Fertility Treatment and Twin Rates: IVF, IUI and Fertility Drug Statistics

Fertility treatment can substantially increase the likelihood of twins, but there is no single “fertility treatment twin rate.” The number depends on the treatment, whether ovarian stimulation is used, how many embryos are transferred, the time period, and whether a study measures twin pregnancies, twin births, or all multiple births.

Modern IVF is especially important to separate from historical IVF. As single embryo transfer has become standard practice in many settings, IVF multiple-birth rates have fallen sharply. At the same time, treatments that stimulate the ovaries can still produce a meaningful twin risk when more than one follicle develops.

Quick takeaways

• In a population-based Ontario study covering 2006–2021, twin pregnancies occurred in 1.4% of unassisted conceptions, 9.4% after ovulation induction or IUI, and 14.7% after IVF. These are twin-pregnancy rates within that study, not universal rates for every patient or clinic.
• Within the same study, the IVF twin-pregnancy rate fell from 27.6% in 2006–2011 to 6.8% in 2016–2021 as single embryo transfer became more prominent.
• UK HFEA data report an IVF multiple-birth rate of 3.2% in 2024, down from 14.4% in 2014. This is a multiple-birth rate among live births, not a twin-pregnancy rate, and the 2024 figure is preliminary.
• Fertility-drug risks also vary. In one randomized trial of ovarian stimulation with IUI, ongoing multiple gestation occurred in 9% of clomiphene pregnancies, 13% of letrozole pregnancies, and 32% of gonadotropin pregnancies. The gonadotropin group included triplets as well as twins.
• Before comparing two percentages, check the treatment, outcome measured, denominator, study period, and population.

Twin Rates by Conception and Fertility Treatment

The table below brings several of the strongest available datasets into one place. It is deliberately not presented as a simple ranking from “lowest” to “highest,” because the studies do not all measure the same outcome. A twin-pregnancy rate, a multiple-gestation rate, and a multiple-birth rate are different statistics.

How to Read the Rates in This Table

The percentages above answer related but not identical questions. The 14.7% IVF figure from the Ontario study describes twin pregnancies across a long 2006–2021 period. The HFEA’s 3.2% figure describes multiple live births after IVF in the UK in 2024. The fertility-drug figures come from a randomized trial in a specific population undergoing ovarian stimulation with IUI.

They should therefore not be treated as competing estimates of one universal number. They show how twin and multiple rates vary across treatments, eras, populations, and outcome definitions.

IVF Twin Rates: Historical vs Modern Data

IVF has a strong historical association with twins because transferring more than one embryo was once much more common. That history still influences many figures quoted online. Modern IVF practice looks different in countries and clinics where single embryo transfer is now widely used.

Modern IVF Twin and Multiple-Birth Rates

In the Ontario population study, IVF was associated with a 14.7% twin-pregnancy rate across the full 2006–2021 study period. Looking only at the later 2016–2021 era, however, the IVF twin-pregnancy rate was 6.8%, compared with 27.6% in 2006–2011.

The same change is visible in national UK data, although the HFEA measures a different outcome. The HFEA reports that the average IVF multiple-birth rate declined from 14.4% in 2014 to 3.2% in 2024. Its definition of multiple-birth rate is the number of live multiple births divided by all live births in the relevant treatment data. The HFEA notes that its 2024 birth-outcome figures are preliminary and have not yet been fully validated.

US data also show the shift toward fewer embryos per transfer. CDC national ART data show that single embryo transfer increased from 30.8% of embryo-transfer cycles in 2013 to 85.9% in 2022.

Important: A 3.2% IVF multiple-birth rate, a 6.8% IVF twin-pregnancy rate, and a percentage of embryo-transfer cycles resulting in twin live birth are different measures. None should be substituted for another.

How IVF Twin Rates Changed Over Time

The direction is consistent even though the metrics differ: modern ART practice increasingly favors single embryo transfer, while twin and higher-order outcomes after IVF have fallen.

Historical US data help explain why fertility treatment became so closely associated with twins. A New England Journal of Medicine analysis estimated that 36% of US twin births in 2011 resulted from medically assisted conceptions. That figure describes the estimated contribution of fertility treatment to all twin births in the population; it is not the probability that an individual fertility-treatment pregnancy would be twins.

Why Single Embryo Transfer Changed the Numbers

In IVF, one of the strongest modifiable drivers of dizygotic, or fraternal, multiple pregnancy is the number of embryos transferred. If more than one embryo is transferred and more than one implants, a twin or higher-order pregnancy can result.

Single embryo transfer (SET), including elective single embryo transfer (eSET), removes that multiple-embryo pathway from a transfer. The American Society for Reproductive Medicine describes SET as the most direct and effective strategy for reducing multiple gestation in ART.

The HFEA reports that 84% of UK embryo transfers involved a single embryo in 2024, alongside an average IVF multiple-birth rate of 3.2%. Importantly, the HFEA also reports that IVF birth rates have increased over time while the multiple-birth rate has fallen. Reducing multiple births therefore does not simply mean accepting the historical success rates associated with older embryo-transfer practice.

IUI and Ovulation-Induction Twin Rates

IUI and ovulation induction are often discussed together, but they are not the same intervention. Intrauterine insemination (IUI) places prepared sperm into the uterus. Ovulation induction (OI) or ovarian stimulation (OS) changes ovarian activity and can cause more than one follicle to mature.

This distinction matters when interpreting a reported “IUI twin rate.” In the Ontario study, OI and IUI were combined into one exposure category. Across 2006–2021, twin pregnancies occurred in 9.4% of pregnancies after OI/IUI. In the later 2016–2021 era, the twin-pregnancy rate for this group was 8.3%.

Because OI and IUI were grouped, these figures cannot tell us that IUI alone produces an 8% or 9% twin rate. The ovarian-stimulation protocol and ovarian response are central to the multiple-pregnancy risk.

An especially useful finding from the Ontario study is that in 2016–2021 the observed twin-pregnancy rate was 8.3% after OI/IUI and 6.8% after IVF. In that population and treatment era, IVF was therefore not automatically the treatment category with the higher twin rate. Modern single embryo transfer can substantially change the comparison.

Why Ovarian Stimulation Matters More Than IUI Alone

Ovulation induction and ovarian stimulation can encourage follicular development. If more than one mature oocyte is released and more than one is fertilized, a dizygotic twin pregnancy can occur.

ASRM identifies multiple follicular development as the dominant risk factor for dizygotic twins and higher-order multiple gestation in OI and OS cycles. The relevant biological pathway is therefore not simply “IUI causes twins.” It is more accurately described as:

ovarian stimulation → multiple developing follicles → more than one ovulated egg → possibility of more than one fertilization → fraternal twins or higher-order multiples.

Follicle Count and Twin Risk

A follicle is the ovarian structure in which an egg develops. During a stimulated cycle, clinicians monitor ovarian response because the number of developing follicles helps indicate how many eggs may potentially ovulate.

As the number of mature follicles increases, there are more opportunities for separate eggs to be fertilized. That is why two people undergoing what may broadly be described as “IUI treatment” can have very different multiple-pregnancy risks depending on the medication used and their ovarian response.

Follicle count is only one part of clinical decision-making, and population studies cannot be converted into a personalized risk from follicle number alone. Individual treatment decisions should be interpreted by the treating fertility team rather than from a population table.

Twin Rates With Fertility Drugs

Fertility medications do not all have the same multiple-pregnancy profile. Some oral agents tend to produce a more limited ovarian response, while injectable gonadotropins can produce multifollicular development more often.

One of the most useful direct comparisons comes from the NIH-funded AMIGOS randomized trial, which enrolled 900 couples with unexplained infertility. Participants underwent ovarian stimulation with gonadotropins, clomiphene, or letrozole, followed by IUI. The percentages below describe multiple gestation among ongoing pregnancies with fetal heart activity in that specific trial.

These figures should not be treated as universal medication-specific probabilities. They come from a defined trial population, treatment protocol, and pregnancy denominator.

Clomiphene / Clomid

Clomiphene citrate, commonly known by the former brand name Clomid, is an oral medication used to stimulate ovulation. NICHD patient guidance summarizes the associated multiple-pregnancy risk as about a 10% chance of twins, with triplets or higher-order pregnancies occurring in less than 1% of cases.

The AMIGOS trial produced a similar but more narrowly defined result: 8 of 85 ongoing clomiphene pregnancies were multiple gestations, or about 9%, and all were twins.

The two numbers are useful for context but are not interchangeable. One is broad patient guidance; the other comes from a specific randomized trial of ovarian stimulation with IUI.

Letrozole

Letrozole is an aromatase inhibitor that can also be used for ovulation induction or ovarian stimulation. In the AMIGOS trial, 9 of 67 ongoing pregnancies in the letrozole group were multiple gestations, or 13%, and all were twins.

The letrozole multiple-gestation rate was significantly lower than the rate in the gonadotropin group but was not significantly different from the clomiphene group. The trial also found lower live-birth rates with letrozole than with gonadotropins, illustrating why multiple-pregnancy statistics should not be interpreted separately from the broader treatment context.

Gonadotropins

Gonadotropins are injectable hormones that stimulate follicular development. Because they can produce a stronger ovarian response and development of multiple follicles, their multiple-pregnancy profile can differ substantially from that of oral agents.

In the AMIGOS trial, 34 of 107 ongoing pregnancies after gonadotropin stimulation were multiple gestations, or 32%. Unlike the clomiphene and letrozole groups, which had only twin multiples, the gonadotropin group included both twins and triplets.

NICHD provides similar broader guidance, stating that about 30% of pregnancies conceived with gonadotropin treatment are multiple births, with about two-thirds of those multiples being twins and the remainder triplets or higher-order multiples.

This is why a statement such as “gonadotropins have a 30% twin rate” would be inaccurate. The cited figure refers to all multiple pregnancies or births, not twins alone.

Why Fertility Treatment Can Result in Twins

Fertility treatment can lead to twins through more than one pathway. The two major routes are developing and fertilizing more than one egg, or transferring more than one embryo during IVF. A third pathway, embryo splitting, explains why identical twins can still occur after a single embryo is transferred.

Multiple Eggs and Fraternal Twins

Ovulation-induction and ovarian-stimulation medications can cause more than one follicle to mature. If multiple oocytes are released and two are fertilized, two genetically distinct embryos can develop.

This produces fraternal twins, medically called dizygotic twins. ASRM identifies multiple follicular development as the primary treatment-related mechanism behind dizygotic twinning in OI and OS cycles.

Multiple Embryos in IVF

IVF creates another pathway because embryos are transferred after fertilization. When two or more embryos are transferred and more than one implants, a dizygotic twin or higher-order pregnancy can result.

This mechanism explains much of IVF’s historical association with twins. As embryo-transfer practice shifted toward SET and eSET, that pathway became substantially less common in many fertility systems.

One Embryo Can Still Produce Identical Twins

Single embryo transfer does not make the possibility of twins exactly zero. One embryo can divide after fertilization and produce identical, or monozygotic, twins.

ASRM notes that ART may be associated with an increased risk of monozygotic twinning and describes the residual multiple-gestation risk after SET as roughly the 1%–2% background risk of monozygotic twins in ART. That is very different from transferring two embryos: SET removes the multiple-embryo pathway, but it cannot prevent a single embryo from splitting.

How to Interpret Fertility-Treatment Twin Statistics

The hardest part of fertility-treatment twin data is often not finding a percentage. It is determining what that percentage actually measures.

Before comparing two published rates, ask five questions:

1. What outcome is being counted? Twins only, or twins plus higher-order multiples?
2. What treatment is being measured? IVF, IUI, ovulation induction, or a particular medication?
3. What is the denominator? All pregnancies, ongoing pregnancies, live births, treatment cycles, or embryo transfers?
4. When were the treatments performed? Older IVF data may reflect very different embryo-transfer practices.
5. Where and in whom was the study performed? Countries, clinics, patient populations, treatment protocols, and policies differ.

Twin Pregnancy Rate vs Twin Birth Rate

The distinction is not technical trivia. It is the reason two reputable sources can report very different percentages without contradicting each other.

Twins vs All Multiple Pregnancies

A multiple pregnancy includes twins, triplets, quadruplets, and higher-order pregnancies. A twin pregnancy includes exactly two fetuses under the study definition.

This matters especially for ovarian-stimulation studies. In the AMIGOS trial, the 9% clomiphene and 13% letrozole multiple-gestation rates consisted entirely of twins. The 32% gonadotropin multiple-gestation rate did not: it included 24 twin gestations and 10 triplet gestations.

Calling all three values “twin rates” would therefore create a misleading comparison.

Why Year and Geography Matter

Fertility treatment changes over time. Embryo-transfer policies, laboratory practice, medication protocols, patient selection, funding systems, and professional guidelines can all affect observed outcomes.

The difference between historical and modern IVF illustrates this clearly. Ontario IVF twin pregnancies fell from 27.6% in the 2006–2011 era to 6.8% in 2016–2021. UK IVF multiple births fell from 14.4% in 2014 to 3.2% in 2024. US single embryo transfer rose from 30.8% of transfers in 2013 to 85.9% in 2022.

These trends are related, but the percentages themselves should still not be compared as though they measure an identical endpoint.

Why Two Studies Can Report Different Rates

Suppose one source reports a 3% rate and another reports 7%. Before concluding that one must be wrong, check whether the first is measuring multiple live births in a recent national registry while the second is measuring twin pregnancies in an earlier treatment cohort.

Differences can arise from:

• pregnancy outcomes versus live-birth outcomes;
• twins alone versus all multiples;
• single versus multiple embryo transfer;
• stimulated versus unstimulated treatment;
• maternal age and ovarian response;
• fresh versus frozen treatment populations;
• clinical policies and treatment era;
• national versus clinic-level data; and
• different study designs and inclusion criteria.

A useful fertility-treatment statistic therefore needs context, not just a percentage.

What These Statistics Can and Cannot Tell You

The rates on this page describe groups of patients and treatment outcomes. They are not personalized predictions.

Individual twin probability can vary with the type of treatment, maternal age, ovarian response, number of developing follicles, medication protocol, number of embryos transferred, embryo-transfer strategy, and other clinical factors. The populations undergoing different treatments also differ. For example, patients in the Ontario IVF and OI/IUI groups were, on average, older than those who conceived without assistance.

Maternal age also matters independently of treatment. Naturally occurring dizygotic twinning varies with age and other population characteristics, while fertility-treatment protocols and ovarian response can modify risk further.

These statistics are most useful for understanding population patterns and treatment differences. They should not be interpreted as a person’s exact chance of twins or used to make treatment decisions without individualized clinical context.

Data Sources and Methodology

This page prioritizes official fertility registries, government health agencies, professional medical organizations, and peer-reviewed primary research. Statistics are presented with their outcome definition, study period, and population wherever those details are necessary for interpretation.

We follow several rules when comparing fertility-treatment twin data:

• A multiple-pregnancy or multiple-birth rate is not relabeled as a twin rate unless the underlying data show that all multiples were twins.
• Pregnancy-based outcomes are kept separate from birth-based outcomes.
• Historical IVF data are labeled by treatment era rather than presented as current universal risk.
• Country and study population are shown when they materially affect interpretation.
• Drug-specific trial results are not generalized to every treatment protocol or patient population.
• Preliminary registry data are identified as preliminary when the source does so.
• Population statistics are not presented as personalized probabilities.

Important HFEA methodology note: HFEA defines multiple pregnancy rate as the number of multiple pregnancies divided by all pregnancies, and multiple birth rate as the number of live multiple births divided by all live births. Its 2024 multiple-birth data are preliminary and may change after validation.

Primary sources used for this page:

• Human Fertilisation and Embryology Authority: Fertility Treatment 2024, Trends and Figures
• HFEA: Fertility Treatment 2024 Quality and Methodology Report
• JAMA Network Open: Multifetal Pregnancy After Implementation of a Publicly Funded Fertility Program
• American Society for Reproductive Medicine: Multiple Gestation Associated With Infertility Therapy
• NICHD: Fertility Treatments for Females
• New England Journal of Medicine: Letrozole, Gonadotropin, or Clomiphene for Unexplained Infertility
• New England Journal of Medicine: Fertility Treatments and Multiple Births in the United States
• CDC: National Assisted Reproductive Technology Summary

Frequently Asked Questions

Can IVF still result in twins after a single embryo transfer?

Yes. Single embryo transfer removes the possibility that two separately transferred embryos will both implant, but one embryo can still divide and form monozygotic, or identical, twins. ASRM describes the residual multiple-gestation risk after SET as roughly the 1%–2% background monozygotic-twin risk seen in ART. SET therefore greatly reduces twin risk but does not reduce it to zero.

Are fertility-treatment twins usually identical or fraternal?

Both are possible, and the mechanism depends on the treatment. Ovulation induction or ovarian stimulation can produce more than one egg, which can lead to dizygotic, or fraternal, twins. Transferring more than one IVF embryo can also result in dizygotic twins if two embryos implant. Monozygotic, or identical, twins result when one embryo splits, which can occur even after a single embryo transfer. ART has also been associated with a modestly increased monozygotic-twinning risk.

Does IUI itself cause twins?

IUI should not be treated as though the insemination procedure by itself creates the entire twin risk. The important factor is often whether IUI is combined with ovulation induction or ovarian stimulation and how many follicles develop. This is also why studies that combine OI and IUI into one category cannot provide a clean twin rate for unstimulated IUI alone.

Why do different sources report different IVF twin rates?

Different sources may be measuring different treatment eras, countries, embryo-transfer practices, age groups, pregnancy outcomes, or birth outcomes. An older IVF twin-pregnancy rate should not be compared directly with a recent multiple-birth rate. Modern use of single embryo transfer is one major reason newer IVF statistics can be substantially lower than historical figures.

Does maternal age affect fertility-treatment twin rates?

Maternal age can affect the baseline likelihood of dizygotic twinning and is also related to fertility-treatment selection, ovarian response, and embryo-transfer decisions. Strong studies therefore often adjust for age or stratify results by age. A population-level treatment rate should not be converted into a personalized estimate without considering those differences.