What Fertility Drugs Increase Chances of Twins?

The fertility drugs most commonly linked to twin pregnancies are clomiphene citrate, letrozole, and injectable gonadotropins, though each carries a different level of risk. Clomiphene, arguably the best-known ovulation-inducing pill, roughly quadruples the odds of a multiple pregnancy compared to unassisted conception. Gonadotropins push the risk even higher, particularly when several follicles mature at once. The relationship between these medications and twins is more layered than most people realize, shaped not just by the drug itself but by dose, monitoring, and even the type of assisted reproduction procedure used alongside it.

Clomiphene Citrate

Clomiphene citrate, sold under brand names like Clomid and Serophene, has been the first-line ovulation drug for decades. It works by blocking estrogen receptors in the brain, which tricks the pituitary gland into releasing more follicle-stimulating hormone (FSH). The extra FSH can coax the ovaries into maturing more than one egg in a single cycle, and when two or more eggs are released and fertilized, fraternal twins result.

A large nationwide cohort study found that the multiple pregnancy rate for clomiphene-exposed pregnancies was about 5.2%, compared to 1.4% in unexposed pregnancies. That translates to roughly a fourfold increase in odds. The vast majority of those multiples were twins, with triplet-or-higher pregnancies occurring in about 0.13% of clomiphene cycles versus 0.03% without the drug.

To put that in everyday terms, if you take clomiphene, you have roughly a 1-in-20 chance of conceiving twins. That is far higher than the natural rate for fraternal twins, which sits between about 1% and 4% depending on ethnicity and other factors.

Clomiphene is typically prescribed for five days early in the menstrual cycle. Many doctors start at a low dose and increase only if ovulation does not happen, partly because higher doses tend to recruit more follicles and push up the twin risk. Even at standard doses, though, the ovaries can overrespond, particularly in younger women or women with polycystic ovary syndrome (PCOS) who already have a high reserve of developing follicles.

Letrozole

Letrozole, originally developed as a breast cancer drug, has become a popular alternative to clomiphene for ovulation induction. It works differently: instead of blocking estrogen receptors, it temporarily reduces estrogen production itself, which similarly prompts the brain to ramp up FSH. The ovaries respond, but letrozole’s effect tends to wear off more quickly, which often leads to a single dominant follicle rather than the two or three that clomiphene sometimes produces.

In a head-to-head trial comparing letrozole, clomiphene, and gonadotropins in women with unexplained infertility, about 9% of ongoing letrozole pregnancies were twins. That was statistically similar to the 9% rate seen with clomiphene in the same trial, but well below the 32% rate seen with gonadotropins alone.

Reviews of the broader evidence suggest that letrozole is associated with higher ovulation and live birth rates than clomiphene, while carrying a lower risk of multiple gestation and promoting a more favorable environment for implantation.

⁴ That combination of better pregnancy rates with fewer twins is a big reason letrozole has moved to the front of the line for PCOS patients in many fertility clinics. The twin risk is not zero, but it is meaningfully lower than what clomiphene or gonadotropins carry.

Injectable Gonadotropins

When oral medications do not produce a pregnancy after several cycles, the next step is often injectable gonadotropins. These are purified or synthetic versions of the hormones FSH and LH that directly stimulate the ovaries, bypassing the brain entirely. Brand names include Gonal-F, Follistim, and Menopur. Because they act on the ovaries with much more force than oral drugs, the risk of developing multiple mature follicles is substantially higher.

In the same trial that compared letrozole and clomiphene, gonadotropin treatment resulted in a 32% multiple gestation rate among ongoing pregnancies. Unlike the oral-drug groups, where all multiples were twins, gonadotropin treatment also produced ten triplet gestations on top of twenty-four twin pregnancies.

The introduction of gonadotropins more than half a century ago was a landmark in infertility medicine, but the surge in high-order multiples it caused was recognized almost immediately.

⁵ That concern has driven decades of effort to lower the twin and triplet rate, primarily through more careful monitoring and cancellation policies when too many follicles develop.

Why Follicle Count Matters More Than the Drug Name

Regardless of which drug is used, the single biggest predictor of a multiple pregnancy is how many mature follicles the ovaries produce in a given cycle. Ultrasound monitoring during a stimulated cycle lets the doctor count these follicles before insemination or timed intercourse. A large study of intrauterine insemination (IUI) cycles with ovarian stimulation showed the relationship starkly in women under 38.

With one mature follicle, the twin-or-higher rate per pregnancy was about 4%. With five mature follicles, over a quarter of all pregnancies were multiples, including a triplet-or-higher rate that jumped from 0.2% to nearly 11%.

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Interestingly, the gain in singleton pregnancies was marginal as follicle number climbed. Going from one follicle to five only nudged the singleton rate from about 14% to 16%, while the multiple rate exploded. That finding underlines something important for anyone weighing the risks: extra follicles mostly increase the chance of twins and triplets rather than substantially improving the chance of a single baby. Many clinics now cancel or convert a cycle to IVF when monitoring reveals more than two or three mature follicles, precisely to avoid high-order multiples.

IVF and Embryo Transfer

In vitro fertilization deserves its own discussion because the twin risk with IVF is driven less by the drugs used and more by how many embryos are transferred to the uterus. When two embryos are placed, the chance of twins rises considerably. Over the past two decades, the field has moved aggressively toward elective single embryo transfer (eSET), which has dramatically reduced twin rates in IVF cycles.

A pilot trial comparing IVF with single embryo transfer to IUI with ovarian stimulation found that the IVF-eSET group had a 14% twin rate, while the IUI group had a 25% multiple rate that included a triplet pregnancy.

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That comparison surprises people who assume IVF always means twins. When a single embryo is transferred, the fraternal twin rate drops to essentially zero because there is no second embryo to implant. Identical twinning can still happen, but that is a separate phenomenon discussed below. The high twin rates people associate with IVF are largely a legacy of the era when clinics routinely transferred two, three, or even four embryos to improve success rates. Today, single embryo transfer is the norm in most high-income countries, and the IVF twin rate has fallen substantially as a result.

Fertility Drugs and Identical Twins

Most conversations about fertility drugs and twins focus on fraternal twins, the kind that result from two separate eggs. But there is a lesser-known phenomenon: fertility treatment also appears to increase the rate of identical (monozygotic) twins, where a single fertilized egg splits into two embryos.

Data from a large Australian population study showed an almost threefold increase in monozygotic twinning for births conceived through assisted reproductive technology compared to naturally conceived births. The risk was even higher following blastocyst-stage embryo transfer, where the embryo is cultured for five days before being placed in the uterus, compared to cleavage-stage transfer at three days.

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Clomiphene citrate specifically has been flagged as well. A population-based survey of multiple births in Belgium found that a greater proportion of monozygotic twins occurred with clomiphene as the sole treatment compared to other ovulation-inducing drugs.

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Why this happens is not fully settled. One theory is that the hormonal environment created by ovulation drugs or the laboratory culture conditions during IVF slightly weaken the zona pellucida, the outer shell of the embryo, making it more prone to splitting. Another idea is that the delay in implantation associated with some assisted reproduction protocols gives the embryo more time in a state where splitting is possible. Whatever the mechanism, it means that even transferring a single embryo does not completely eliminate the chance of twins, because that one embryo can still become two.

What About Metformin?

Metformin is an insulin-sensitizing medication widely used in women with PCOS to restore ovulation. Because it helps the body respond to insulin more effectively, it can allow the ovaries to resume normal cycling in women whose hormonal imbalance was suppressing ovulation. Some women wonder whether metformin alone raises the chance of twins.

The evidence suggests it does not, at least not meaningfully. Reports have noted increased ovulation and pregnancy rates in PCOS patients taking metformin, but without an accompanying increase in high-order pregnancies.

⁴ A case report of triplets after metformin exists, but isolated case reports do not establish a pattern. Metformin works by restoring a more normal hormonal environment, so it generally leads to the release of a single egg rather than the multi-egg response that clomiphene or gonadotropins provoke. When metformin is combined with clomiphene or letrozole, the twin risk is driven by the ovulation drug, not the metformin.

How Age and Body Weight Shift the Odds

Your baseline chance of conceiving twins before any medication enters the picture depends partly on your age and body mass. Older women naturally produce more FSH as ovarian reserve declines, which occasionally leads to the release of two eggs in a single cycle. Data from U.S. birth records shows that twins accounted for about 6.9% of neonates born to women aged 40 and over, compared to just 2.2% for women aged 18 to 24 and about 3.1% for women 25 to 29.

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Body weight plays a role too. A large study found that twin delivery rates climbed with increasing pre-pregnancy BMI. Women with the highest degree of obesity had a 41% higher rate of twin delivery compared to women with a normal BMI, even after adjusting for other factors. Part of that association was explained by the fact that women with higher BMI were also more likely to use assisted reproductive technology, but in the most obese group, the link between weight and twins persisted independently of fertility treatment.

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The practical takeaway is that when a 40-year-old woman with a higher BMI takes clomiphene, her twin risk is compounded by factors already pushing her toward multiple ovulation. Fertility specialists factor age and weight into their monitoring and dosing decisions for exactly this reason.

Health Risks Worth Knowing About

Twins conceived through fertility treatment are not just more common than spontaneous twins; they may carry slightly higher medical risks. A systematic review and meta-analysis comparing IVF twins to spontaneously conceived twins found that IVF twins had a roughly 23% higher risk of preterm birth and a 14% higher risk of low birth weight, even after accounting for maternal age and other variables.

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It is worth noting that all twin pregnancies carry elevated risk compared to singletons, regardless of how they were conceived. Twins are more likely to be born early and at lower weights, and the mother faces higher rates of complications like preeclampsia and gestational diabetes. But the added bump in risk for fertility-treatment twins is real and is one reason why the field has invested so much in reducing multiple pregnancies through single embryo transfer policies and cycle cancellation guidelines.

The Difference Between Wanting Twins and Getting Them

A lot of people searching for information about fertility drugs and twins are actually hoping for twins. The idea of two babies at once appeals to couples who have struggled with infertility and want to “catch up,” or who simply like the idea of siblings close in age. Fertility clinics regularly encounter patients who ask for higher drug doses or request transfer of two embryos specifically to increase their twin odds.

The medical community’s position has shifted firmly against this. Professional guidelines in reproductive medicine now emphasize that a singleton pregnancy is the safest outcome for both mother and babies, and that the goal of fertility treatment should be one healthy baby at a time. The push toward single embryo transfer in IVF and conservative follicle thresholds in stimulated IUI cycles reflects this philosophy. When a cycle produces four or five mature follicles, many clinics will cancel the cycle entirely rather than proceed, even when the patient is eager to go ahead.

That does not mean twins never happen. Even with careful monitoring and conservative protocols, oral drugs like clomiphene and letrozole can produce an extra follicle that goes undetected, or two eggs can release from one follicle. The twin risk is reducible but not eliminable when fertility drugs are involved, which is part of the informed consent conversation every patient should have before starting treatment.

How Monitoring Protocols Vary by Drug

The level of monitoring you receive during a fertility drug cycle depends heavily on which medication is prescribed and what procedure is planned. Clomiphene and letrozole cycles for timed intercourse are sometimes unmonitored, particularly in general gynecology practices rather than specialty fertility clinics. That means no ultrasound is done to count follicles before ovulation, and the patient simply takes the pills, uses an ovulation predictor kit, and proceeds. In these unmonitored cycles, neither the patient nor the doctor knows how many eggs are about to be released, which means the twin risk is less controlled.

In fertility clinics, oral drug cycles typically include at least one mid-cycle ultrasound to count mature follicles. If three or more are seen, the doctor may recommend converting to IVF, using barrier protection to avoid conception that cycle, or simply canceling. Gonadotropin cycles almost always include serial monitoring with both ultrasound and blood estrogen levels, because the risk of ovarian hyperstimulation and high-order multiples is too great to leave unmonitored. Even with close monitoring, though, a follicle can grow rapidly between appointments, and real-world twin rates remain stubbornly above zero.

If you are starting fertility treatment and your provider does not plan to monitor your cycle with ultrasound, it is reasonable to ask why, particularly if you are young, have PCOS, or are taking gonadotropins. Monitoring is the single most effective tool for managing multiple pregnancy risk alongside the choice of drug and dose.