Single vs Double Embryo Transfer: Twins and ASRM

You have a small number of blastocysts in the freezer, your clinic has recommended transferring one, and your math brain is asking whether two would double your chance. This is the single vs double embryo transfer conversation in its honest form. Not the "twins are a blessing" version, but the version I would have with you in clinic, with the data and the obstetric risk in plain view.

The decision in one sentence. For patients under age 38 with a euploid blastocyst, transfer one embryo at a time. That is the ASRM 2021 guideline¹, and the reason is that per-transfer pregnancy rates are similar between single and double transfer when the embryo is euploid, while multiple-gestation risk multiplies neonatal and maternal complications. The clinic offering you elective single embryo transfer (eSET) is not giving you a worse chance. They are splitting your cumulative live-birth probability across two transfers with lower obstetric risk.

Single vs double embryo transfer: the data in numbers

The per-transfer pregnancy-rate comparisons that matter for this decision:

• Single euploid blastocyst transfer, age under 38: Per-transfer pregnancy rate roughly 55 to 65 percent.
• Double untested blastocyst transfer: Per-transfer pregnancy rate roughly 50 to 65 percent, with a 25 to 35 percent twin rate.
• Double euploid blastocyst transfer: Per-transfer pregnancy rate roughly 65 to 75 percent, with a 40 to 50 percent twin rate.
• Cumulative live birth across two sequential single euploid transfers: Roughly equal to a single double euploid transfer.

That last point is the one most patients miss in the heat of the decision. Two single transfers, done sequentially, use the same embryos as one double transfer. Your cumulative live-birth chance is similar. The math is not "one versus two for the same money." It is "two transfers spaced out, with normal singleton pregnancies as the goal" versus "one transfer with a substantial twin probability."

The Forman et al. 2013 randomized controlled trial of single euploid blastocyst transfer was the landmark study that established this equivalence in the PGT-A era². The McLernon et al. BMJ 2010 individual-patient-data meta-analysis of elective single versus double transfer reached the same conclusion in pre-PGT data⁵. The Cochrane review on number of embryos transferred is consistent³.

The ASRM 2021 guideline, in plain English

The ASRM committee opinion on the limits to the number of embryos to transfer is the document your RE is working from¹. The guideline differs by age and by whether the embryo has been PGT-A tested. A simplified version:

• Age under 38 with a euploid blastocyst: One embryo. Strongly recommended.
• Age under 35 with an untested blastocyst: One to two blastocysts.
• Age 35 to 37 with untested blastocysts: One to two blastocysts.
• Age 38 to 40 with untested blastocysts: Up to three blastocysts per case.
• Age 41 to 42 with untested blastocysts: Up to three blastocysts per case judgment.

For cleavage-stage (day 3) embryos, the caps are slightly higher because day-3 embryos have lower per-embryo implantation rates than blastocysts. The guideline is not a prescription; it is a frame within which your RE makes a case-by-case recommendation based on your history.

The 2021 update tightened the eSET recommendation for younger patients with euploid embryos because the PGT-A and vitrification data had matured. The cumulative live-birth math made the case clearer than the 2017 version did.

Why twins are not just "two babies at once"

The phrase "twins would be a blessing" comes up in nearly every transfer-number consultation. I respect the feeling. I also explain, carefully, that twin pregnancy is a complication, not a bonus.

The numbers behind that statement:

• Preterm birth: Twin pregnancies have a preterm birth rate around 60 percent, compared with roughly 10 percent for singletons. About half of twins are born before 37 weeks.
• Preeclampsia: Roughly three times the singleton rate.
• Gestational diabetes: Higher than in singleton pregnancy.
• Cesarean delivery: Substantially higher rate.
• NICU admission: Common, sometimes for weeks.
• Cerebral palsy: Risk roughly doubled relative to singletons, driven largely by prematurity.
• Maternal hemorrhage and hypertension: Higher rates of postpartum hemorrhage, gestational hypertension, and longer recovery time.

The ASRM committee opinion on multiple gestation associated with infertility therapy lays out these risks in detail⁴. The cost dimension is also real. NICU stays, prematurity sequelae, and the lifetime care load of severely preterm twins are substantial.

I will share something from my own clinic experience. I have transferred two embryos and watched a patient celebrate a positive test, and then watched twins land in NICU at 31 weeks with consequences that followed the family for years. I do not push double embryo transfer (DET) for the math anymore. The math is not what it looks like at first glance, and the obstetric ledger does not flatter the choice.

Why eSET is medically generous, not stingy

This framing matters because many patients walk into the conversation feeling cheated. The clinic recommending eSET is not robbing you of a better chance. They are splitting the same cumulative probability across two transfers with much lower obstetric risk.

A simple example. If your two euploid blastocysts each give you a 60 percent per-transfer pregnancy chance, then:

• One eSET, plus a second eSET if the first does not work, gives you roughly an 84 percent cumulative chance of pregnancy (1 - 0.4 × 0.4).
• One DET with both euploid embryos gives you roughly a 65 to 75 percent per-transfer chance, with a 40 to 50 percent twin probability.

The cumulative numbers are close, and the obstetric profile of two singletons is dramatically safer than one twin pregnancy. The clinic is not gatekeeping. They are doing the obstetrics-aware math.

Where DET still gets considered

I want to be honest that DET is not always wrong. The ASRM guideline allows it in specific situations, and there are profiles where it is the right call:

• Repeated failed eSETs with euploid embryos (recurrent implantation failure). When two or three good-quality euploid blasts have not implanted, the embryo-side explanation weakens and the uterine-side explanation strengthens, but the math on transferring two becomes more reasonable.
• Untested embryos in advanced maternal age with a limited cohort: At 40+ with only one or two day-5 embryos, both untested, the per-embryo euploid likelihood is lower (often under 30 percent), and DET caps closer to the singleton range.
• Strong patient preference after counseling: If you have heard the obstetric risks, you have heard the cumulative math, and you still want DET, you are an adult making an informed decision. Your RE should not bulldoze you.
• Coverage constraints: Some insurance plans cover only one transfer. If you have one coverage attempt and limited self-pay capacity, DET changes the calculus, though the twin risk does not.
• Patient with a poor-prognosis embryo cohort (low morphological grade, mosaic-only options) where each embryo's implantation chance is low enough that twin risk becomes acceptable.

These are real situations. The default is still eSET for good-prognosis cycles, but "default" is not the same as "always."

The MFPR conversation

Some clinics raise multifetal pregnancy reduction (MFPR) at the point of DET counseling. The idea is that if a DET produces a twin or higher-order pregnancy, the patient can opt for reduction to a singleton. This is medically possible and obstetrically protective, but it carries serious emotional and ethical weight. Not every patient will entertain it. Some will not even discuss it as a hypothetical.

The point of raising MFPR before transfer, not after, is that the decision to transfer two embryos and the decision to reduce a twin pregnancy should not be downstream of each other. If you would not consider reduction, that is information that should shape the transfer number choice now, not later.

What to ask your RE before transfer day

The questions worth bringing to the pre-transfer appointment:

• What is your clinic's eSET rate for someone with my age and embryo profile?
• What is my expected per-transfer pregnancy rate with one euploid blastocyst?
• What is my cumulative live-birth rate across both euploid blasts if we do eSET twice?
• What is my twin pregnancy rate if we transfer two?
• Why are you recommending what you are recommending for me specifically?
• If we do DET and end up with twins, what is your maternal-fetal medicine referral pathway?

Write the answers down. The pre-transfer appointment is short and the decision is dense.

Single vs double embryo transfer: what this means for you

Three things to take with you from the single vs double embryo transfer decision.

First, the default for good-prognosis cycles in 2024 is single embryo transfer, not because clinics are being cautious for liability reasons, but because the cumulative math and the obstetric data both point there. The ASRM 2021 guideline is the explicit version of that consensus¹.

Second, twin pregnancy is a complication, not a bonus. The numbers on preterm birth, preeclampsia, NICU stays, and cerebral palsy are real, and they are not improved by good intentions. The decision to transfer two embryos should be made with full knowledge of the obstetric ledger.

Third, DET is not always wrong. There are profiles where it is reasonable, and patient autonomy matters. If you have heard the data and still want DET, your RE should counsel you carefully, not refuse you outright.

What's next

• If you are heading into transfer prep, read embryo transfer prep.
• If you want the larger map of the transfer cycle, return to embryo transfer explained.
• If you are still weighing fresh versus frozen, read fresh vs frozen embryo transfer.
• If a prior transfer did not work and you are deciding what is next, read failed IVF, decoding the next step.

Sources

1. Practice Committee of the American Society for Reproductive Medicine and the Practice Committee for the Society for Assisted Reproductive Technologies. Guidance on the limits to the number of embryos to transfer: a committee opinion. Fertility and Sterility 2021;116(3):651-654. https://doi.org/10.1016/j.fertnstert.2021.06.050
2. Forman EJ, Hong KH, Ferry KM, et al. In vitro fertilization with single euploid blastocyst transfer: a randomized controlled trial. Fertility and Sterility 2013;100(1):100-107.e1. https://doi.org/10.1016/j.fertnstert.2013.02.056
3. Pandian Z, Marjoribanks J, Ozturk O, Serour G, Bhattacharya S. Number of embryos for transfer following in vitro fertilisation or intra-cytoplasmic sperm injection. Cochrane Database of Systematic Reviews 2013;7:CD003416. https://doi.org/10.1002/14651858.CD003416.pub4
4. Practice Committee of the American Society for Reproductive Medicine. Multiple gestation associated with infertility therapy: a committee opinion. Fertility and Sterility 2012;97(4):825-834. https://doi.org/10.1016/j.fertnstert.2011.11.048
5. McLernon DJ, Harrild K, Bergh C, et al. Clinical effectiveness of elective single versus double embryo transfer: meta-analysis of individual patient data from randomised trials. BMJ 2010;341:c6945. https://doi.org/10.1136/bmj.c6945
6. Society for Assisted Reproductive Technology (SART). National Summary Report: CORS data on multiple gestation rates. https://www.sartcorsonline.com/