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.