Interpretation
Our finding of a positive association between the COVID-19 lockdown and
risk of PTB was consistent with some previous studies (8,12,28). Several
reasons possibly explained the increased risk of PTB. First, the lack of
medical resources during the COVID-19 pandemic and lockdown measures
might interrupt the timely antenatal care for pregnant women (9,29).
Secondly, fear and panic about the pandemic could make pregnant women
reluctant to seek help from medical institutions, and further impacted
the timely detection and diagnoses of pregnancy complications (9,30).
For example, we observed a higher rate of GDM in the exposed group than
the unexposed group. This suggested a potential mediation role of GDM,
as GDM is a critical risk factor of PTB (10). In addition, pregnant
women have always been considered a susceptible population to mental
disorders (31). The lockdown and restriction measures could increase
psychological problems in pregnant women through concomitant financial
problems and increased stress (30,32), particularly if they were
socioeconomically disadvantaged (33). The closure of entertainment
venues also reduced the outlets for negative feelings (34). A previous
study observed a more pronounced increase in depression and anxiety in
pregnant women during the COVID-19 pandemic than in the general
population (35). Lastly, the nutritional status of pregnant women was
also of concern. During the lockdown period, the decreased supply of
fresh foods could lead to inadequate intake of vegetables and high-fiber
foods. Meanwhile, the intake of high-carbohydrate foods might have
increased because they were relatively easier to obtain and store (9).
It was reported that the overweight and obesity rates increased during
the lockdown period due to unbalanced diets and less exercise (36). This
suggested that the maternal stress and obesity during the lockdown might
influence the risk of PTB (9,35).
We further observed that women in early and middle pregnancy during the
Level I lockdown had a greater risk of PTB, which also contributed to
the health effects of the COVID-19 lockdown. Zhang et al. reported that
women in the first and second trimesters of pregnancy during the
lockdown had more severe psychological disorders (16). A simple
explanation could be that these mothers continued to experience Level II
and III lockdown after the Level I lockdown, which may have led to more
cumulative effects on their fetal health. This was supported by our
observed positive association between PTB risk and cumulative exposure
to lockdown of all levels in the first 24 GWs. An alternative
explanation could be that early and middle pregnancy is a critical
period for fetal development because the majority of fetal organ and
tissues retain plasticity at that time (37). As a result,
lockdown-induced poor diet, depression, and anxiety problems in early
and middle pregnancy may substantially interrupt fetal development
(38-40).
It should be noted that several other previous studies reported a
reduction (rather than an increase as in our study) in rate of PTB
during the COVID-19 lockdown (11,13). Although the mechanisms underlying
these negative associations were unclear, several socio-environmental
and behavioral modifiers were proposed (5,41). First, the lockdown
measures increased company and support from partners and family, which
could reduce the existing psychological stress in pregnant women.
Second, working from home increased their rest time at home and
decreased work-related stress. Third, the reduced anthropogenic
emissions improved the air quality, which could benefit maternal and
fetal health. Fourth, precautionary behavioral changes were promoted
during the lockdown, including social distancing, enhanced hand hygiene,
and use of face masks. These behavioral changes could potentially reduce
the chances of other common viral infections in addition to COVID-19
during pregnancy. Finally, lockdown measures also reduced daily
commuting, road traffic incidents, and consumption of cigarettes,
coffee, alcohol, prescription drugs, and street drugs due to limited
accessibility (5,41).
Previous studies reported inconsistent associations of lockdown exposure
with maternal and fetal health (5,11,13). These inconsistencies may have
a few explanations. First, some studies (11) had small sample sizes and
potentially inadequate statistical power to detect an association
between lockdown exposure and PTB. Second, the seemingly decreased risk
of adverse pregnancy outcomes related to lockdown might be partially
related to the reduced number of ultrasound scans and screening, which
increased the possibility of under-diagnoses of early pregnancy loss,
miscarriages, or stillbirths. Third, the health effects of lockdown may
last for several months, but previous studies did not track participants
long enough to assess the total effects of lockdown, which could have
led to underestimations. In this study, we used the data of pregnant
women who experienced the Level I lockdown until the end of 2020 and
were able to obtain birth outcomes of all exposed women by covering the
entire pregnancy. Fourth, air quality improvement during the lockdown
was proposed as a major contributor to the reduced risk of PTB. In this
study, we also found a substantial reduction in air pollution during the
lockdown (Table S4), which was consistent with previous studies (2,42).
Fifth, seasonal effects and pregnancy stages were not considered in most
previous studies, which could lead to biased results. To evaluate this
potential bias, we estimated the difference in PTB rates between new
births during the Level I lockdown and all previous births during the
entire years (rather than matching the calendar months) from 2015-2019.
We did not find a significant association between lockdown and PTB risk
(Table S5). Finally, although the lockdown measures may increase company
and support from partners and family, the potential increase in family
conflicts and domestic abuse should also be considered (43). These
findings suggest that the health effects of COVID-19 lockdown were
comprehensively affected by socio-environmental changes and behavioral
modifications, and that improvement in one factor could not make up for
the overall disadvantage (11,28).