A few things genuinely raise the chance of a twin pregnancy — family history on the mother's side, age over 30, having been pregnant before, fertility treatment, and even nutrition and height. None of these make twins certain; they only raise the chance, never a guarantee either way.
What raises the chance of fraternal (non-identical) twins
Fraternal twins happen when two separate eggs are released and both get fertilised — so anything that makes a woman more likely to release more than one egg in a cycle raises the chance:
- Family history on the mother's side — a tendency to release more than one egg (hyperovulation) can run in families, passed down through the mother's line
- Age over 30 — follicle-stimulating hormone (FSH) tends to rise with age, which can occasionally trigger more than one egg to mature and release in the same cycle
- Having been pregnant before — women who have already had one or more pregnancies have a somewhat higher chance of a twin pregnancy in a later one
- Fertility treatment — ovarian stimulation medicines are designed to help follicles grow, and can result in more than one mature egg releasing in a cycle, which is one reason fertility treatment carries a higher chance of twins than natural conception
- Nutrition and height — population studies associate taller women and certain diet patterns with a slightly higher chance of hyperovulation, though this is a minor factor next to family history, age and fertility treatment
- Ethnicity — the baseline chance of a natural twin pregnancy varies somewhat between ethnic groups, independent of the other factors above
- Conceiving while breastfeeding — rare, but possible, and occasionally associated with twin pregnancies
Identical twins are a different story
Identical twins happen when a single fertilised egg splits into two early on — and this split is essentially random chance. None of the factors above are known to raise the chance of identical twins specifically; they only apply to the fraternal kind, where two separate eggs are involved.
These factors raise the chance of a twin pregnancy — they never make it a certainty, in either direction.
Frequently asked
Does having twins run in families?
A tendency toward fraternal twins can run in families, but specifically through the mother's side — it relates to a tendency to release more than one egg in a cycle. A family history of twins on the father's side doesn't carry the same effect, since it's the mother's ovulation pattern that determines whether two eggs are released.
Does fertility treatment make twins more likely?
Yes. Ovarian stimulation medicines used in IUI and IVF are designed to encourage follicle growth, and this can result in more than one mature egg releasing in a cycle — one reason fertility treatment carries a higher chance of a twin pregnancy than conceiving naturally. Your doctor monitors follicle growth through the cycle specifically to manage this, including through follicular monitoring and decisions around how many embryos to transfer.
Can anything increase the chance of identical twins?
Not that's established — identical twins come from a single fertilised egg splitting into two, which is understood to happen by chance rather than being influenced by family history, age, or any of the factors that raise the chance of fraternal twins.
If I'm over 30, does that mean I'm likely to have twins?
No — age over 30 is one of several factors that can slightly raise the chance of releasing more than one egg in a cycle, but it's a modest effect on its own, not a strong predictor. Most pregnancies after 30, including first pregnancies, are still single births.