The latest U.S. data show a clear maternal-age pattern in twin births. In 2024, the twin birth rate ranged from 16.1 twin births per 1,000 total births among mothers younger than 20 to 35.6 per 1,000 among mothers age 40 and older. Mothers ages 35–39 had a nearly identical rate of 35.4 per 1,000.
These figures come from the National Center for Health Statistics (NCHS), using National Vital Statistics System (NVSS) birth data. The overall U.S. twin birth rate in 2024 was 30.1 per 1,000 total births.
Primary data source: National Center for Health Statistics, A Decade of Decline in Twin Childbearing in the United States, 2014–2024.
Quick answer: Twin birth rates generally rise across maternal-age groups, but the latest U.S. figures do not show a large additional increase after the late 30s. In 2024, the rate was 35.4 per 1,000 births for mothers ages 35–39 and 35.6 for mothers age 40 and older. These are population birth statistics, not an individual’s probability of conceiving twins.
Twin Birth Rates by Maternal Age in 2024
The table below shows the latest final age-specific twin birth rates reported by NCHS. The rate is expressed as twin births per 1,000 total births within each maternal-age group.
| Maternal age | Twin births per 1,000 total births |
|---|---|
| Younger than 20 | 16.1 |
| 20–24 | 22.7 |
| 25–29 | 28.9 |
| 30–34 | 33.2 |
| 35–39 | 35.4 |
| 40 and older | 35.6 |
Source: NCHS/NVSS, 2024 natality data. Rates are twin births per 1,000 total births.
Which Maternal Age Group Has the Highest Twin Birth Rate?
Among the age groups reported by NCHS, mothers age 40 and older had the highest numerical twin birth rate in 2024 at 35.6 per 1,000 total births. However, the rate for mothers ages 35–39 was 35.4, only 0.2 per 1,000 lower.
That distinction matters. Older material about twins can give the impression that twin rates continue rising sharply after age 40. The latest U.S. data show a much flatter pattern at the upper end of the maternal-age range.
How Much Do Twin Birth Rates Rise With Age?
In 2024, the observed twin birth rate among mothers age 40 and older was about 2.2 times the rate among mothers younger than 20: 35.6 compared with 16.1 per 1,000 births.
The increase is gradual across the middle age groups. The rate rises from 22.7 among ages 20–24 to 28.9 at ages 25–29, then to 33.2 at ages 30–34. The increase becomes much smaller after that, reaching 35.4 at ages 35–39 and 35.6 at age 40 and older.
This comparison describes twin births observed in different maternal-age populations. It should not be interpreted as saying that a particular woman age 40 or older has 2.2 times the personal probability of conceiving twins as a particular woman younger than 20.
How Twin Birth Rates Changed From 2014 to 2024
The age pattern becomes more informative when current rates are compared with 2014, the recent U.S. high point in twin childbearing. The overall twin birth rate declined from 33.9 per 1,000 births in 2014 to 30.1 in 2024, an 11% decline.
The decline was not distributed evenly across maternal-age groups.
| Maternal age | 2014 rate | 2024 rate | Absolute change | Approx. relative change |
|---|---|---|---|---|
| Younger than 20 | 16.0 | 16.1 | +0.1 | +0.6% (not statistically significant) |
| 20–24 | 23.0 | 22.7 | -0.3 | -1.3% (not statistically significant) |
| 25–29 | 30.5 | 28.9 | -1.6 | -5% |
| 30–34 | 40.3 | 33.2 | -7.1 | -18% |
| 35–39 | 48.6 | 35.4 | -13.2 | -27% |
| 40 and older | 66.0 | 35.6 | -30.4 | -46% |
Source rates: NCHS/NVSS. Absolute changes and approximate percentage changes shown here are calculated from the published rates. NCHS reports that changes for mothers younger than 20 and ages 20–24 were not statistically significant.
Key trend: The difference is most striking among older mothers. The rate for mothers age 40 and older fell from 66.0 per 1,000 births in 2014 to 35.6 in 2024, while the rate for mothers ages 35–39 fell from 48.6 to 35.4.
The Largest Declines Were Among Older Mothers
The strongest changes occurred among mothers age 30 and older. NCHS reported an 18% decline for ages 30–34, a 27% decline for ages 35–39, and a 46% decline for mothers age 40 and older.
The 40-and-older group therefore changed much more than the younger age groups. Its rate fell from 66.0 twin births per 1,000 total births in 2014 to 35.6 in 2024, a reduction of 30.4 per 1,000.
By comparison, NCHS found no statistically significant 2014–2024 change among mothers younger than 20 or ages 20–24.
What the 35–39 and 40+ Rates Show Today
In 2014, the twin birth rate was substantially higher among mothers age 40 and older than among mothers ages 35–39: 66.0 versus 48.6 per 1,000. By 2024, those rates had moved much closer together, at 35.6 and 35.4 respectively.
The data establish that convergence, but they do not by themselves establish a single cause for it. Changes in fertility-treatment practices, maternal characteristics, reproductive behavior, and other population-level factors may be relevant. The NCHS age data should therefore be used to describe the observed trend rather than to assign a cause the dataset does not isolate.
What a Twin Birth Rate Actually Measures
A twin birth rate is a population measure. In the NCHS report, it is expressed as twin births per 1,000 total births.
This distinction matters because a birth-based statistic answers a different question from a conception probability. It describes how frequently twin births appear among recorded births in a population. It does not measure the probability that a particular person will conceive twins during a menstrual cycle or pregnancy attempt.
Twin birth rate does not equal personal conception probability. For example, a reported rate of 35.6 per 1,000 births should not be interpreted as a 3.56% personal chance of conceiving twins. It is a population birth statistic with a different denominator.
How the Rate Is Calculated
At a simplified level, the reported measure can be understood as:
Twin births ÷ total births × 1,000
The denominator is total births, not menstrual cycles, conceptions, pregnancies attempted, or women trying to become pregnant. That difference is one reason population birth statistics cannot be directly converted into an individual’s conception odds.
Twin Birth Rate vs Personal Chance of Conceiving Twins
An individual’s probability of twins depends on more than maternal age. Relevant context may include whether conception is spontaneous or treatment-assisted, the type of fertility treatment used, reproductive history, genetics, and other factors.
The NCHS age-specific rates average across people with different characteristics and circumstances. They are therefore most useful for describing population patterns rather than estimating one person’s probability.
There is another important distinction: birth statistics describe pregnancies that reached delivery. A conception-based probability would begin earlier in the reproductive process and use a different denominator.
Why Maternal Age Is Associated With Fraternal Twinning
The maternal-age relationship is more clearly established for fraternal, or dizygotic, twins than for identical, or monozygotic, twins.
Fraternal twins develop when two separate ova are fertilized in the same pregnancy. Because this requires more than one egg to be available for fertilization, factors associated with multiple follicle development and multiple ovulation can influence dizygotic twinning.
Research has long identified higher maternal age as one factor associated with dizygotic twinning. The relationship should not be confused with overall fertility: fertility generally declines with advancing reproductive age even though multiple follicle development may become more common during part of that age range.
FSH, Multiple Follicle Development, and Multiple Ovulation
One proposed biological mechanism involves follicle-stimulating hormone, usually abbreviated FSH. FSH helps stimulate ovarian follicles during the reproductive cycle.
Research examining spontaneous ovulatory cycles found multifollicular development more frequently in older women and associated it with higher basal FSH concentrations. If more than one follicle produces an egg and multiple eggs are ovulated and fertilized, a dizygotic twin pregnancy can result.
This helps explain how advancing maternal age may be associated with fraternal twinning even while overall fertility potential declines. It does not mean an older woman will necessarily release multiple eggs in any given cycle.
Supporting evidence: Human Reproduction: The paradox of declining fertility but increasing twinning rates with advancing maternal age.
Why Identical Twins Are Different
| Fraternal twins | Identical twins |
|---|---|
| Also called dizygotic twins | Also called monozygotic twins |
| Begin with two separately fertilized ova | Begin when one early embryo divides |
| More clearly associated with maternal age and multiple ovulation | Natural rates are much less clearly associated with maternal age |
Because monozygotic twinning occurs through embryo splitting rather than fertilization of two separately ovulated eggs, the maternal-age mechanism that helps explain fraternal twins does not transfer directly to identical twins.
Assisted reproductive technology can add complexity to monozygotic twinning, so this comparison is most useful for understanding the underlying biological distinction rather than every treatment-specific scenario.
How Fertility Treatment Affects Interpretation of Age-Based Twin Rates
The national age-specific birth rates should not be interpreted as rates of spontaneous twinning alone. They describe twin births observed across the U.S. birth population rather than isolating only pregnancies conceived without fertility treatment.
Fertility treatment can affect the frequency of multiple gestation. Treatments involving ovarian stimulation can increase the possibility that more than one egg is available for fertilization, while transferring multiple embryos during in vitro fertilization can increase the chance of a multiple pregnancy.
Modern fertility practice has increasingly emphasized reducing treatment-associated multiple gestation. The American Society for Reproductive Medicine notes that limiting the number of embryos transferred, including use of single-embryo transfer where appropriate, is an important strategy for reducing ART-associated multiple gestation.
This does not establish fertility treatment as the sole cause of the 2014–2024 decline in every maternal-age group. The NCHS report documents the statistical changes but does not isolate one causal explanation.
Supporting evidence: American Society for Reproductive Medicine: Multiple gestation associated with infertility therapy.
Spontaneous vs Treatment-Associated Twinning
Spontaneous twinning refers to twin pregnancies occurring without fertility treatment intended to stimulate ovulation or assisted reproduction. Treatment-associated twinning may occur in connection with fertility therapies such as ovarian stimulation or assisted reproductive technology, including IVF.
When interpreting national age-specific statistics, those categories should not be silently treated as interchangeable. An observed population birth rate can include people with very different routes to conception and therefore does not represent a pure biological effect of maternal age.
Are Twin Birth Rates by Age the Same Everywhere?
No. The U.S. rates on this page should not be assumed to apply unchanged to other countries or populations.
Twinning rates vary geographically, and much of the variation in natural twinning rates is associated with differences in dizygotic twinning. Maternal-age patterns can still appear across populations even when their absolute rates differ.
For example, a Human Reproduction analysis covering 39 low-income countries found substantial differences in overall twinning rates between countries while also finding an association between maternal age at birth and the probability of twinning. In its modeled pattern, twinning probability increased with maternal age, reached a peak around age 39, and then declined slightly.
That international evidence provides useful context, but it should not replace the current U.S. NCHS figures. Population characteristics, fertility patterns, medically assisted reproduction, time period, and study methods can all affect comparisons.
Supporting evidence: Human Reproduction: More twins expected in low-income countries with later maternal ages at birth and population growth.
Data Source, Methodology, and Limitations
Primary Data Source
The main statistics on this page come from the National Center for Health Statistics report A Decade of Decline in Twin Childbearing in the United States, 2014–2024, published in June 2026.
The report is based on birth certificate data from the National Vital Statistics System. NCHS reports twin birth rates as twin births per 1,000 total births and compares maternal-age groups for 2014 and 2024.
For this report, NCHS defines a twin birth as a pregnancy in which two fetuses are delivered, whether alive or dead, regardless of gestational age; the two deliveries may also occur at different times during the pregnancy.
The historical comparison table on this page uses the published NCHS rates. Absolute differences and approximate percentage changes were calculated from those values. Where NCHS identifies an age-group change as not statistically significant, that qualification is retained.
Methodology source: NCHS Health E-Stat 117, Data Source and Methods.
What the Data Can Tell Us
The data are useful for answering population-level questions such as:
- How common were twin births within different maternal-age groups?
- Which age groups had the highest observed twin birth rates?
- How did age-specific rates change between 2014 and 2024?
- Which maternal-age groups experienced the largest declines?
- How does the current age pattern differ from statistics published a decade earlier?
What the Data Cannot Tell Us
The age-specific birth rates cannot, by themselves, determine:
- an individual’s probability of conceiving twins;
- the probability of twins in a particular menstrual cycle;
- a spontaneous-only twin conception rate;
- the probability associated with a particular fertility treatment;
- whether maternal age caused a particular twin pregnancy;
- the exact cause of the decline in twin rates within each age group.
The figures should therefore be interpreted as descriptive population statistics rather than personalized reproductive predictions.
Frequently Asked Questions
What is the twin birth rate for mothers ages 35–39?
In the United States in 2024, the twin birth rate for mothers ages 35–39 was 35.4 twin births per 1,000 total births. In 2014, it was 48.6 per 1,000, so NCHS reports a 27% decline over the decade.
What is the twin birth rate for mothers age 40 and older?
The 2024 rate was 35.6 twin births per 1,000 total births. This was the highest numerical maternal-age rate reported for 2024, but it was only slightly above the 35.4 rate among mothers ages 35–39.
Why do older sources show much higher twin rates after age 40?
Data year matters. In 2014, the U.S. rate for mothers age 40 and older was 66.0 per 1,000 births. By 2024 it had fallen to 35.6, a 46% decline. Older articles using earlier datasets can therefore quote substantially higher figures that no longer describe the latest U.S. pattern.
Does 35.6 twin births per 1,000 mean a 3.56% chance of conceiving twins?
No. It means the reported population had a twin birth rate of 35.6 per 1,000 total births under the NCHS measure. It is not a per-cycle conception probability, a spontaneous twin conception rate, or a personalized probability for an individual.
Does maternal age affect fraternal twins more than identical twins?
The age association is clearer for fraternal, or dizygotic, twinning. Dizygotic twins require two separately ovulated eggs, and maternal age has been associated with multiple follicle development and dizygotic twinning. Identical, or monozygotic, twins originate from division of a single embryo and do not show the same straightforward natural maternal-age pattern.
Do the U.S. age-specific twin rates represent spontaneous twins only?
No. The NCHS figures are population birth statistics and are not presented as spontaneous-only conception rates. Fertility treatment can influence multiple gestation, so the national rates should not be interpreted as the biological effect of maternal age in isolation.
Sources and Data Notes
- National Center for Health Statistics: A Decade of Decline in Twin Childbearing in the United States, 2014–2024. Primary source for the 2014 and 2024 U.S. twin birth rates, maternal-age comparisons, definitions, and methodology.
- Human Reproduction: The paradox of declining fertility but increasing twinning rates with advancing maternal age. Supporting research on maternal age, FSH, and multifollicular development in spontaneous ovulatory cycles.
- Twin Research and Human Genetics: Dizygotic twinning. Supporting review of factors associated with dizygotic twinning.
- Human Reproduction: More twins expected in low-income countries with later maternal ages at birth and population growth. International evidence on maternal age and variation in twinning rates.
- American Society for Reproductive Medicine: Multiple gestation associated with infertility therapy. Professional guidance on fertility treatment, ART, and treatment-associated multiple gestation.
Data reviewed: September 2026. The principal age-specific statistics are based on final 2024 U.S. birth data published by NCHS in June 2026.