What the evidence supports
Maternal age is associated with spontaneous dizygotic (fraternal) twinning. One proposed pathway is age-related change in follicle-stimulating hormone and multifollicular development. Beemsterboer and colleagues observed this pattern in 959 spontaneous cycles from 507 women [2].
Official US data provide useful context but not a clean spontaneous-conception coefficient. In 2024, the twin-birth rate rose from 16.1 twin babies per 1,000 births among mothers under 20 to 35.4 at ages 35–39 and 35.6 at 40 or older [1]. Those rates include treatment-related births and count babies, not twin pregnancies.
How model v2 uses age
TwinCalc applies age weights only to the 0.85% dizygotic component of its 1.25% spontaneous baseline:
- Under 25: ×0.70
- 25–29: ×0.95
- 30–34: ×1.10
- 35–39: ×1.20
- 40 or older: ×1.20
These are conservative heuristics, not coefficients copied from one clinical study. The earlier ×4.0 peak was removed because it overstated what current population evidence can support in an individual educational model.
What this does not mean
Age-related twinning patterns do not make waiting to conceive safer or easier. Overall fertility, miscarriage risk and pregnancy complications follow different age curves. TwinCalc does not advise when to conceive and cannot estimate an individual treatment outcome. Discuss timing and risk with a qualified clinician.