’ supervision. Once an embryo is ready for transfer, it can be placed into the uterus.
If, after this stage, the couple has several suitable embryos left, there is no need to transfer them all at once. They can be frozen and kept for future attempts.
This offers an important advantage: if the first transfer does not result in pregnancy, going through ovarian stimulation and egg retrieval again is not always necessary. A previously stored embryo can be used for the next transfer.
How is the number of embryos decided at Silk Medical?
At Silk Medical, the transfer strategy is shaped individually, taking into account medical history, age, embryo quality and characteristics, the results of previous attempts, and other relevant factors.
The main goal of an IVF program is not just a positive pregnancy test, but creating the conditions for a safe pregnancy and the birth of a healthy baby.
If the couple already has frozen embryos, or if genetic testing is being used, this information also informs the choice of further treatment strategy.
If you are planning IVF and want to understand which transfer option fits your situation, the Silk Medical team can help assess the prognosis and work out an individual treatment plan.
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