Chance of Twins Calculator

Estimate your likelihood of twins based on age, family history (maternal side), height/BMI, ethnicity, prior pregnancies, fertility treatment, and past twins. Results show a baseline risk with a fraternal (dizygotic) vs identical (monozygotic) split, using population studies. This tool is informational and not medical advice.

Basics
Height and Weight
Units:
Fertility treatment (if any)
Methodology & Limits

What this estimates
The calculator combines (1) a natural-conception baseline split into dizygotic (DZ, fraternal) and monozygotic (MZ, identical) components, and (2) multiplicative modifiers for known DZ risk factors (maternal age, parity, maternal family history, height/BMI, ethnicity, prior twins). For treatment scenarios (ovulation induction, IUI with stimulation, IVF), it overlays typical contemporary twin rates from large trials/registries. Educational estimates only.

Baseline and zygosity
We set a constant MZ background of ~0.35–0.4% per pregnancy (≈3–4 per 1,000), which is relatively stable worldwide and largely independent of age or family history. DZ varies by population and biology and is modeled separately. Monden & Smits 2021, Human Reproduction.

DZ modifiers (natural conception)
A DZ baseline (~1.2%) is adjusted by literature-based factors: higher with advanced maternal age (peak early–mid 30s), higher parity, maternal family history of twins, taller stature, and higher BMI; population differences are incorporated (higher average DZ in many African populations, lower in East/South Asian populations). Key sources: review of DZ twinning determinants Hoekstra et al., 2008; anthropometry association (height/BMI) Reddy et al., 2005; global patterns (ethnicity/geography) Monden & Smits, 2021.

Prior twins and familial predisposition
Recurrence reflects an underlying hyper-ovulation tendency. Mothers who have had DZ twins (or with maternal-line DZ twins) show substantially higher subsequent DZ odds; genetic studies and family data support this (e.g., FSH-receptor pathways). We up-weight DZ when “prior twins = yes”. Mbarek/FSHR familial DZ twinning, The Lancet 2000; see also GWAS on spontaneous DZ twinning Mbarek et al., 2016.

Treatment scenarios
Ovulation induction (oral agents) / IUI with stimulation: Oral agents produce low but elevated twin rates; in unexplained infertility the AMIGOS RCT found multiple-gestation rates ≈9% with clomiphene, ≈13% with letrozole, and ≈32% with gonadotropins (all with IUI). The tool maps “OI/IUI” to this band and applies mild age adjustments. Sources: NEJM 2015 AMIGOS trial with protocol summary Yale synopsis.

IVF (embryo transfer policy): Elective single-embryo transfer (SET) yields very low twin rates, largely due to rare MZ splitting (~1–2%); double-embryo transfer (DET) drives much higher twin rates (often ~25–30% in younger patients). The calculator sets IVF-SET to a small MZ component and IVF-DET to a higher DZ-dominant twin probability with an age-sensitive range. Evidence: MZT after SET ≈1.5% in large registries Paul et al., 2025 (ANZ registry); UK HFEA reporting shows ~1–3% with SET and >30% twin rate with DET in under-35s HFEA 2019 report.

Computation details
Natural scenario: total twins = DZbase × (modifiers for age, parity, family history, height/BMI, ethnicity, prior twins) + MZbase. Treatment scenario: replace DZ with regimen-specific typical rates (IUI/OS, IVF-SET, IVF-DET) and keep an MZ floor appropriate to technique (slightly higher with blastocyst transfer). Results are clamped to plausible bounds and displayed as percentages with a small uncertainty band.

Limitations
Population averages cannot capture individual clinic protocols, dosing, embryo stage, PGT, or transfer policy; real-world risk can differ. Use for education, not clinical decision-making.

What’s next?

Twin & Triplet Due Date IVF/FET Due Date Track hCG Levels

Not pregnant yet? Try the Pregnancy Chance Calculator or see your daily chance calendar.

Key points

  • Fraternal (dizygotic) twins happen when two eggs are fertilized. Their odds vary with factors like maternal age, family history on the maternal side, prior pregnancies, height/BMI, ethnicity, and fertility treatment.
  • Identical (monozygotic) twins come from one embryo splitting. Rates are relatively stable across groups and are less influenced by age or family history.
  • Single-embryo transfer (SET) in IVF lowers twin odds; transferring multiple embryos increases twin odds.
  • Your result is a population-based estimate. Only ultrasound can confirm twins.

What affects the odds?

  • Maternal age: Twin rates (especially fraternal) tend to rise through the early–mid 30s.
  • Maternal family history: A mother with female relatives who had fraternal twins may have higher odds.
  • Parity/prior twins: Previous pregnancies—and especially previous fraternal twins—are linked with higher odds.
  • Body size: Taller body size and higher BMI have been associated with more fraternal twinning.
  • Ethnicity: Twin rates differ by population in published registries.
  • Fertility treatment: Ovulation-inducing meds and transferring multiple embryos can raise twin odds. Discuss embryo-transfer policies with your clinic.

Important: None of these factors “cause” twins on their own. They shift probabilities only.

FAQ

Do identical twins run in families?
Identical twinning is mostly random and not strongly tied to family history. Fraternal twins are the type more associated with maternal family history.
Does IVF always mean twins?
No. Many clinics recommend single-embryo transfer (SET). Twin likelihood depends on your clinic’s protocol and number of embryos transferred.
Can high hCG prove twins?
Higher hCG can occur with twins but is not diagnostic. Only ultrasound can confirm twins.
Do twins skip a generation?
That’s a myth. What matters most for fraternal twinning is ovulation of multiple eggs—which can run on the maternal side.