One Embryo or Two? How the Transfer Number Decision Is Made

Few IVF decisions feel as personal as the one made a few minutes before transfer: one embryo, or two? After months of injections, money and waiting, a second embryo can look like cheap insurance. Many patients, especially those paying out of pocket or travelling for treatment, ask for two because they cannot imagine doing all of this again.

That feeling is understandable. It is also the reason professional bodies in the US and UK have written detailed guidance on this one question. This article explains what that guidance says, why twins are treated as a risk rather than a bonus, which patients the guidance gives more room to, and how to have the conversation with your doctor without feeling pushed either way.

The short answer from the guidelines

The American Society for Reproductive Medicine and the Society for Assisted Reproductive Technology publish a joint opinion on the maximum number of embryos to transfer. The current version (ASRM/SART Practice Committees, 2021) sets out a few clear points:

• A euploid embryo (one that tested chromosomally normal with PGT-A) should be transferred alone, at any age.
• For patients under 35, single-embryo transfer is strongly encouraged whatever the embryo's stage.
• For patients aged 35 to 37, single-embryo transfer should be strongly considered.
• Older age groups and patients without favourable features have higher upper limits, but those are ceilings, not targets.
• Exceeding the limits is described as a rare exception that needs documented counselling and a recorded justification.

In the UK, the Human Fertilisation and Embryology Authority describes single embryo transfer as usual best practice and sets clinics a target of fewer than 10% of IVF births being multiples (HFEA, 2016).

The ASRM upper limits, laid out

The 2021 opinion groups patients by prognosis and age, then gives a maximum for cleavage-stage (day 2 to 3) embryos and for blastocysts (day 5 to 6). The table below summarises it. "Favourable" has a specific meaning explained in the next section.

For people 43 and older using their own eggs, the committee says there is not enough data to set a limit and advises caution. These are US professional recommendations, not laws, and other countries have their own rules or none at all.

What counts as a "favourable" prognosis

The opinion defines it narrowly. In a fresh cycle, favourable means the patient is expected to have at least one good-quality embryo left over to freeze, or has had a previous live birth after an IVF transfer. In a frozen cycle, any one of these qualifies: the embryos were vitrified at day 5 or 6, they are euploid, it is the patient's first frozen transfer, or the patient has had a previous IVF live birth.

That definition covers a lot of people. If you are doing a frozen transfer of a vitrified blastocyst, you very likely fall into the favourable group, which puts you at a limit of one up to age 37.

Rules that override age

• Donor eggs: use the donor's age, not the recipient's.
• Frozen embryos: use your age when the embryos were frozen, not your age at transfer.
• Gestational carriers: single embryo transfer is strongly recommended.
• Medical conditions that make a twin pregnancy especially dangerous: no more than one embryo.

Why twins are treated as a complication

Twins can sound like a happy shortcut to the family size you want. The medical view is different because the risks land on both the pregnant person and the babies. The HFEA notes that at least half of twins are born premature and underweight, and that mothers carrying multiples have higher rates of high blood pressure, gestational diabetes, anaemia and haemorrhage than mothers of single babies (HFEA, 2016). The ASRM opinion similarly describes twin pregnancies as carrying significantly more morbidity than singletons.

Two other points from the ASRM document are worth knowing before transfer day. First, reducing a multiple pregnancy later does not fully remove its risks and can lead to loss of the whole pregnancy. Second, transferring two embryos does not split the odds evenly: you may get no pregnancy, one baby or two, and you cannot choose which.

Does transferring one embryo lower the chance of a baby?

This is the question underneath most requests for two. The honest answer depends on whether you are measuring one transfer or your whole course of treatment.

The HFEA states that elective single embryo transfer has no significant effect on success rates, while noting it may not be right for everyone, particularly some older patients, and advising people to discuss it with their clinic. The ASRM opinion points to evidence in the same direction: in national US data, clinics that did more elective single transfers in patients under 38 had fewer multiple pregnancies without a significant drop in clinic-level live birth rates. It also cites research in which one euploid blastocyst gave pregnancy rates similar to two untested blastocysts in women around 42, with far fewer twins.

The way to think about it: if you have more than one good embryo, a single transfer does not throw the second one away. It goes into the freezer for a later frozen transfer. Your cumulative chance, across both embryos, is what matters, and a frozen transfer of a vitrified blastocyst is a routine procedure in most modern labs.

Where the trend has gone

The ASRM opinion reports that in 2017, 12.4% of women under 38 with a successful IVF cycle had a twin pregnancy, down from 23% in 2014. It describes that fall as progress while noting twin rates were still well above what happens without treatment. It also found that nearly half of US IVF multiple pregnancies in women under 35 followed the transfer of two blastocysts, which is exactly the group the guidance now steers towards one.

The document also notes that insurance coverage for IVF was linked to fewer embryos being transferred. The likely reason is intuitive: when each attempt is not a financial cliff, patients and doctors feel less pressure to put everything into one transfer.

The money and travel pressure, named honestly

If you are paying cash, or flying to another country for treatment, the pull towards two embryos is stronger. A second frozen transfer means another preparation cycle, possibly another trip, and more cost. Those are real burdens and it is fair to say them out loud in your consultation.

Some practical ways patients handle that tension without automatically transferring two:

1. Ask for frozen transfer pricing up front. Knowing the real cost of a second transfer turns a vague fear into a number you can plan for.
2. Ask whether a frozen transfer could be done closer to home, with embryos shipped, or with local monitoring and a short trip. Our guide to shipping frozen embryos internationally covers the logistics and the failure points.
3. Price in the cost of a twin pregnancy too. Higher-risk pregnancies tend to mean more appointments, more monitoring, and a greater chance of early delivery. That cost does not appear on the IVF invoice, but it is real.

When a clinic suggests two, or refuses two

You may meet either situation. A clinic abroad might offer two embryos as standard, or a clinic might decline your request for two. Neither is automatically wrong, but both deserve a conversation.

If two are offered

Ask why. The guidance allows more than one embryo for older age groups and for patients without favourable features, so there may be a reason grounded in your situation. If the answer is simply "it gives you better odds", ask about the twin rate as well. The HFEA advises people considering treatment abroad to ask each clinic what its multiple birth rate is and what it does to reduce it (HFEA guidance on treatment abroad).

If two are refused

Ask what the clinic's policy is based on and whether there is any situation in which it would transfer two. If you are under 35 with a good blastocyst, a refusal is consistent with mainstream guidance. If you are older or have had several failed transfers, a more detailed discussion of your own embryos and history is reasonable.

Questions to bring to your transfer consultation

• Which ASRM prognosis group would I fall into, and why?
• What is my estimated chance of a live birth from one embryo versus two, and of twins in each case?
• If I transfer one, how many embryos remain frozen, and what would the next transfer involve?
• Do I have any health conditions that would make a twin pregnancy riskier for me?
• What is this clinic's own multiple pregnancy rate?

You will also be asked to confirm the number in writing. Our plain-language guide to IVF consent forms explains what those documents usually cover.

The bottom line

For most people with a good-quality embryo, especially under 38 or with a blastocyst or tested embryo, current guidance points to one at a time. That is not about being cautious with your chances; it is about getting to a baby with fewer serious risks along the way, while keeping your other embryos for later. Older patients and those with a harder history have more room in the guidance, and that is a decision to make with your doctor using your own embryos and numbers, not a general rule.