The asymmetry that confuses people
The folk saying “twins skip a generation” sits on a real biological asymmetry: only women ovulate, so a paternal family history of dizygotic twins can only be expressed through the daughters of the man in question. A man whose mother had fraternal twins might carry hyperovulation alleles, pass them to his daughters, and only then see them expressed.
The asymmetry is well documented. Maternal-line family history of dizygotic twins is associated with a roughly two- to three-fold increase in twin probability for the woman trying to conceive. Paternal-line history is associated with a smaller, indirect increase — typically estimated at around 1.2× [3][4].
For our calculator, we apply ×2.5 for maternal-line history and ×1.2 for paternal-line history. They can compound, so a woman whose mother and father both have twins in their families can sit at roughly 3× baseline before any other factor.
What is actually inherited
The heritable component is hyperovulation — releasing more than one egg per cycle. Two large genome-wide association studies, including a multi-cohort study by Mbarek and colleagues, identified variants near the FSHB and SMAD3 genes that explain a portion of the inter-individual variance in dizygotic twinning [2]. These variants modulate the FSH–follicle axis, which is the same biological lever pulled by maternal age and superovulation drugs.
Importantly, no equivalent heritable component exists for monozygotic (identical) twinning. The splitting of a fertilised egg into two embryos appears to be a stochastic event with no documented genetic predisposition. Globally, monozygotic twin rates sit close to 0.4% and barely vary across populations.
Reading your own family tree
When users complete the calculator, the most common confusion is which “side” to check. A simple rule:
- Are you trying to conceive yourself? Your mother’s relatives are your maternal line. Your father’s relatives are your paternal line.
- Twins on your father’s side? That is paternal — and it counts, just less.
- Twins on your husband’s mother’s side? That is not in our model. The hyperovulation gene is expressed in the woman who is ovulating. Your partner’s family history matters far less for twin probability than yours does.
This last point is counter-intuitive but important: the hyperovulation gene needs an ovary to act on. Your partner’s genome contributes to the embryo, but cannot make you release two eggs.
Practical takeaway
Family history is one of the few factors you cannot change but can quickly verify with a phone call. If you find a strong maternal-line history of dizygotic twins, treat it as a real signal in conjunction with age and other factors — and ask whether your fertility clinician adjusts protocols (e.g. embryo number) based on this information.