Twin probability factors — what science tells us
Eight sourced evidence topics, each stating whether it is weighted, excluded or requires protocol-specific data.
Maternal age
Spontaneous fraternal-twin rates generally rise with age, but model v2 uses a deliberately compressed weight.
Read the deep diveIVF & fertility treatments
Risk varies by treatment, stimulation, embryo-transfer count, age and clinic; model v2 applies no universal multiplier.
Read the deep diveFamily history
The clearest association concerns a first-degree female relative with spontaneous fraternal twins; the questionnaire uses one broad family-history proxy.
Read the deep divePopulation & ancestry context
Twin rates differ by population — highest in West Africa, lowest in East Asia, with Europe in the middle.
Read the deep diveHeight & BMI
Observational associations exist, but model v2 compresses both weights to reflect residual uncertainty.
Read the deep diveContraception history
Evidence is old and inconsistent, so recent contraception cessation was removed from model v2.
Read the deep diveDiet & nutrition
Dietary observations are vulnerable to confounding and are not used to calculate an individual estimate.
Read the deep divePrevious pregnancies
Parity receives a small, conservative weight; breastfeeding was removed because the evidence is insufficient.
Read the deep diveTry the calculator
Seven questions, an educational spontaneous-twinning estimate, and the limitations beside every number.
Run the full calculator