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Secondary sex ratio in multiple births
Johan Fellman1, Aldur W Eriksson
1Folkhälsan Institute of Genetics, Department of Genetic Epidemiology, Helsinki, Finland. johan.fellman@shh.fi
Summary
The sex ratio at birth varies by multiple births, with singletons having the highest ratio and triplets the lowest. Historical data shows a convergence of stillbirth sex ratios towards live birth levels over time.
Area of Science:
- Demography
- Reproductive Biology
- Human Genetics
Background:
- The sex ratio at birth (SRB) is a key demographic indicator.
- Factors influencing SRB, including prenatal loss, are of significant interest.
- Previous hypotheses suggest an inverse relationship between SRB and prenatal loss frequency.
Purpose of the Study:
- To analyze long-term trends in the secondary sex ratio (SSR) in Sweden.
- To investigate the influence of birth multiplicity on SSR.
- To examine changes in SSR among stillbirths and live births over time.
Main Methods:
- Analysis of Swedish birth data from 1869 to 2004.
- Stratification of data by birth multiplicity (singletons, twins, triplets, etc.).
- Examination of SSR trends in live births and stillbirths, considering historical periods.
Main Results:
- Live birth SSR was highest in singletons (105.9), intermediate in twins (103.2), and lowest in triplets (99.1).
- Stillbirth SSR for singletons and twins decreased from ~130 in the 1860s to approach live birth levels by the 1990s.
- Marital status and residence (urban/rural) showed no significant influence on twin SSR; higher-order multiples had low SSR.
Conclusions:
- Birth multiplicity significantly influences the secondary sex ratio at birth.
- Declining stillbirth rates, particularly for males, contributed to the convergence of stillbirth and live birth SSR.
- Long-term demographic data provides valuable insights into factors affecting human reproductive patterns.
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