Chances of Having Twins Calculator
Use this chances of having twins calculator to estimate your odds of twins from your age, family history and any fertility treatment.
Updated October 22, 2026
Medical Disclaimer: This calculator provides a general estimate based on established research and is for educational purposes only, not a substitute for medical advice. Individual results can vary based on health conditions, so consult a qualified healthcare professional or registered dietitian before making significant changes to your diet, exercise, or health routine. Read our full Medical Disclaimer.
How Your Chances of Having Twins Are Calculated
Your chance of having twins is the share of pregnancies like yours that end in a twin birth. In the US in 2024, about 30 of every 1,000 babies born were twins¹. Since each twin pregnancy gives two babies, that is about 1 in 65 pregnancies. This calculator starts from the rate for your age group, then adjusts it for family history or replaces it with figures for fertility treatment.
What Your Result Measures
Your result is the estimated chance that a pregnancy ends in a twin birth, averaged over women with the same answers. For natural conception, the result is also split into identical twins (one fertilized egg that splits in two) and fraternal twins (two eggs, each fertilized by its own sperm).
The Formula
Chance per pregnancy (t) = twin birth rate per 1,000 babies / (2,000 - twin birth rate)
Identical twins = 0.4% (4 in 1,000 pregnancies, at any age)
Fraternal twins = (t - 0.4%) x 2 with a family history, or (t - 0.4%) without
Your chance = identical twins + fraternal twins
The identical twin rate stays close to 4 per 1,000 pregnancies around the world, while fraternal twinning changes with age, family and ancestry².
How This Calculator Computes Your Result
Enter your age (15 to 50) and whether you have a family history of fraternal twins. You can also choose a fertility treatment.
No treatment: the tool looks up the 2024 US twin birth rate for your age group¹ and converts it to a chance per pregnancy. It doubles the fraternal part if your mother, sister or daughter had fraternal twins, if your mother is a fraternal twin, or if you are one.
Ovulation pills or injections: the tool uses the published figure for that treatment instead, with no age or family adjustment³.
IVF: with one embryo it uses 3%³. With two embryos it uses a figure that falls with age, from about 40% at age 25 to 21% at 40⁴. Enter your age when the eggs were collected, or the egg donor's age.
You see your chance as a percent and as "about 1 in N", how it compares with the US average of 1.5%, and the identical and fraternal split.
What Your Result Means
Without treatment, your chance depends mostly on your age¹:
Your age | US twin birth rate per 1,000 babies (2024) | Chance per pregnancy | With a family history |
Under 20 | 16.1 | 0.8% (1 in 123) | 1.2% (1 in 82) |
20 to 24 | 22.7 | 1.1% (1 in 87) | 1.9% (1 in 53) |
25 to 29 | 28.9 | 1.5% (1 in 68) | 2.5% (1 in 39) |
30 to 34 | 33.2 | 1.7% (1 in 59) | 3.0% (1 in 34) |
35 to 39 | 35.4 | 1.8% (1 in 55) | 3.2% (1 in 31) |
40 and over | 35.6 | 1.8% (1 in 55) | 3.2% (1 in 31) |
Fertility treatment changes the chance far more than age does:
Treatment | Estimated chance of twins |
Ovulation pills (clomiphene or letrozole) | Up to about 8% (1 in 12) |
Ovulation injections (gonadotropins) | Up to about 30% (1 in 3) |
IVF with one embryo | About 3% (1 in 33) |
IVF with two embryos | About 21% to 40%, lower with age |
The pill, injection and one-embryo figures come from the American Society for Reproductive Medicine³, and the two-embryo range from a US prediction model⁴. A result near 1.5% is about the US average. Most natural twins are fraternal: about 27 in 100 sets of twins are identical at ages 25 to 29, and about 22 in 100 from age 35.
Why Age, Family and Treatment Matter
Fraternal twins happen when the ovaries release two eggs in the same cycle. As women age, rising levels of FSH (the hormone that tells the ovaries to ripen eggs) are thought to make this more likely, and the fraternal twinning rate rises about fourfold from age 15 to 35². The tendency also runs through the mother's family. In two studies, women with a sister who had fraternal twins had 1.7 to 2.5 times the usual rate, and the mothers and daughters of women with fraternal twins had about twice the rate². A father can pass the tendency on to his daughters, but his family history does not change the chance for his partner's pregnancy².
Ovulation drugs can ripen more than one egg. IVF twin rates depend mostly on how many embryos are placed. In 2022, about 92 in 100 first IVF transfers for US patients under 35 used one embryo by choice, and twins made up 3.3% of those births⁵. With one embryo, twins come from that embryo splitting, so they are identical.
Other Factors Not in the Calculator
These are linked to fraternal twins, but the evidence gives no figures we could add reliably²:
Previous births: more previous pregnancies mean more twins, even after allowing for age, but the size of the effect is unclear.
Height and weight: in one study, tall women had 1.5 to 2 times the twin rate of short women. A BMI of 30 or more has also been linked with more twins.
Ancestry: twinning is most common in sub-Saharan Africa (about 23 per 1,000 pregnancies) and least common in Asia (5 to 6 per 1,000). In the US in 2024, the twin birth rate was 42.0 per 1,000 babies for non-Hispanic Black mothers, 31.3 for non-Hispanic white mothers and 23.4 for Hispanic mothers¹.
Chance of Twins by Age Without Treatment
The chance per pregnancy of having twins for each age group, with and without a family history of fraternal twins.
- No family history
- With a family history
Show data table
| Your age | No family history (%) | With a family history (%) |
|---|---|---|
| Under 20 | 0.8 | 1.2 |
| 20 to 24 | 1.1 | 1.9 |
| 25 to 29 | 1.5 | 2.5 |
| 30 to 34 | 1.7 | 3.0 |
| 35 to 39 | 1.8 | 3.2 |
| 40 and over | 1.8 | 3.2 |
Chance of Twins With Fertility Treatment
The estimated chance of twins with each treatment. Without treatment, the US average is about 1.5%.
Show data table
| Treatment | Estimated chance of twins (%) |
|---|---|
| IVF with one embryo | 3 |
| Ovulation pills, up to | 8 |
| IVF with two embryos, age 40 | 21 |
| Ovulation injections, up to | 30 |
| IVF with two embryos, age 25 | 40 |
A Note on Accuracy
This is an approximation that combines figures from different studies and assumes their effects multiply. The US age rates include births after fertility treatment, so for natural conception the true figure, mainly from age 35, is likely a little lower. Treatment results vary by clinic, dose and embryo. Only an ultrasound can tell you whether you are having twins.
Frequently Asked Questions
What are the chances of having twins naturally?
In the US, about 1 in 65 pregnancies is a twin pregnancy, based on 30.1 twin babies per 1,000 births in 2024¹. The chance rises from about 1 in 123 under age 20 to about 1 in 55 from age 35.
Do twins run in the family?
Fraternal twins can run in the mother's family, and women whose mother or sister had fraternal twins have roughly twice the usual chance². Identical twins happen at about the same rate in all families.
Does IVF increase the chance of twins?
Mostly when two embryos are transferred, which leads to twins in about 21 to 40 of every 100 births, depending on age⁴. With one embryo, the chance is about 3%³.
How accurate is a twins calculator?
It gives an average for people with your answers, not a prediction for your pregnancy, so treat the result as a rough guide.
Sources
A Decade of Decline in Twin Childbearing in the United States, 2014 to 2024, Martin and Osterman (CDC, NCHS), 2026
Dizygotic Twinning, Hoekstra et al., 2008
Fertility Drugs and the Risk of Multiple Births, American Society for Reproductive Medicine, 2023
Application of a Validated Prediction Model for In Vitro Fertilization, Luke et al., 2015
Final National Summary Report for 2022, Society for Assisted Reproductive Technology