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.