How many embryos should be transferred?

The success and safety of an embryo transfer depend on how many embryos are chosen for transfer, however, it is a balancing act. On the one hand, transferring more than one embryo may seem like it can increase the chances of achieving pregnancy. Still, on the other hand, it can lead to multiple pregnancies and put the mother and babies at risk of many serious complications. 

The European Society of Human Reproduction and Embryology (ESHRE) recently published a new guideline helping doctors and embryologists decide on how many embryos to transfer back to their patients so that the treatment risks are reduced and the chances of success are maximised.

What is an embryo transfer?

Embryo transfer is a procedure during fertility treatment when the created embryo is placed in the womb. It is usually done 5 days after the egg collection in case of a fresh fertility cycle or during a frozen embryo transfer cycle when a previously created and frozen embryo is thawed. 

Embryo transfer is a much simpler procedure compared to egg collection and can resemble a smear appointment. The procedure itself takes up to 10-15 minutes and the patient will be able to go home straight away.

How many embryos are usually transferred?

During a fertility treatment, the fertilisation of multiple eggs may lead to creation of more than one embryo. Historically, multiple embryos (more than two embryos) were transferred to the womb to increase the chances of achieving a successful pregnancy, this is called multiple embryo transfer. The thinking was that by replacing a higher number of embryos, the overall chances of pregnancy were higher. However, eventually improvements in treatment quality have reduced the need to transfer multiple embryos.

Currently, the two types of embryo transfer that are mostly used in fertility treatment are:

• elective single embryo transfer (eSET) - in which only a single embryo is selected and transferred

• double embryo transfer (DET) - in which two embryos are selected and transferred to the uterus. 

What do you mean by ‘embryo selection’?

During the time the embryos are in the IVF lab, embryologists regularly monitor their development. This is done by grading them, meaning that embryologists assess their growth and appearance every day and give them a particular ‘grade’. On the day of embryo transfer or freezing, embryologists will look at the grading and it will help them to select the best quality embryos.

In some clinics embryologists are using artificial intelligence (AI) to help them select which embryos to transfer.

Why it isn’t recommended to transfer multiple embryos?

Transferring more than one embryo can lead to a multiple pregnancy, such as twins and triplets, which can have serious complications for both the mother and the babies. These risks may include:

• Ectopic pregnancy

• Gestational diabetes

• Preeclampsia and placental complications

• Preterm birth

• Low birth weight

• Long-term development issues, such as cognitive impairments, learning disabilities, and behavioural challenges

Moreover, having twins can cause 

• Increased stress levels 

• Financial strain

• Postpartum depression

• Parental bonding difficulties

Extensive research undertaken by ESHRE fertility professionals has shown that transferring a single embryo benefits patients more because it is selected as the best quality embryo from a larger number of available embryos while the remaining embryos can be stored and used later in a frozen embryo transfer (FET). 

How is the decision of how many embryos to transfer made?

While the new guideline recommends eSET in all circumstances, in practice it is expected that DET will still occur in practice at clinics around the world. 

The decision on how many embryos to transfer will depend on unique patient and embryo factors, such as:

• maternal age

• embryo quality

• ovarian response to medication

• number of previous cycles

• duration of infertility

• medical history 

This new guideline helps doctors and embryologists provide personalised advice on whether single or double embryo transfer is the most suitable approach for their patients. 

The recommendations emphasise that eSET is the best practice for achieving a healthy pregnancy and minimising the risks. It is because all of the research indicates that there is no evidence that overall birth rates after transferring one embryo would suffer when compared to transferring two embryos. Since there is very strong evidence that transferring a single embryo is much safer for both the mother and a baby, the recommendation is to practise eSET. 

Image source: Alteri, Alessandra & Arroyo, Gemma & Baccino, Giuliana & Craciunas, Laurentiu & De Geyter, Christian & Ebner, Thomas & Koleva, Martina & Kordic, Klaudija & Mcheik, Saria & Mertes, Heidi & Baldani, Dinka & Rodriguez-Wallberg, Kenny & Rugescu, Ioana & Santos-Ribeiro, Samuel & Tilleman, Kelly & Woodward, Bryan & Vermeulen, Nathalie & Veleva, Zdravka. (2024). ESHRE guideline: number of embryos to transfer during IVF/ICSI. Human Reproduction. 39. 10.1093/humrep/deae010.

You can find more information on the ESHRE website https://www.eshre.eu/Guidelines-and-Legal/Guidelines/Embryo-transfer