Single vs Double Embryo Transfer: What Current Evidence Recommends

Medically reviewed by the fertility specialists at Mannat Fertility  

For decades, transferring two embryos instead of one was the default way IVF clinics tried to improve the odds of a baby. It often worked — but it also meant a much higher chance of twins, and twins come with real medical risks that aren’t always obvious from the outside. Today, a large and growing body of research has changed how fertility specialists think about this decision. This article walks through what that evidence actually shows, where major medical bodies stand, and the factors that go into deciding between single embryo transfer (SET) and double embryo transfer (DET) for a specific patient.

Quick takeaways

• Transferring two embryos at once raises the live birth rate from that single attempt, but it also raises the chance of a twin pregnancy substantially — and twin pregnancies carry meaningfully higher risks for both mother and babies.
• When the comparison is done properly — one embryo now, with a second frozen embryo available if needed — single embryo transfer achieves a comparable overall (cumulative) chance of a baby, with far fewer complications.
• Major professional bodies, including ASRM in the US and ESHRE in Europe, now recommend single embryo transfer as the default for most patients, with double transfer reserved for specific, defined situations.
• The right choice still depends on individual factors — age, embryo quality and genetic testing status, prior IVF history, and uterine factors — which is why this is a conversation to have with your own fertility specialist rather than a one-size-fits-all rule.

This article is for general education and isn’t a substitute for individualised advice from your treating doctor.

SET and DET: what the terms actually mean

Single embryo transfer (SET) means placing one embryo into the uterus during an IVF or ICSI cycle. Double embryo transfer (DET) means placing two. Both can be done with fresh embryos (transferred a few days after egg collection) or frozen embryos (thawed and transferred in a later cycle).

The decision matters because it directly affects two very different outcomes: the chance of having a baby from that one transfer, and the chance of that pregnancy being a twin pregnancy.

Why this became a major clinical question

In the early years of IVF, success rates per transfer were low, so clinics routinely transferred two, three, or more embryos to improve the odds. As embryo culture and selection techniques improved, success rates rose — and so did the multiple pregnancy rate. At its peak, twin pregnancy was far more common after IVF than after natural conception, almost entirely because of the number of embryos placed in the uterus at once.

That mattered because twin pregnancies are not simply “two healthy babies instead of one.” They carry meaningfully higher rates of preterm birth, low birth weight, neonatal intensive care admission, gestational hypertension, pre-eclampsia, and Caesarean delivery compared with singleton pregnancies. A 2020 meta-analysis pooling roughly 60 studies on outcomes after IVF found that mothers carrying twins had several-fold higher odds of Caesarean delivery, pregnancy-induced hypertension, and preterm labour, while their babies had substantially higher odds of preterm birth, low birth weight, and NICU admission, compared with mothers carrying a single baby. You can read more about these specific risks on our page on IVF and multiple-pregnancy risks.

This is the core tension behind the SET vs DET decision: more embryos can mean a better chance per transfer, but also a meaningfully higher chance of a higher-risk pregnancy.

What the evidence actually shows

Live birth rate per transfer

Randomized trials comparing one embryo to two consistently show that, in a single fresh cycle, double embryo transfer produces a higher live birth rate than single embryo transfer. One of the foundational trials, published in the New England Journal of Medicine, found a live birth rate of about 43% after double embryo transfer compared with about 30% after a single fresh embryo transfer. A 2020 update to the Cochrane systematic review of randomized trials reached a similar conclusion: live birth was less likely after a single fresh transfer than after a double transfer.

Cumulative live birth rate — the number that matters more

Per-transfer numbers only tell part of the story, because a single embryo transfer that doesn’t result in a pregnancy can usually be followed by transferring a second, previously frozen embryo. When researchers add that second frozen transfer into the calculation — known as the cumulative live birth rate — the gap between SET and DET narrows considerably. In the same NEJM trial, the cumulative live birth rate after one fresh single transfer followed by one frozen single transfer was close to 39%, compared with about 43% after one double transfer — a small difference achieved without anywhere near the same multiple-pregnancy risk. This is also why embryo quality matters so much for this strategy to work: good-quality blastocysts that survive freezing and thawing well make “one now, one in reserve” a realistic plan. You can read more about how frozen transfers perform in our article on frozen embryo transfer success rates.

Multiple pregnancy risk

This is where the numbers are most striking. An early Cochrane analysis pooling four randomized trials found that double embryo transfer increased the odds of a multiple pregnancy more than sixty-fold compared with single embryo transfer. More recent, larger reviews comparing single and double embryo transfer across dozens of studies confirm the same pattern: the multiple pregnancy rate rises sharply with DET, even though the live-birth-rate advantage of DET is comparatively modest once cumulative outcomes are considered.

Cost

A 2016 US-based cost analysis found that hospital delivery and neonatal costs were considerably higher for double embryo transfer pregnancies than for single embryo transfer pregnancies, almost entirely driven by complications related to twin births. These figures reflect US healthcare costs specifically, but the underlying point — that multiple pregnancies are more resource-intensive to manage safely — holds generally.

What ASRM (United States) recommends

The American Society for Reproductive Medicine’s 2021 committee opinion (developed jointly with SART) sets out age- and prognosis-based limits rather than a single blanket rule:

• Genetically tested (euploid) embryos: ASRM recommends transferring only one embryo at a time, regardless of the patient’s age.
• Untested embryos, favourable prognosis, age 37 or younger: single embryo transfer is strongly recommended.
• Older age groups or less favourable prognosis: the guidance allows for transferring more than one untested embryo, with the specific limit rising gradually with age (for example, patients aged 41–42 may be considered for up to three blastocysts or four cleavage-stage embryos in select cases).
• Donor egg cycles with a young, favourable-prognosis donor: single embryo transfer is recommended.
• Gestational carrier cycles: single embryo transfer is strongly recommended at any age, specifically because of the health risks a multiple pregnancy poses to the carrier.

In short, ASRM’s framework tries to balance the safety benefits of single embryo transfer against the reality that success rates fall with age, and allows some flexibility for older or lower-prognosis patients rather than mandating SET universally.

What ESHRE (Europe) recommends

The European Society of Human Reproduction and Embryology published an extensive evidence-based guideline in 2023 that takes a more uniform position: it recommends single embryo transfer as the preferred approach in essentially every situation it examined — regardless of maternal age, embryo quality, day of transfer, use of PGT-A/genetic screening, donor eggs, gestational carrier cycles, or a history of previous failed transfers. ESHRE’s position is that no single clinical or embryological factor justifies routinely choosing DET over single embryo transfer, largely because the multiple pregnancy rate consistently rises with DET even in patient groups where the live-birth benefit is uncertain.

Why the guidance differs between regions

It’s worth being upfront that these two major bodies don’t fully agree, and the reasons are informative rather than purely scientific:

• Healthcare and insurance structures. In many European countries, IVF cycles (including frozen transfers) are publicly funded or covered with limited patient cost, which makes a “transfer one now, freeze the rest for later” strategy financially realistic for patients. Where IVF and FET cycles are paid out-of-pocket, repeating transfers is a bigger burden, which factors into how flexible a guideline can reasonably be.
• Population data limitations. Much of the strongest randomized trial data comes from younger patients with relatively good prognoses. There’s comparatively little high-quality trial data on SET vs DET specifically in women over 40, which is part of why ASRM leaves more room for clinical judgement in that age group.
• Differing weight given to patient-perceived benefit. Some patients view a twin pregnancy as a desirable outcome rather than a risk to be avoided; both ASRM and ESHRE guidance assume that part of this decision involves informed counselling about risk, not just clinical eligibility.

Factors that shape the decision for an individual patient

Guidelines set the general direction, but the right choice for any one person depends on a combination of factors that your fertility specialist will weigh together:

• Maternal age and ovarian reserve. Younger patients with good ovarian reserve typically have enough embryos and high enough per-embryo success rates that SET with a frozen backup is the safer, evidence-supported choice. The calculation can look different for patients with diminished reserve or advanced age, where each embryo represents a more limited resource.
• Embryo quality and genetic testing status. A single good-quality blastocyst — particularly one confirmed to be chromosomally normal through pre-implantation genetic testing — has a high enough implantation potential on its own that adding a second embryo mainly adds multiple-pregnancy risk without a proportional benefit.
• History of previous failed transfers. This is one of the more debated areas: ASRM allows it to factor into the decision for patients with high-quality embryos who haven’t achieved pregnancy after a prior single transfer, while ESHRE’s review found insufficient evidence to support transferring two embryos purely because of past failures. If you’ve had a transfer that didn’t result in pregnancy, it’s worth discussing the range of possible reasons — embryo, uterine, or other factors — with your specialist; our article on reasons IVF cycles don’t succeed covers this in more depth.
• Uterine and endometrial factors. Conditions affecting the lining of the uterus, such as a thin endometrium, don’t change the basic guidance — both ASRM and ESHRE still favour single embryo transfer in these cases — but they’re an important part of the broader workup before any transfer.
• Donor egg or donor embryo cycles. When a young, well-screened donor’s eggs are used, per-embryo success rates tend to be high, which is part of why single embryo transfer is generally recommended in donor egg cycles as well.
• Number and quality of available embryos overall. The broader picture of factors that influence IVF success — egg and sperm quality, lab conditions, and uterine receptivity — all feed into whether a “single embryo now, frozen embryo later” plan is realistic for a given patient.
• Alternative transfer strategies. Some clinics also offer approaches such as sequential transfer of embryos from different developmental stages in the same cycle; this is a separate strategy from simply transferring two embryos together, and is discussed in our piece on sequential embryo transfer.

The practical takeaway

The weight of current evidence — multiple randomized trials, two major systematic reviews, and the formal guidance of both ASRM and ESHRE — points the same direction: for most patients, particularly those with good-quality embryos and a reasonable number of embryos available, single embryo transfer followed by a frozen transfer if needed gives a similar overall chance of a baby to double embryo transfer, with a substantially lower risk of a complicated twin pregnancy.

That doesn’t mean DET is never appropriate. It can still have a role for carefully selected patients — for example, some older patients with a limited number of available embryos and a realistic, counselled understanding of the higher twin-pregnancy risk. This is exactly the kind of decision that benefits from a detailed conversation with a fertility specialist who knows your full medical history, embryo quality, and treatment goals, rather than a generic rule applied to everyone.

If you’re weighing this decision for an upcoming transfer, our team at Mannat Fertility can walk you through your specific embryo report, age-related considerations, and the trade-offs in plain terms. You can read more about our approach on our doctors’ page or book a consultation to discuss your own treatment plan.

Sources

• American Society for Reproductive Medicine & Society for Assisted Reproductive Technology. Guidance on the limits to the number of embryos to transfer: a committee opinion (2021). Fertility and Sterility, 2021;116(3):651–654.
• ESHRE Guideline Group on the Number of Embryos to Transfer. Evidence-based guideline: number of embryos to transfer during IVF/ICSI. Human Reproduction, 2024;39(4):647–657.
• 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.
• Pandian Z, Templeton A, Serour G, Bhattacharya S. Number of embryos for transfer after IVF and ICSI: a Cochrane review. Human Reproduction, 2005;20(10):2681–2687.
• Thurin A, et al. Elective single-embryo transfer versus double-embryo transfer in in vitro fertilization. New England Journal of Medicine, 2004;351:2392–2402.
• Ma S, Peng Y, Hu L, et al. Comparisons of benefits and risks of single embryo transfer versus double embryo transfer: a systematic review and meta-analysis. Reproductive Biology and Endocrinology, 2022;20:20.
• Carpinello OJ, et al. cost-comparison study of single versus double embryo transfer delivery and neonatal costs, 2016 (PMID: 26969653).

This article is for general educational purposes and does not replace a personal consultation with a qualified fertility specialist. Treatment decisions, including the number of embryos to transfer, should be made jointly with your treating doctor based on your individual medical history and test results.