Natural twin rates start from baseline fraternal and identical probabilities. This calculator adjusts fraternal twin odds based on maternal age, prior births, family history (especially maternal side), body size indicators, and ethnicity. Identical twinning remains comparatively stable and mostly random.
Natural vs treatment mode
With no fertility treatment selected, the model uses natural-cycle adjustments. If you select OI/IUI or IVF, treatment-specific rates are used because assisted reproduction can meaningfully change twin probability.
Your result includes fraternal %, identical %, and combined twin probability. Treatment selections also show typical ranges to reflect real-world clinical variability.
Frequently Asked Questions
Common questions answered by our medical team
The tool starts from baseline rates and applies multipliers to fraternal (dizygotic) risk based on maternal factors and treatment. Identical (monozygotic) probability is kept relatively stable and low compared with fraternal changes.
