Previous pregnancies
Parity—the number of previous pregnancies—has been associated with dizygotic twinning in several population datasets, but the evidence is mixed and maternal age is a stronger predictor [1]. Parity can also correlate with age, fertility and family characteristics, making a large independent coefficient difficult to defend.
Model v2 therefore uses compressed weights on the dizygotic component:
- No previous pregnancy: ×1.00
- One: ×1.05
- Two: ×1.10
- Three or more: ×1.15
These are heuristic model choices, not clinical coefficients.
Why breastfeeding was removed
The former model used a ×1.4 breastfeeding multiplier based mainly on a small observational report [2]. We did not find sufficiently robust replication to support that number. Breastfeeding is no longer asked or weighted; legacy values are neutral.
Pregnancy spacing and breastfeeding decisions involve recovery, feeding goals, contraception and individual health. They should not be changed to influence twin probability.