Twin rates depend on age, family history, BMI, and whether you're using fertility assistance. Here's your number.
The US twin birth rate has hovered around 31–33 per 1,000 live births for the last decade (CDC National Vital Statistics — about a third of those are identical, two-thirds dizygotic). Twin probability rises with maternal age until about 40 because older ovaries sometimes release more than one egg per cycle.
Family history matters because the tendency to double-ovulate is heritable, and it's inherited through the mother's side. A father whose mother had twins doesn't pass it on. Painter et al. (2010, Nat Genet) mapped this to common variants near FSHB, which regulates follicle-stimulating hormone.
For IVF, modern clinics typically transfer one embryo at a time — so the eye-popping twin rates you may have heard about are largely from older protocols. SART's current recommendation is single-embryo transfer for most patients under 38.
Sources: CDC National Vital Statistics Reports (twin births); Painter JN et al. Nat Genet 42:543–4 (2010), "A genome-wide association study identifies a locus for non-syndromic spontaneous dizygotic twinning"; ASRM/SART practice committee guidance on single-embryo transfer.