Birth sex ratio in Western Sydney during the COVID-19 pandemic: associations by maternal country of birth

The sex ratio at birth (SRB)—the proportion of male to female births—is a vital demographic indicator with implications for societal stability. SRB can be influenced by a range of factors including socio-economic conditions, parental health, cultural preferences, with acute population-level stress recognised as a highly significant influence. This study examined temporal changes in SRB during the COVID-19 pandemic and associated maternal factors in a large, multicultural population. Birth data from women residing in the Western Sydney Local Health District (WSLHD) between 2016 and 2023 were analysed using multivariable logistic regression and time series analysis. Analyses adjusted for maternal demographic, socio-economic, health, and reproductive factors. Of 119,756 singleton births, 61,832 were male, resulting in a sex ratio at birth (SRB) of 0.516 (106.7 males per 100 females). No significant overall change in SRB was observed during the early post-pandemic period. The SRB during the 3–5 months following the onset of the COVID-19 pandemic increased only among births to Chinese mothers, as indicated in both time series analyses and logistic regression (SRB = 0.621 vs. 0.519; adjusted odds ratio [aOR] = 1.557; 95% CI: 1.194–2.030 compared to other periods). Other factors associated with an increased SRB included having an Indian mother (SRB = 0.524; adjusted odds ratio [aOR] = 1.038; 95% CI: 1.001–1.076), compared to having a non-Indigenous Australian mother (SRB = 0.516). Higher SRBs were also observed among mothers with one previous pregnancy (SRB = 0.519 vs. 0.513; aOR = 1.031; 95% CI: 1.004–1.059) or two previous pregnancies (SRB = 0.522; aOR = 1.050; 95% CI: 1.012–1.090), compared to mothers with no previous pregnancies. Conversely, mothers with hepatitis B had a lower SRB than those without the condition (SRB = 0.483 vs. 0.517; aOR = 0.858; 95% CI: 0.755–0.976). The COVID-19 pandemic was not associated with an immediate aggregate shift in SRB in WSLHD. However, subgroup-specific variation by maternal country of birth was observed, highlighting the importance of maternal background in shaping SRB patterns. These findings underscore the need for culturally informed public health approaches in diverse populations during major societal disruptions. This study examined whether the COVID-19 pandemic was associated with changes in the number of boys born compared to girls in Western Sydney, Australia, and whether this differed across maternal groups. Overall, there was no clear change in the balance between boys and girls during the early pandemic period. However, among mothers from China, a higher proportion of boys was observed a few months after the pandemic began. Mothers born in India had the highest proportion of boys throughout the study period, but no distinct change related to the early pandemic was identified for this group. The study also found that women who had more previous pregnancies were slightly more likely to have boys, while mothers with hepatitis B were less likely to have boys. Among Chinese mothers, those living in wealthier areas were less likely to have boys compared to those in less wealthy areas. Older Australian mothers and those who were underweight were also less likely to have boys. These findings suggest that changes in the sex ratio during the pandemic differed across cultural and social groups. Supporting families in culturally appropriate ways may help maintain a balanced number of boys and girls during future societal disruptions.