Sex differences and sustained excess in mortality among discordant twins in Matlab, Bangladesh: 1977-1985

A scoping review of methods for assessment of sex differentials in early childhood mortality

BACKGROUND

While assessment of sex differentials in child mortality is straightforward, their interpretation must consider that, in the absence of gender bias, boys are more likely to die than girls. The expected differences are also influenced by levels and causes of death. However, there is no standard approach for determining expected sex differences.

METHODS

We performed a scoping review of studies on sex differentials in under-five mortality, using PubMed, Web of Science, and Scopus databases. Publication characteristics were described, and studies were grouped according to their methodology.

RESULTS

From the 17,693 references initially retrieved we included 154 studies published since 1929. Indian, Bangladeshi, and Chinese populations were the focus of 44% of the works, and most studies addressed infant mortality. Fourteen publications were classified as reference studies, as these aimed to estimate expected sex differentials based upon the demographic experience of selected populations, either considered as gender-neutral or not. These studies used a variety of methods - from simple averages to sophisticated modeling - to define values against which observed estimates could be compared. The 21 comparative studies mostly used life tables from European populations as standard for expected values, but also relied on groups without assuming those values as expected, otherwise, just as comparison parameters. The remaining 119 studies were categorized as narrative and did not use reference values, being limited to reporting observed sex-specific estimates or used a variety of statistical models, and in general, did not account for mortality levels.

CONCLUSION

Studies aimed at identifying sex differentials in child mortality should consider overall mortality levels, and report on more than one age group. The comparison of results with one or more reference values, and the use of statistical testing, are strongly recommended. Time trends analyses will help understand changes in population characteristics and interpret findings from a historical perspective.

Twinning rates and survival of twins in rural Nepal

BACKGROUND

Twin pregnancies are common but there are few data on rates of twinning or survival of liveborn twin infants in developing countries.

METHODS

The rates of multiple births were calculated in a population-based cohort of married women of childbearing age who were enrolled in a randomized community trial to assess the impact of vitamin A or beta-carotene on maternal and infant health and survival.

RESULTS

The rate of twinning was 16.1 per 1000 pregnancies (7.4 if only twin pregnancies resulting in two liveborn infants were used). The rate for triplets and quadruplets was 0.19 and 0.06 per 1000 pregnancies. Twinning rates were higher among women of higher parity, but were not associated with maternal age. Twinning rates among twins where at least one was live born (or increased in utero survival) were 30% (95% CI : -1%, 71%) and 44% (95% CI : 9%, 89%) higher among women receiving vitamin A and beta-carotene supplements than placebo, after adjusting for maternal age, gestational age, and parity. The perinatal mortality rate was 8.54 times higher for twins than singletons, 7.32 higher for neonatal mortality, and 5.84 higher for cumulative 24-week mortality. This difference was reduced but not erased by adjusting for gestational age. No difference in survival of liveborn twin infants was seen by supplement group. A higher mortality rate among male twins was largely explained by gestational age.

CONCLUSIONS

Multiple births are relatively common occurrences in rural Nepal, and carry a much higher mortality risk for the infants than for singletons. Vitamin A or beta-carotene supplementation appeared to increase the rate of twinning, or improve the survival of twins in utero, but did not increase twin survival after birth.