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Why Do Clinics Recommend Single Embryo Transfer?

Single embryo transfer avoids the higher risks of twins and triplets. One embryo then a frozen one may give a similar overall chance to two at once.

By Vireon Medical Editorial TeamFirst published Evidence checked through

Short answer

Mainly to avoid twins and triplets, which carry higher risks for both the pregnant person and the babies. Transferring two embryos at once raises the chance of a birth in that single cycle, but trials suggest that transferring one embryo and then a frozen one if needed gives a similar overall chance with far fewer multiple pregnancies. How strongly single embryo transfer is advised depends on age, embryo quality and how many embryos are available.

Key points

  • Multiple pregnancy is a recognised risk of IVF for both parent and babies.
  • Around 6 in 10 twins are born before 37 weeks.
  • In the UK, 88% of IVF cycles in 2023 used single embryo transfer, and the multiple birth rate fell to 3.4%.
  • One embryo followed by a second single transfer may give a similar cumulative chance to transferring two at once.
  • Freezing spare embryos is what allows the one-at-a-time approach to keep the overall chance up.

Why twins are treated as a risk

Many people hoping for a family would welcome twins, so it can seem odd that clinics try to avoid them. The reason is medical. The NHS says that in a twin pregnancy the pregnant person is at higher risk of complications such as iron deficiency anaemia and pre-eclampsia (a condition involving high blood pressure that can be dangerous).3

For the babies, the main risks are being born too early and too small. Around 6 in 10 twins are born before 37 weeks, and almost 8 in 10 triplets are born before 35 weeks.3 The NHS lists having more than one baby as a recognised risk of IVF.4

Single versus double embryo transfer

A 2020 Cochrane systematic review pooled 17 randomised trials with 2,505 women.2 It found three main things.

OutcomeDouble embryo transferSingle embryo transfer
Live birth in one fresh cycleAbout 46%About 27% to 35% (low-certainty evidence)2
Cumulative live birth (one double transfer vs two single transfers)About 42%About 34% to 46%; may be little or no different (low certainty)2
Multiple pregnancyAbout 13% to 15%About 0% to 4% (moderate certainty)2
Results from a 2020 Cochrane review of randomised trials; figures are illustrative estimates from the review.

Put simply: two embryos at once gives a better chance in that one transfer, but much of the gap closes if the single transfer is followed by a further single transfer, usually of a frozen embryo. Meanwhile, the multiple pregnancy rate drops sharply.2

The certainty of evidence for the live birth comparisons is low, partly because many trials reported their methods poorly. The evidence that single transfer reduces multiple pregnancy is stronger, rated moderate by the reviewers, so the label here is Moderate.2

What happened when clinics changed practice

The UK offers a large real-world example. Single embryo transfers rose from around 10% of IVF cycles in the early 2000s to 88% in 2023. Over the same period, the multiple birth rate from IVF fell from around 28% in the early 1990s to 3.4% in 2023, which the HFEA (the UK fertility regulator) describes as one of the lowest rates in the world.1

Birth rates did not fall over the same period. Between 2013 and 2023, the average birth rate rose from 19% to 25% for fresh transfers using a patient’s own eggs, and from around 18% to 33% for frozen transfers.1 These are observational national data rather than a trial, so they cannot prove cause and effect, but they fit with the Cochrane findings.

Does it apply to everyone?

Not equally. The Cochrane review notes that the reduction in multiple pregnancy has been shown mainly in women with a good prognosis, such as younger patients with good-quality embryos.2 For older patients, or when embryo quality is poorer, the balance can be different, and a clinician may discuss transferring more than one.

The NHS describes this as a shared decision: if more than one embryo is available, the doctor should explain the risks and benefits of transferring more than one.4 Age matters to that conversation because birth rates per embryo fall steeply with age; see how age affects IVF success.

Common questions

Will single embryo transfer lower my chances?

Per transfer it may, but over a fresh transfer plus a frozen one the overall chance may be similar.2 That depends on having more than one good embryo to freeze.

Can a single embryo still result in twins?

Yes, occasionally; the Cochrane review estimates a multiple pregnancy risk of about 2% to 4% after a single fresh transfer.2

Are frozen embryos as good as fresh ones?

In UK data, average birth rates from frozen transfers rose to 33% in 2023, compared with 25% for fresh transfers using a patient’s own eggs, though the groups are not directly comparable.1 Our page on frozen versus fresh embryo transfer explains the nuance.

Does genetic testing of embryos help choose the one to transfer?

It is promoted for that, but whether it improves birth rates is debated; see what PGT-A is and whether it improves outcomes.

Bottom line

Single embryo transfer is recommended because twin and triplet pregnancies carry real risks, and transferring one embryo at a time, with freezing for later, preserves much of the overall chance of a baby. The case is strongest for people with a good prognosis and more than one good-quality embryo.

Sources

  1. Human Fertilisation and Embryology Authority. Fertility treatment 2023: trends and figures. HFEA. https://www.hfea.gov.uk/about-us/publications/research-and-data/fertility-treatment-2023-trends-and-figures/. Accessed 5 November 2026.
  2. Kamath MS, Mascarenhas M, Kirubakaran R, Bhattacharya S. Number of embryos for transfer following in vitro fertilisation or intra-cytoplasmic sperm injection. Cochrane Database of Systematic Reviews; 2020. doi:10.1002/14651858.CD003416.pub5. https://cochranelibrary.com/cdsr/doi/10.1002/14651858.CD003416.pub5/pdf/CDSR/CD003416/CD003416_abstract.pdf
  3. NHS. Antenatal care with twins. NHS website. https://www.nhs.uk/pregnancy/finding-out/antenatal-care-with-twins/. Accessed 5 November 2026.
  4. NHS. IVF: risks. NHS website. https://www.nhs.uk/conditions/ivf/risks/. Accessed 5 November 2026.

Article history

  • — First published.