Seasonality and Sex-Biased Fluctuation of Birth Weight in Tibetan Populations

Abstract

Birth weight (BW) is a key determinant of infant mortality. Previous studies have reported seasonal fluctuation of BW. However, the responsible environmental factors remain disputable. High-altitude environment provides a great opportunity to test the current hypotheses due to its distinctive climate conditions. We collected BW data of ~ 9000 Tibetan singletons born at Lhasa (elevation: 3660 m) from 2014 to 2018. Using regression models, we analyzed BW seasonality of highland Tibetans. Multivariate models with meteorological factors as independent variables were employed to examine responsible environmental factors accounting for seasonal variation. We compared BW, low-BW prevalence and sex ratio between highland and lowland populations, and we observed a significant seasonal pattern of BW in Tibetans, with a peak in winter and a trough in summer. Notably, there is a marked sex-biased pattern of BW seasonality (more striking in males than in females). Sunlight exposure in the 3rd trimester and barometric pressure exposure in the 2nd trimester are significantly correlated with BW, and the latter can be explained by seasonal change of oxygen partial pressure. In particular, due to the male-biased BW seasonality, we found a more serious BW reduction and higher prevalence of low-BW in males, and a skewed sex ratio in highlanders. The infant BW of highland Tibetans has a clear pattern of seasonality. The winter BW is larger than the summer BW, due to the longer sunlight exposure during the late-trimester. Male infants are more sensitive to hypoxia than female infants during the 2nd trimester, leading to more BW reduction and higher mortality.

Supplementary information: The online version contains supplementary material available at 10.1007/s43657-021-00038-7.

Keywords: Birth weight; High altitude; Seasonality; Sex-bias; Tibetans.

Fig. 1

Monthly birth weight changes of Tibetans and fluctuation of meteorological factors in Lhasa. a Mean birth weight (sex separately) by month of birth after adjusted by maternal age, gestational age and parity. ANOVA is used to evaluate statistical significance. b Seasonal change of five meteorological factors in Lhasa (elevation: 3660 m)

Fig. 2

Seasonal fluctuation of BW and its correlation with multiple meteorological parameters. a Mean birth weight comparisons among the four birth seasons after adjusted by maternal age, gestational age and parity. ANOVA is used to evaluate statistical significance. Bonferroni corrections are conducted to adjust multiple comparisons. *p < 0.05; ***p < 0.001; ****p < 0.0001. b, c The correlations of sunlight and BarP exposure in the 3rd and the 2nd trimester with BW, respectively. Linear regression models are used to test the direction and significance of the associations (see “Methods”). β: regression coefficient;  p value adjusted by covariates and Bonferroni corrections

Fig. 3

Sex-biased BW reduction and sex ratio comparison among multiple datasets. a Comparisons of BW among Shanghai Han Chinese (HAN), Lhasa Tibetans (TBN) and Lhasa Han Chinese. ANOVA is used to test the significance. Bonferroni corrections are conducted to adjust multiple comparisons. ****p < 0.0001. b Comparison of low birth weight (low-BW) ratio among three populations. Male/female low-BW ratios are indicated, respectively. c Comparison of sex ratios (male/female) of Lhasa highlanders with other populations.*Sex ratio of lowland Han Chinese and highland Tibetans in China are from published data (Jiang et al. 2020)