Secondary sex ratio in multiple births.
Level 3 - non-randomized controlled study
Retrospective population-based registry cohort study
PubMed 20158313 · doi:10.1375/twin.13.1.101
What was done
The authors analyzed historical Swedish national birth registry records from 1869 to 2004 to evaluate the secondary sex ratio (males per 100 females) across singletons, twins, triplets, and higher-order multiple births. They compared live births and stillbirths over time and conducted sub-analyses on data from 1869–1878 and 1901–1967 to evaluate the potential influences of maternal marital status and urban versus rural residence.
What was found
Among live births from 1869 to 2004, the average secondary sex ratio was 105.9 for singletons, 103.2 for twins, and 99.1 for triplets. For higher-order sets registered between 1901 and 1967 (20 quadruplet, two quintuplet, and one sextuplet sets), the sex ratio was approximately 92.0. The secondary sex ratio among stillbirths for singletons and twins was around 130 in the 1860s but converged toward live-birth levels by the 1990s alongside declining stillbirth rates. Neither marital status nor urban versus rural residence had a marked influence on the secondary sex ratio in twins.
Why it matters
This long-term national registry analysis provides empirical support for the hypothesis that the secondary sex ratio is inversely related to plurality and prenatal loss risk.
Limits
The abstract does not report the total sample size of births. As an observational demographic study, it cannot directly establish biological mechanisms or control for confounders such as chorionicity, zygosity, or assisted reproductive technologies. Findings from Swedish historical records across 135 years may also be affected by temporal changes in vital registration practices.
Cited by
- supports In the general human population, the secondary sex ratio is approximately 105 male births for every 100 female births.