What can run in families
The familial signal concerns spontaneous dizygotic twins, which result from two ova. Linkage and genome-wide studies support a heritable component related to multiple ovulation [1][2]. The 2010 family study notes that women with a first-degree female relative who had dizygotic twins have about twice the population rate [1].
Monozygotic twins result from one embryo splitting. Their rate is much more stable across populations, and TwinCalc does not apply a family-history weight to that 0.40% baseline component.
Why the questionnaire changed
Model v1 asked separately about maternal and paternal sides and applied ×2.5 and ×1.2. The evidence does not provide a sufficiently stable, independently quantified paternal-side coefficient for that design.
Model v2 asks one broader question: whether fraternal twins occur in the user’s biological family. A “yes” answer applies ×2.0 to the dizygotic component. This is still a heuristic proxy: it does not encode degree of relation, zygosity certainty, treatment use in the relative, or family size.
How to answer
Use spontaneous fraternal twins in the biological family as the relevant signal. If the twins were identical, treatment-related, or their type is unknown, the evidence is less directly applicable. A father can pass variants to a daughter, but that biological possibility is not the same as a validated paternal-side individual multiplier.
Family history can inform a conversation with a clinician. It is not a genetic test and does not determine an individual outcome.