Twinning causative origin investigated by Sartwell's biometrical method

Area of Science:

• Biometrics
• Epidemiology
• Reproductive Biology

Background:

• Sartwell's biometrical model analyzes incubation times for diseases of unknown etiology.
• The model, assuming lognormal distribution, has been applied to infectious, neoplastic, and genetic diseases.
• This study applies the model to twin births to investigate potential causal factors.

Purpose of the Study:

• To test hypotheses regarding the causal origins of unlike-sex and like-sex twins using biometrical analysis.
• To investigate the maternal prenatal origin of unlike-sex (dizygotic/DZ) twins.
• To explore the menopausal basis for like-sex (monozygotic/MZ and DZ) twins.

Main Methods:

• Population-based study with a matched control group.
• Application of Sartwell's biometrical model with lognormal distribution assumption.
• Analysis of time windows (induction periods): age at onset, time between births, and time to last birth.
• Statistical methods included Box-Cox transformations, Fisher's cumulants, and normal probability plots.

Main Results:

• Unlike-sex (DZ) twins show a lognormal age of onset, supporting a maternal prenatal origin with a median induction time of 31 ± 6 years.
• This suggests a multiplicative, age-related causal process possibly involving hypophyseal hormones.
• The analysis for like-sex twins indicated a less clear-cut menopausal origin, with evidence suggesting some MZ twins may be menopause-related, but further research is needed due to group heterogeneity.

Conclusions:

• The maternal prenatal period is a likely causal origin for unlike-sex (DZ) twins, linked to an age-related hormonal process.
• A menopausal basis for some like-sex (MZ) twins is suggested but requires more extensive study.
• Biometrical modeling provides insights into the etiology of twin types, differentiating potential causal pathways.