One of the most important decisions in an IVF cycle — and one that is sometimes not discussed in sufficient depth — is how many embryos to transfer. For many patients, the instinct is to transfer two: it feels like it improves the chances of success, and the idea of having twins is not unwelcome. The medical evidence tells a more complex story.

What the Numbers Show

Transferring two embryos does increase the chance of pregnancy from a single transfer — but not by doubling it. The reason is that embryos do not compete for implantation independently; uterine capacity and receptivity are shared factors.

For a 35-year-old with two good-quality blastocysts:

  • Single embryo transfer (eSET): approximately 35–40% live birth rate
  • Double embryo transfer (DET): approximately 40–50% live birth rate

The incremental gain in live birth rate from transferring two rather than one is real but modest — typically 5–15 percentage points. However, the multiple pregnancy rate is substantially higher: approximately 20–25% of DET cycles that result in pregnancy produce twins, compared to 1–2% for eSET cycles (from the rare case of an embryo splitting).

Why Twin Pregnancy Is a Medical Risk

Twin pregnancy is consistently higher risk than singleton pregnancy — for the babies and for the mother:

For the babies:

  • Preterm birth rate: approximately 50% of twins are born before 37 weeks (vs 8% of singletons)
  • Very preterm birth (<32 weeks): significantly more common
  • Lower birthweight, NICU admission, and associated complications are all more frequent
  • Long-term developmental and educational outcomes are lower on average for premature twins

For the mother:

  • Higher rates of gestational diabetes, pre-eclampsia, and hypertension
  • Higher caesarean section rate
  • Greater risk of postpartum haemorrhage
  • Significantly higher physical demands of twin pregnancy

The HFEA has described multiple pregnancy as "the single biggest health risk of IVF" — not because twins are always seriously harmed, but because the elevated risk is large, predictable, and largely avoidable.

The HFEA and NICE both recommend elective single embryo transfer (eSET) as the standard approach for most patients in most cycles. The underlying logic is that:

  1. The per-cycle live birth rate reduction from transferring one rather than two is modest
  2. The subsequent frozen single embryo transfer cycle (using a frozen embryo from the same stimulation) restores cumulative live birth rates close to those from a fresh double transfer
  3. Twin pregnancy risk is substantially reduced

Cumulative success with eSET strategy: If you transfer one embryo (fresh), and if unsuccessful, transfer a second frozen embryo from the same collection, the cumulative live birth rate is comparable to transferring two fresh embryos at once — but with a near-elimination of twin pregnancy risk.

When Double Transfer May Be Appropriate

eSET is not a universal rule. The HFEA and most UK fertility specialists recognise specific situations where double transfer may be clinically appropriate:

  • Older patients with lower-quality embryos. For patients over 40 with no high-grade blastocysts, where each embryo's individual implantation potential is lower, the benefit of double transfer shifts more in its favour.
  • Poor historical response. Where multiple previous eSET cycles have failed and embryo quality has been good, some clinics will discuss DET as a next step.
  • Limited frozen embryos. Where only two lower-grade embryos are available and no frozen backup exists, there is less downside to transferring both.
  • Cleavage-stage transfer (Day 3). At Day 3, embryo developmental potential is harder to predict than at blastocyst stage. Some clinics are more willing to consider DET at Day 3. Most UK clinics have moved toward blastocyst transfer as the default, which improves selection and supports eSET.

Having the Conversation with Your Clinic

UK clinics are required by HFEA licensing conditions to have a Multiple Births Minimisation Strategy and to report on their multiple birth rates. Clinics with high DET rates and high multiple birth rates face scrutiny from the HFEA.

This means that in practice, most UK clinics will recommend eSET for younger patients with good-quality blastocysts. If you wish to transfer two, you should expect the clinic to explain the risk and may be asked to sign an informed consent document.

The conversation is worth having clearly. Ask your clinic:

  • What is your recommendation for my case, and why?
  • If we transfer one now, how many frozen embryos do I have available as a backup?
  • What is my expected cumulative live birth rate with eSET + subsequent FET, versus DET?

Frequently Asked Questions

Q: I want twins. Can I request double embryo transfer?

A: You can discuss this with your clinic, and in some circumstances DET may be agreed. However, the HFEA requires clinics to counsel you about twin pregnancy risks and document that the decision was made with full information. Clinics may decline DET in younger patients with good-quality embryos, citing their HFEA obligations around multiple birth minimisation. Wanting twins is understandable, but the medical evidence is clear that twin pregnancy carries significantly elevated risk.

Q: If I do eSET, will I definitely get another chance with a frozen embryo?

A: Only if you have frozen embryos available. Not all stimulated IVF cycles produce surplus blastocysts for freezing — particularly in older patients or those with lower ovarian reserve. Before deciding on eSET vs DET, confirm with your embryologist how many good-quality embryos are available for freezing and what their expected thaw survival rate is.

Q: Does it make a difference whether it's a Day 3 or Day 5 transfer?

A: Blastocyst (Day 5) transfer allows better selection of embryos than Day 3 transfer, because more of the embryo's developmental programme is visible. This makes eSET at Day 5 more efficient than eSET at Day 3 — there is more confidence that the selected embryo is the highest quality one. Most UK clinics now favour Day 5 transfer where embryo development allows.

Q: I'm 41 with two mediocre-grade embryos. Should I transfer both?

A: This is a discussion to have directly with your consultant and embryologist, who know the specific grades and your history. In general, older patients with lower-grade embryos are a population where DET is more often considered appropriate. Your consultant will balance the modest twin risk (which is lower when individual implantation potential is lower) against the cumulative benefit.

Q: Does transferring two embryos increase the risk of identical twins?

A: Transferring two embryos does not increase the risk of identical twins (monozygotic twins from embryo splitting) — that risk is the same whether one or two embryos are transferred, and is approximately 1–2% per cycle regardless. The substantially higher twin rate with DET comes entirely from both transferred embryos implanting (dizygotic or fraternal twins).

This article is for information only and does not constitute medical advice. Always make transfer decisions in consultation with your fertility clinic based on your specific embryo quality and history.