Twin probability factors — what science tells us
Eight evidence-based factors, each with a deep-dive page citing the studies behind the number.
Maternal age
Twin rates rise sharply after 30 and peak between 35 and 39, driven by higher FSH levels.
Read the deep diveIVF & fertility treatments
IVF, IUI and ovulation induction multiply baseline twin probability — driven by multi-embryo transfer and superovulation.
Read the deep diveFamily history
A maternal-side history of dizygotic twins predicts hyperovulation more strongly than a paternal history.
Read the deep diveEthnicity
Twin rates differ by population — highest in West Africa, lowest in East Asia, with Europe in the middle.
Read the deep diveHeight & BMI
Taller women and women with BMI ≥ 30 show modest but consistent increases in dizygotic twinning, likely IGF-mediated.
Read the deep diveContraception history
A short FSH rebound in the first cycles after stopping the pill can briefly raise twin probability.
Read the deep diveDiet & nutrition
Folic acid supplementation and dairy consumption have been linked to slightly higher twin rates in observational studies.
Read the deep divePregnancy number & breastfeeding
Multiparous women and those breastfeeding when ovulation resumes show small but consistent increases in twin probability.
Read the deep diveTry the calculator
Eight factors, 90 seconds, your personalised probability with the evidence behind every number.
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