Introduction
The ongoing COVID-19 pandemic has spread throughout the world and
affected billions of people(1). Various measures have been implemented
around the world to control the pandemic, including restricting large
social movements and gatherings, closing international and interstate
borders, controlling travel, and implementing partial or full lockdown
of cities and regions. These measures have effectively controlled the
spread of COVID-19 and reduced the anthropogenic emissions of air
pollution (2), which have resulted in substantial health benefits (3).
However, these measures have also caused huge economic loss,
unemployment, shortage of medical resources, and psychological stress,
(4-7) which may lead to adverse health outcomes.
Pregnant women and fetuses may be susceptible populations to the effects
of lockdown and restriction measures. A few studies have reported that
the COVID-19 lockdown measures may increase the risk of adverse birth
outcomes such as stillbirth and cesarean delivery (8,9). Preterm birth
(PTB) is one of the most important adverse birth outcomes and a major
cause of death in children under 5 years of age(10). Several studies
have examined the associations of COVID-19 lockdown measures with the
risk of PTB, but the results were inconsistent(8,9,11-14). A study in
London reported an increase in the incidence of PTB during the COVID-19
pandemic period over the pre-pandemic period(12). Another study
conducted in Nepal also observed a greater risk of PTB during the
COVID-19 lockdown than before lockdown.(8) In contrast, studies
conducted in Denmark and the Netherlands observed a substantial
reduction in the risk of PTB during the COVID-19 periods than before
lockdown (11,13). The other two studies conducted in China and Botswana
did not find any significant association between the COVID-19 lockdown
and the risk of PTB (9,14). The inconsistent findings across these
studies may be attributable to differences in study design, sample size,
demographic characteristics of study subjects, and socioeconomic
developments of societies.
Although the aforementioned studies have preliminarily estimated the
associations between COVID-19 lockdown and PTB, several research issues
or gaps need to be addressed. First, the susceptibility of pregnant
women to environmental factors largely depends on the stage of pregnancy
(15,16). Previous studies estimated the overall rate of PTB in pregnant
women exposed to COVID-19 lockdown measures (8,9,11,12,14,17-19), but
did not consider their pregnancy stage when lockdown occurred. This may
lead to an underestimation of PTB risk during the lockdown if pregnant
women with a gestational age > 36 weeks were also included.
Second, lockdown intensity usually varied over time. However, none of
previous studies considered the change in intensity of lockdown
exposures. Third, previous studies have suggested a seasonal variation
in the incidence of PTB (20,21). The seasonal effects should be
considered in selecting the control periods for the COVID-19 lockdown.
However, some previous studies applied the annual or multiple years’
average incidence of PTB as the reference (9,11,14), which might lead to
biased findings. Fourth, the follow-up time (2-4 months) in previous
studies was not long enough to capture the birth outcomes of pregnant
women who experienced the lockdown in their early pregnancy
(8,9,11,12,14).
To fill these research gaps, we comprehensively elucidated the
association of the COVID-19 lockdown on gestational length and PTB risk
in South China by quantifying the timing and intensity of exposure,
considering seasonal effects, and allowing sufficient follow-up time.
This study could provide in-depth insights to inform management
practices regarding pregnancy and childbirth during and after lockdown.