Women in their mid-to-late thirties are, in fact, significantly more likely to conceive fraternal twins than younger women. The peak occurs around ages 35 to 39, driven by hormonal shifts that make releasing more than one egg per cycle increasingly common as ovarian reserve declines. But the relationship between maternal age and twinning is more nuanced than a simple upward trend, and it applies almost entirely to one type of twin.
Why Age Raises the Odds of Fraternal Twins
The connection between age and twinning centers on a hormone called follicle-stimulating hormone, or FSH. As a woman ages and her supply of eggs diminishes, the ovaries become less responsive to FSH. The brain compensates by producing more of it. Researchers have described this as an “overshoot” response: the pituitary gland, detecting weakening signals from the ovaries, ramps up FSH output beyond what is needed to mature a single egg. The result is that two or more follicles develop to the point of ovulation in the same cycle, setting the stage for fraternal twins.
A study examining this mechanism directly found that women who developed multiple large follicles were, on average, older (about 36 versus 35 years) and had higher baseline FSH levels (around 10 versus 8 IU/l) than women who developed just one follicle. The prevalence of this multi-follicle development increased with age, leading the researchers to conclude that fraternal twinning is tied to the more frequent occurrence of double ovulation in older women.1PubMed. The paradox of declining fertility but increasing twinning rates with advancing maternal age This creates an interesting paradox: fertility is declining overall as the egg supply drops, but the chance of conceiving twins is going up at the same time.
How the Numbers Change by Age
The increase is not subtle. A large study of U.S. births found that twins accounted for roughly 1.3% of neonates born to the youngest mothers (ages 15 to 17), rising to about 2.2% for women 18 to 24, 3.1% for women 25 to 29, 4.1% for women 30 to 34, 5.0% for women 35 to 39, and 6.9% for women 40 and older.2PubMed Central. The Role of Maternal Age in Twin Pregnancy Outcomes That means a woman over 40 is roughly five times as likely to have a twin birth as a teenager, and about twice as likely as a woman in her late twenties.
These numbers include all twin types and all conceptions, including those from fertility treatments, so they somewhat overstate the natural effect of age alone. Still, the age gradient has been documented consistently across populations. Studies of twinning rates in Japan and across developed countries have all found that the fraternal twinning rate peaks in the 35-to-39 age group before declining sharply.3PubMed. Twinning rates among women at the end of their reproductive span and their relation to age at menopause4PubMed. Analysis of multiple birth rates in Japan. 1. Secular trend, maternal age effect, and geographical variation in twinning rates That sharp drop after 39 reflects the fact that the overshoot mechanism eventually loses the race against overall fertility decline: FSH may still be elevated, but fewer and fewer viable eggs remain to respond to it.
Identical Twins Are a Different Story
Everything described so far applies to fraternal (dizygotic) twins, which come from two separately fertilized eggs. Identical (monozygotic) twins, which come from a single fertilized egg that splits, follow different rules entirely. A study using logistic regression to tease apart the effects of age and the number of previous pregnancies found a significant positive link between maternal age and fraternal twinning, but no association between maternal age and identical twinning.5PubMed. Maternal age and parity as predictors of human twinning The same study found an interesting reverse pattern for parity: having had more pregnancies was linked to slightly lower rates of identical twinning, but had no independent effect on fraternal twinning once age was accounted for.
So if someone tells you that twins “run in the family” and points to identical twins, age is probably not the explanation. The age effect is specific to the hormonal mechanism that leads to double ovulation, which can only produce fraternal twins.
Fertility Treatments Amplify the Age Effect
It is impossible to talk about twinning rates and maternal age without addressing assisted reproduction. Fertility treatments such as IVF, ovulation-stimulating drugs, and intrauterine insemination all increase the chance of multiples. Women who use these treatments also tend to be older, which compounds the natural age effect. When researchers in Norway tracked twinning rates from 1967 to 2004, they found a 50% increase in multiple pregnancies between 1988 and 2004. Strikingly, this increase persisted even after excluding twins conceived through assisted reproduction, suggesting that rising maternal age in the general population was independently pushing twinning rates up.6PubMed. Increasing twinning rates in Norway, 1967-2004: the influence of maternal age and assisted reproductive technology (ART)
A parallel finding from a broader time-trend analysis confirmed that both the natural fraternal twinning rate and the average maternal age at twin birth increased in a linear fashion from 1969 to 2009.7Human Reproduction. Time trends in the natural dizygotic twinning rate In other words, even setting IVF aside, the societal trend toward later childbearing has been reliably producing more twins for decades. Fertility treatments are a major driver of twin pregnancies, but they are layered on top of a real biological age effect, not creating it from scratch.
Other Factors That Influence Twinning
Age is probably the most talked-about factor, but it does not work in isolation. Several other traits have been linked to fraternal twinning rates.
- Body size: Women with a BMI of 30 or higher have significantly elevated odds of fraternal twins, and taller women also show a higher rate. The trend for increasing twinning with increasing BMI held up even after adjusting for race, age, parity, and height.8PubMed. Relationship of maternal body mass index and height to twinning
- Genetics: A genome-wide association study identified specific gene variants near FSHB (the gene encoding a subunit of FSH) and in SMAD3 that were significantly associated with fraternal twinning. The FSHB variant makes biological sense given the hormonal mechanism already described.9The American Journal of Human Genetics. Genome-wide Association Study Identifies Variants Associated with Dizygotic Twinning These variants replicated across multiple populations, adding confidence that the genetic component is real.
- Ethnicity and geography: Twinning rates vary widely across populations, with some West African populations having rates several times higher than East Asian populations. These differences likely reflect genetic variation in the same hormonal pathways.
The practical takeaway is that a tall, older woman with a family history of fraternal twins and a higher BMI stacks several independent risk factors. None of these factors guarantee twins, but they all nudge the probability upward through the same general pathway of increasing the likelihood that two eggs will be released and fertilized in the same cycle.
Does the Father’s Age Matter?
There is some evidence that paternal age has an independent, if smaller, effect. A study from the Jerusalem Perinatal Study found that fathers aged 35 and older had roughly double the rate of twin deliveries compared to fathers under 25. Adjusting for the mother’s age reduced this association but did not eliminate it: older fathers still had a significantly elevated twin delivery rate after the adjustment.10PubMed Central. Paternal age and twinning in the Jerusalem Perinatal Study The mechanism behind this is less clear. It may involve age-related changes in sperm that affect fertilization dynamics, or it may partly reflect assortative mating (older fathers tend to have older partners). Either way, the effect is smaller than the maternal age effect and is not well understood.
Health Risks When Age and Twins Coincide
Carrying twins is harder on the body than carrying a singleton at any age, and doing so in your late thirties or forties raises additional concerns. A study comparing twin and singleton pregnancies in women 45 and older found dramatically higher rates of complications in the twin group. Hypertensive complications occurred in about 41% of twin pregnancies versus 14% of singleton pregnancies, and preeclampsia rates were roughly 29% versus 12%. Delivery before 37 weeks happened in 56% of twin pregnancies compared to just 8% of singletons, and low birth weight was far more common among twins.11PubMed. Twin versus singleton pregnancy in women ≥ 45 years of age: comparison of maternal and neonatal outcomes
That said, the picture is not universally grim. A population-based study found that while twins born to older mothers were more likely to arrive late preterm (between 34 and 37 weeks), they were not at increased risk of very early preterm birth, perinatal death, or need for mechanical ventilation compared to twins of younger mothers.12Journal of Perinatology. Effect of older maternal age on birth outcomes in twin pregnancies: a population-based study Interestingly, in that same study, the risk of neonatal intensive care admission was elevated for older multiparous women (those who had given birth before) but not for older first-time mothers. A separate analysis of a large cohort found that the relationship between maternal age and some complications actually looked different in twin pregnancies than in singletons, with some complication rates not following the same upward age trajectory seen in singleton births.13PubMed. Maternal age and pregnancy outcomes in twin compared with singleton gestations
The lesson here is that twin pregnancies in older women deserve careful monitoring, but older mothers carrying twins should not assume the worst. The risks are real and elevated, but they are not uniform across all outcomes, and individual health status matters enormously.
An Evolutionary Explanation for Age-Related Twinning
The fact that twinning increases with age and then drops off has struck evolutionary biologists as a puzzle worth explaining. Twins are riskier for both mother and babies, so why would natural selection favor releasing two eggs as women age? One research team proposed that double ovulation is actually an adaptive insurance strategy. As eggs age, the chance that any single fertilized egg will fail early in development goes up. By releasing two eggs instead of one, an older woman improves the probability that at least one will result in a successful pregnancy. The “ideal” outcome from a fitness standpoint would be to always double ovulate but only carry one baby. Twinning, in this framework, is not something selected for directly. It is a by-product of selection for double ovulation as a hedge against rising rates of early embryo loss.14Nature Ecology & Evolution. An age-dependent ovulatory strategy explains the evolution of dizygotic twinning in humans
A complementary idea draws on the “terminal investment hypothesis” from life-history theory. Because menopause sets a hard endpoint on fertility, evolutionary theory predicts that women should invest more heavily in reproduction as they approach that endpoint, since there is less future reproduction to save resources for. Twin pregnancies in the late thirties could represent this kind of last-push strategy, where the body shifts toward maximizing the current reproductive attempt.15Human Reproduction. Why twin pregnancies are more successful at advanced than young maternal age? A potential role of ‘terminal reproductive investment’ These evolutionary models are speculative in the way that all adaptationist explanations are, but they offer a coherent story for why the hormonal changes of aging lean toward double ovulation rather than simply shutting down one follicle at a time.
Vanishing Twins and the Hidden Rate of Twinning
One thing worth knowing is that the twin conception rate is considerably higher than the twin birth rate, especially for older women. Vanishing twin syndrome, where one twin is lost spontaneously in the first trimester, has been reported in roughly 15 to 35% of twin pregnancies. Advanced maternal age is among the identified risk factors for this outcome.16PubMed Central. The vanishing twin: Diagnosis and implications With early ultrasound now routine, many women learn they were initially carrying twins before one embryo is reabsorbed. This means the biological tendency toward double ovulation in older women is even more pronounced than twinning statistics at birth would suggest. Some of those “singleton” pregnancies in older mothers started as twins.
The Oral Contraceptive Rebound
A lesser-known finding that sometimes surprises people involves what happens shortly after stopping birth control pills. An early study published in the New England Journal of Medicine found that women who conceived within a few months of stopping oral contraceptives they had used for more than six months had a twinning rate roughly double the expected value. The excess twins were predominantly fraternal, consistent with what you would expect from a temporary FSH rebound as the body readjusts to cycling naturally.17PubMed. Fetal loss, twinning and birth weight after oral-contraceptive use This finding has not been robustly replicated in modern large-scale studies, and today’s lower-dose pills may not produce the same effect, but it illustrates how any disruption that temporarily elevates FSH can tip the scales toward double ovulation. For an older woman stopping long-term contraception to try conceiving, the natural age-related FSH rise and any rebound effect could theoretically stack, though no study has directly tested that combination.
The rebound observation also reinforces the central theme of twinning biology: what drives fraternal twins is not some mysterious trait but a straightforward hormonal event. Anything that increases FSH at the right moment in the cycle, whether aging ovaries, genetic variants near the FSHB gene, or coming off hormonal suppression, makes double ovulation and therefore fraternal twins more likely.