Statistics
Last Updated
Jun 9, 2026
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Clomid, the brand name for clomiphene citrate, has been the world's default fertility drug since 1967. It is cheap, taken as a pill, and it works by a clever trick: it blocks estrogen receptors in the brain so the body thinks estrogen is low and pumps out the hormones that drive ovulation. The catch is that ovulating and having a baby are two very different numbers, and age moves them more than anything else. Here is the data, updated for 2026.
The headline numbers
Clomid in four figures.
How well it works
Ovulation is common, live births less so.
Clomid is good at its core job, triggering ovulation, but the gap between ovulating and taking home a baby is wide. Most suitable candidates ovulate, a much smaller share conceive over several cycles, and fewer still reach a live birth.
Approximate Clomid outcomes, drawn from different measures: ovulation among suitable candidates, cumulative conception across cycles, and live birth rate in a major head-to-head trial.
The variable that matters most
Success falls off a cliff with age.
In a study of more than 4,100 Clomid-with-insemination cycles, the per-cycle pregnancy rate dropped steadily with age and then collapsed after 42. The line tells the whole story: a woman under 35 had more than ten times the per-cycle odds of a woman past 42.
Pregnancy rate per completed cycle, clomiphene citrate with intrauterine insemination. Source: Dovey et al., Fertility and Sterility (4,199 cycles).
A woman under 35 had more than ten times the per-cycle odds of a woman past 42.
Across a full course of treatment
The cumulative odds, by age.
Stacking several cycles raises the odds, but the same age pattern holds. Across a full course of treatment, about a quarter of women under 35 conceived, versus fewer than 1 in 50 past 42. That cliff is why most clinics steer women over 40 toward IVF rather than more Clomid.
Cumulative pregnancy rate per patient treated, across all cycles. Source: Dovey et al., Fertility and Sterility.
Clomid vs letrozole
Which drug wins depends on the diagnosis.
Clomid's main rival is letrozole. The honest answer to which is better is that it depends on why a woman is not conceiving. For unexplained infertility, Clomid edged out letrozole on live births in a large trial. For PCOS, the most common reason it is prescribed, letrozole clearly won. That split is why many clinics now start PCOS patients on letrozole instead.
Live birth rate by drug and diagnosis. Sources: NIH multi-center trial (unexplained infertility) and a randomized PCOS trial. Clomid leads for unexplained infertility, letrozole for PCOS.
The multiples question
The twin rate is real, but smaller than its reputation.
Because Clomid can prompt the ovaries to release more than one egg, it raises the odds of twins, but less than its reputation suggests. Most Clomid pregnancies are still single births, the twins are almost always fraternal, and triplets or higher occur in under 1% of cases.
Twin pregnancy rate. Triplets and higher occur in under 1% of Clomid pregnancies. Sources: CCRM, CNY Fertility, Fertility and Sterility.
How it actually works
A six-decade-old trick on the brain.
Clomid belongs to the same chemical family as the breast-cancer drug tamoxifen, both are selective estrogen receptor modulators. In the brain, Clomid blocks the receptors that sense estrogen, so the hypothalamus reads estrogen as low and tells the pituitary to release more FSH and LH. Those are the signals that grow and release an egg. It is an elegant piece of biology, and it has barely changed since the drug was approved in 1967.
Two practical rules fall out of the data. Timing is finite: if Clomid is going to work it usually does so within the first three or four cycles, which is why treatment is capped around six. And it is increasingly one option among several rather than the automatic first step, with letrozole now preferred for PCOS and injectables or IVF recommended past 40.
Sources: Dovey et al., Fertility and Sterility (per-cycle and cumulative pregnancy rates by age, 4,199 cycles); CCRM Fertility and CNY Fertility (ovulation and twin rates); an NIH multi-center trial reported via CBS News and a randomized PCOS trial (Clomid vs letrozole live birth rates). Figures are rounded and vary by study, patient age, and diagnosis.
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