Strengths and Limitations
We applied a large dataset with detailed individual information to investigate the association between lockdown and PTB risk. The dataset covered a wide enough time, in which birth outcomes of all women who have experienced the lockdown were recorded. We used strict contemporaneous controls to reduce the impact of seasonal effects on birth outcomes. In addition, we quantified the cumulative exposure by designing weightings to quantitatively aggregate the effects of different phases of lockdown measures. These strengths could provide a stronger causal argument for our findings.
There were several limitations that need to be addressed. First, as the COVID-19 pandemic and associated lockdown measures occurred unanticipatedly, we had to collect data from medical records that might miss some other gestation-length-related outcomes such as early pregnancy losses, miscarriages, and stillbirths. Previous studies reported an increased rate of stillbirth related to the COVID-19 lockdown (8,12). Our supplemental analysis also showed a higher stillbirth risk in the exposed group than in the unexposed group (Table S6). Second, several individual behaviors such as smoking, alcohol consumption, nutrition, and physical activity were not obtained. Their potential mediation roles were not evaluated in our analyses. Third, this study was conducted in only five cities in South China, which limited the generalization of our findings. Fourth, due to the coexistence of the COVID-19 pandemic and the lockdown status, we could not separate their induvial impacts on the outcomes.