Strengths and limitations
Our population-based study,
together with the recent one of
Gurol-Urganci et
al.15 in UK, is the largest ever done as regards both
the number of PTB and stillbirths during the COVID-19 pandemic. This is
due not only to the fact that both Italy and UK have a larger number of
births in comparison to those in northern European countries which
previously published nationwide results like Denmark,4The Netherlands,16 and Sweden,17 but
also to the wider time span considered as pandemic period.
The longer pandemic period considered, besides increasing the sample
size, made it possible to study women who were exposed to mitigation
strategies during their whole pregnancy.
The large sample size allowed us to
study the different categories of
PTB and to analyze separately singletons and multiples, though the
relative low number of multiple pregnancies precludes definitive answers
in this subgroup.
As a further limitation, the dataset used does not contain information
on lifestyle and social behaviors of pregnant women, which precludes an
analysis of possible important and widespread causes for the observed
decrease of PTB among the general population.
In estimating the total effect of COVID-19 pandemic on pregnancy
outcomes we did not consider the effect of the SARS-CoV2 infection on
pregnant women – for which the data were not available. The COVID-19
infection is however known to increase PTB1,2 so that
excluding COVID-19-positive women would probably yield further reduced
PTB rates.
Finally, our study was a retrospective one using routinely collected
data, which are prone to registration errors, although data are filled
in by midwifes and doctors soon after birth and are annually checked for
the CeDAP report from the Ministry of Health.