Factors

How height, bmi and twin probability affects twin probability

Observational associations exist between maternal anthropometry and fraternal twinning, but they do not prove causation and model v2 uses small weights.

Effect size: ×0.95–1.20 vs. baseline

Last updated:

Association, not instruction

Reddy and colleagues reported associations between maternal BMI, height and dizygotic twinning after adjustment for race, age, parity and height or BMI as appropriate [1]. A Danish analysis also examined twinning across height and BMI [2]. Observational adjustment reduces some confounding but cannot establish that changing weight or height would change an individual outcome.

Conservative model weights

Model v2 applies small heuristics only to the 0.85% dizygotic component:

  • Height: ×0.95 below 160 cm, ×1.00 at 160–170, ×1.10 at 170–175, ×1.20 above 175
  • BMI: ×0.95 below 25, ×1.05 at 25–30, ×1.15 above 30

The former maximum weights of ×1.5 for height and ×1.3 for BMI were compressed to reflect residual uncertainty and the risk of multiplying correlated population associations.

Health decisions

These fields are descriptive inputs, not recommendations. Do not gain or lose weight to influence twin probability. BMI is also an imperfect screening measure and does not capture body composition or an individual pregnancy risk profile. Discuss preconception health with a qualified clinician.

See how these small observational weights combine in the educational twin-chance calculator .

Source

How we calculated this

See the multiplier and how this factor combines with the rest of the model.

References

  1. [1] Reddy UM, Branum AM, Klebanoff MA. (2005). Relationship of maternal body mass index and height to twinning. Obstetrics & Gynecology.
  2. [2] Basso O, et al. (2004). Risk of twinning as a function of maternal height and body mass index. JAMA.

Related factors

×0.70–1.20

Maternal age

Spontaneous fraternal-twin rates generally rise with age, but model v2 uses a deliberately compressed weight.

Read the deep dive
Protocol-specific

IVF & fertility treatments

Risk varies by treatment, stimulation, embryo-transfer count, age and clinic; model v2 applies no universal multiplier.

Read the deep dive
×2.0

Family history

The clearest association concerns a first-degree female relative with spontaneous fraternal twins; the questionnaire uses one broad family-history proxy.

Read the deep dive