Chances of Having Twins Calculator

Twin birth rates by age

Twin rates rise with age into the late 30s and stay high after 40. One reason is that levels of follicle-stimulating hormone (FSH) increase as the ovaries age, making it more likely that two eggs are released in one cycle. Older parents are also more likely to have used fertility treatment, which these figures include. The overall US twin birth rate fell 11% between 2014 (33.9 per 1,000) and 2024 (30.1), largely as IVF clinics moved to transferring one embryo at a time.

Identical vs fraternal twins

Identical (monozygotic) twins

Identical twins form when one fertilised egg splits into two embryos. This appears to happen essentially at random: the rate is fairly constant worldwide, at about 4 per 1,000 births, and is not inherited in most families. Identical twins share the same DNA and are always the same sex.

Fraternal (dizygotic) twins

Fraternal twins come from two separate eggs fertilised by two sperm in the same cycle – genetically as alike as any siblings. Releasing two eggs (hyperovulation) is influenced by age, genetics, ancestry, body size and fertility drugs, so almost all of the variation in twin rates comes from fraternal twins. In the US and Europe they occur in about 8 per 1,000 naturally conceived births.

Fertility treatment and twin odds

Because twin pregnancies carry higher risks, professional guidance strongly favours elective single embryo transfer for most IVF patients and careful monitoring of stimulated cycles – cycles are often cancelled if too many follicles develop. If you are having treatment, ask your clinic for its own multiple-pregnancy rates, which can differ from national averages.

Other factors that affect fraternal twin odds

Without a validated formula that combines these factors, the calculator does not invent a personal percentage from them – it shows whether each one moves your odds up or down from the published baseline.

Twins around the world

A 2021 Oxford-led study of 165 countries found the global twinning rate had risen by a third since the 1980s, from 9 to 12 per 1,000 births – about 1.6 million twins a year, or 1 in every 42 babies – mainly because of fertility treatment and older parenthood. Africa has the highest rates, largely from naturally high fraternal twinning. In the US, CDC data show twin birth rates of 42.0 per 1,000 for Black non-Hispanic mothers, 31.3 for White non-Hispanic mothers and 23.4 for Hispanic mothers in 2024.

What a twin pregnancy means for care

Twin pregnancies are monitored more closely because they carry higher risks of preterm birth, high blood pressure, gestational diabetes and growth problems. Most twins are born before 37 weeks. If an early ultrasound shows twins, your maternity team will check whether they share a placenta (chorionicity), which decides how often you need scans.

How the calculator chooses your figure

1. If you conceived naturally, it shows the CDC 2024 US twin birth rate for your age group, and – for comparison – the average natural rate in the US and Europe of about 12 per 1,000 (8 fraternal + 4 identical).
2. If you had IVF, it replaces the age figure with the ASRM figure for the number of embryos transferred.
3. If you had ovarian stimulation with IUI, it shows the ASRM range by number of mature follicles.
4. It then lists the factors you ticked and whether each raises or lowers the odds – without inventing a combined percentage.

Types of twin pregnancy

Twins who share a placenta need more frequent scans because of complications such as twin-to-twin transfusion syndrome. Your team will tell you which type you have from an early ultrasound.

Identical twins after IVF

Even when only one embryo is transferred, a small share of IVF pregnancies are twins because the embryo splits. Studies suggest embryo splitting is somewhat more common after IVF than in natural conception, which is why the single-embryo figure is about 2% rather than zero.

Early signs and how twins are confirmed

Some people with twin pregnancies notice stronger nausea, more tiredness or a bump that measures larger than expected, and pregnancy hormone (hCG) levels are often higher on average. None of these signs is reliable on its own – plenty of single pregnancies have them too. An ultrasound scan, usually from about 6–8 weeks or at the routine dating scan, is the only way to confirm twins.

Sources

1. CDC NCHS Health E-Stat 117 – A Decade of Decline in Twin Childbearing in the United States, 2014–2024 — cdc.gov/nchs/data/hestat/hestat117.htm
2. ASRM Practice Committee – Multiple gestation associated with infertility therapy — asrm.org
3. Monden C., Pison G., Smits J. (2021) Twin Peaks: more twinning in humans than ever before. Human Reproduction – ox.ac.uk news 12 Mar 2021