Twins & Multiples After IVF — Risks & Single Embryo Transfer

"Could we transfer two embryos? We'd love twins."

Fertility clinicians hear this request frequently — and it is completely understandable. After months or years of infertility, the idea of completing your family in a single pregnancy feels enormously appealing. Two embryos means two chances, two babies, and the hope of being done with the emotional and financial burden of treatment.

But twins and higher-order multiples carry serious medical risks that are not always communicated clearly before treatment. The IVF community — led by ASRM, SART, and the CDC — has spent the last two decades working to reduce multiple pregnancy rates because of those risks. The transition to single embryo transfer (SET) as the standard of care is one of the most important quality improvements in the history of reproductive medicine.

This guide explains the data clearly: what the risks are, why SET is recommended, and what the cumulative success rates tell you about your real chances over multiple transfers.

Why IVF Produces Multiple Pregnancies

Multiple pregnancies after IVF occur almost exclusively because more than one embryo is transferred simultaneously. Unlike natural conception (where releasing two eggs is required for non-identical twins), IVF gives clinicians direct control over how many embryos enter the uterus.

Historically, transferring 2 or 3 embryos was common practice because individual embryo survival and implantation rates were lower. The logic was: more embryos transferred = higher probability of at least one implanting = higher per-transfer success rate.

That logic was not wrong at the time — but it traded short-term per-cycle success rates for higher rates of twin and triplet pregnancies, with all the associated complications. As IVF laboratory quality improved, embryo vitrification (freezing) became highly effective, and PGT-A (genetic testing) became more accessible, the rationale for transferring multiple embryos has largely disappeared. Understanding IVF success rates by age can help you see why sequential single transfers often deliver comparable cumulative outcomes to double transfers.

Medical Risks of Multiple Pregnancy

The medical community does not view twin pregnancies as simply "two singletons at once." Multiple pregnancy carries a distinct risk profile that applies to both the mother and the babies — even when both are healthy.

Risks to the Babies

Preterm birth is the most serious and common complication of multiple pregnancy. While the average singleton pregnancy is 40 weeks, the median gestational age at delivery for twins is approximately 35–36 weeks, and for triplets, approximately 32 weeks.

Preterm birth is the leading cause of neonatal mortality and the leading cause of lifelong developmental disabilities in children. The consequences of being born premature can include:

• Respiratory distress syndrome (underdeveloped lungs)
• Intraventricular hemorrhage (brain bleeds)
• Necrotizing enterocolitis (intestinal damage)
• Retinopathy of prematurity (eye damage)
• Cerebral palsy
• Long-term neurodevelopmental delays

The earlier the birth, the more severe the potential consequences. Triplets and higher-order multiples face dramatically higher rates of these complications than twins.

NICU admission: Approximately 57% of twins require NICU admission compared to about 9% of singletons. For triplets, NICU admission is nearly universal.

Low birth weight: Twins are frequently low birth weight (<2,500 grams), which is associated with infant health complications and long-term metabolic and cardiovascular risk.

Intrauterine growth restriction (IUGR): In multiple pregnancies, competition for placental nutrients can result in one or more babies growing below their expected weight.

Risks to the Mother

Gestational hypertension and preeclampsia: Twin pregnancies carry approximately 2–3 times the risk of preeclampsia compared to singletons. Severe preeclampsia can cause seizures (eclampsia), organ damage, and is a leading cause of maternal mortality worldwide.

Gestational diabetes: Multiple pregnancies increase placental hormone production, which disrupts insulin function and raises the risk of gestational diabetes.

Cesarean delivery: The majority of twin pregnancies are delivered by C-section, which carries its own risks including surgical complications and longer recovery.

Postpartum hemorrhage: Multiple pregnancies are associated with higher rates of postpartum hemorrhage due to uterine overdistension.

Maternal mortality: The risk of maternal death is higher in multiple pregnancies than singleton pregnancies across all categories.

Hospitalization during pregnancy: Preterm labor, cervical incompetence, and other complications of multiple pregnancy often require extended bed rest or hospitalization during the third trimester.

The Emotional and Financial Reality

The risks above do not disappear when babies are born healthy — they create a different kind of difficulty:

• NICU stays are traumatic for families and financially costly
• Raising multiples is significantly more demanding and expensive than raising children sequentially
• The healthcare costs associated with preterm births are measured in hundreds of thousands of dollars per child

Many couples who wanted twins do not anticipate the degree to which a complicated twin pregnancy can affect their own health, the babies' health, and the family's financial stability.

The Shift Toward Single Embryo Transfer (SET)

ASRM's committee opinion on the number of embryos to transfer has been progressively updated to recommend single embryo transfer for most patients. Current ASRM guidelines (2021) state:

• Women under 35: Transfer 1 euploid (PGT-A tested) embryo; if no PGT-A, transfer 1 blastocyst
• Women 35–37: Transfer 1–2 blastocysts (1 preferred if euploid)
• Women 38–40: Transfer 1–2 blastocysts
• Women over 40: Transfer no more than 3 blastocysts given lower expected success per embryo
• Donor egg cycles: Transfer 1 euploid embryo; the donor's age, not the recipient's, governs embryo quality

The guidelines are more liberal for older patients because lower per-embryo success rates shift the risk-benefit calculation — but SET is still recommended wherever euploid embryos are available.

SART data shows that the rate of elective SET (eSET) in the United States has increased from approximately 30% in 2013 to over 70% in recent years, with corresponding decreases in IVF twin rates.

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Cumulative Success Rates: SET Over Two Cycles vs. Double Transfer

The most persuasive argument for SET is cumulative success data. When patients compare "transfer 2 embryos in cycle 1" vs. "transfer 1 embryo in cycle 1, freeze the other for cycle 2," the cumulative live birth rates are comparable — but the multiple pregnancy rate is dramatically lower with sequential SET.

This has been demonstrated in multiple randomized trials and large observational studies. A landmark analysis:

The cumulative rate from two SET cycles approaches or equals the per-cycle rate of DET — with a multiple pregnancy rate that is 10–15 times lower. You are not sacrificing your overall probability of pregnancy by choosing SET; you are spreading that probability across two lower-risk transfers.

The key insight is that frozen embryo transfer (FET) technology is now excellent. Vitrification has achieved post-thaw survival rates of 95% or higher for blastocysts. Frozen embryos are no longer a consolation prize — they perform comparably to fresh in most studies, and better in some protocols.

The Patient Preference Tension

Despite the medical evidence, some patients still prefer to transfer two embryos. The reasons are real:

• Emotional exhaustion from treatment and desire to be "done"
• Financial concerns about paying for a second transfer cycle
• Uncertainty about whether their embryos will survive freezing and thawing
• Age-related urgency
• The genuine desire for twins

These preferences deserve to be heard — not dismissed. The role of a reproductive endocrinologist is to ensure that the patient has accurate, complete information about the medical risks and then support their decision. In most cases, after a thorough discussion of the data, most patients choose SET. In some cases — particularly older patients, those with lower-quality embryos, or those with prior failures — DET may be medically appropriate.

But the conversation must include the real risk numbers, not platitudes like "twins are a blessing." The medical evidence is clear about the risk profile.

Monochorionic Twins: Higher Risk Even After SET

A rare but important phenomenon: even after transferring a single embryo, there is approximately a 1–3% chance that the embryo splits into identical twins (monozygotic twins). This is slightly higher after IVF than in natural conception, possibly due to the manipulation involved in embryo culture and transfer.

Identical twins who share a placenta (monochorionic twins) face additional serious risks beyond those of fraternal twins:

Twin-to-twin transfusion syndrome (TTTS): Blood flows unevenly between the twins through shared placental vessels. Without intervention, TTTS has high mortality rates.

Selective intrauterine growth restriction (sIUGR): One twin receives a disproportionate share of placental nutrients, growing much larger than the other.

Twin anemia-polycythemia sequence (TAPS): A more gradual form of blood imbalance between monochorionic twins.

Monochorionic twin pregnancies require specialized monitoring — typically at a maternal-fetal medicine practice with experience in complex twin management.

The point is that even SET carries a small risk of high-risk identical twins — underscoring that transferring two embryos adds substantially to an already-existing multiple pregnancy risk.

Donor Egg Multiples

Donor egg IVF follows the same SET principles, but with one important note: donor eggs from young donors have very high success rates per transfer. Transferring two donor egg embryos frequently produces twins — sometimes in patients who were told their chances of any pregnancy at all were low.

ASRM guidelines are explicit: in donor egg cycles with euploid embryos, transfer 1 embryo. The success rate with a single donor embryo transfer is typically 60–70% per transfer, leaving little clinical justification for the added risk of DET.

SART Data on SET Rates by Clinic

SART's online IVF outcomes database (sart.org) reports SET rates alongside success rates for each member clinic. When evaluating clinics, looking at both metrics together is informative:

• A clinic with a high success rate and a high multiple pregnancy rate may be achieving that success by routinely transferring multiple embryos — at a cost to patient safety
• A clinic with a high success rate and a low multiple pregnancy rate demonstrates both clinical skill and adherence to best-practice guidelines

Choosing a clinic that strongly recommends SET and achieves good cumulative success rates through sequential single transfers is the safest approach for most patients. Our fertility clinic directory lets you search for clinics by SET rate and live birth outcomes.

Reducing Multiple Pregnancy Risk in Stimulated IUI

IVF is not the only fertility treatment that produces multiples. Stimulated IUI — using gonadotropins (injectable follicle-stimulating hormone) to produce multiple follicles before insemination — can cause multiple follicle release, with each mature egg capable of fertilization.

Gonadotropin-stimulated IUI cycles that produce 3 or more mature follicles are typically cancelled or converted to IVF to avoid high-order multiple pregnancy. This is standard protocol at reputable clinics.

Oral agents (letrozole or clomiphene) produce lower multiple pregnancy rates than gonadotropins, which is part of why they are preferred for ovulation induction in IUI protocols.

Frequently Asked Questions

Q: Does transferring two embryos double my chances of having a baby? A: No. Landmark cumulative data shows that two sequential single embryo transfer (SET) cycles achieve live birth rates of approximately 70–80%, approaching or equaling the per-cycle rate of double embryo transfer (55–65%). The critical difference is the multiple pregnancy rate: two sequential SETs produce a 2–4% multiple rate, while double embryo transfer produces a 25–35% multiple rate.

Q: What are the main medical risks of twin pregnancy after IVF? A: The median gestational age at delivery for twins is 35–36 weeks (vs. 40 for singletons), and approximately 57% of twins require NICU admission compared to 9% of singletons. Mothers face approximately 2–3 times the risk of preeclampsia, higher rates of gestational diabetes, cesarean delivery, and postpartum hemorrhage. These are not rare complications — they represent the expected risk profile of a twin pregnancy.

Q: Can identical twins occur even after transferring a single embryo? A: Yes, though rarely. After single embryo transfer, there is approximately a 1–3% chance that the embryo splits into identical (monozygotic) twins — a rate slightly higher than in natural conception, possibly related to embryo manipulation during culture and transfer. Identical twins who share a placenta (monochorionic twins) face additional serious risks including twin-to-twin transfusion syndrome.

Q: What does ASRM recommend for the number of embryos to transfer? A: ASRM's 2021 guidelines recommend transferring 1 euploid (PGT-A tested) embryo for women under 35 and strongly prefer single embryo transfer for women up to 40 when euploid embryos are available. For women over 40, up to 3 blastocysts may be considered given lower per-embryo success rates. In donor egg cycles, ASRM recommends transferring 1 embryo due to the high success rates with donor eggs.

Q: How can I evaluate a clinic's approach to multiple embryo transfer? A: SART's online IVF outcomes database (sart.org) reports SET rates alongside success rates for each member clinic. A clinic with high success rates and a low multiple pregnancy rate demonstrates both clinical skill and adherence to best-practice guidelines. A clinic achieving high success through routine double transfers is doing so at a cost to patient safety.

Key Takeaways

• Twins and multiples after IVF are primarily caused by transferring more than one embryo, not by the treatment itself
• Multiple pregnancy carries significant medical risks for both mother and babies, including preterm birth, NICU admission, preeclampsia, and gestational diabetes
• Transferring two embryos does not double your chance of success — cumulative success through two sequential SET cycles approaches the same rate as a single double transfer, with far fewer multiple pregnancies
• ASRM recommends single embryo transfer for most patients; elective SET rates have exceeded 70% at US clinics
• Frozen embryo transfer technology is excellent — embryos frozen for subsequent single transfers perform comparably to fresh
• Even after SET, there is a small chance of identical twins (monozygotic splitting), which carries its own elevated risk profile

This article is for informational purposes only and does not constitute medical advice. Please consult a board-certified reproductive endocrinologist for personalized guidance.