Interpretation
Our data on a reduction in PTB concomitant with the COVID-19 pandemic
period are concordant with those of two recent national-based studies
published after the systematic review and meta-analysis of Yang and
coll.,9 which concluded that a reduction in the odds
of PTB was observed only in single-center studies. The first of these
studies was conducted in Israel,18 on birth data from
the Israel national newborn screening program and showed a 10% decline
in all preterm deliveries during the COVID-19 pandemic national lockdown
period. The second one, in UK,15 used administrative
hospital records and found a slightly lower frequency of preterm birth
rates (from 6.1% to 6.0%) during the entire pandemic period compared
to pre-lockdown.
In our study, we considered events (PTB and stillbirths) up to the end
of March 2021, when most mitigation strategies stopped in Italy. Most of
the previous nationwide studies in Europe4,16,17 and
large regional studies in other countries,19-21 were
instead restricted to 2020- mostly to the first months of the year. The
larger time span considered allowed us to also study women who
experienced changes in care and social activities for most of or all the
pregnancy. This is not trivial, as multiple factors at different times
during pregnancy might have had an impact on the rate of preterm
deliveries. Though there are not, so far, studies available on this
interesting topic, we can speculate in accordance with
others15,18 that lifestyle and behavior (more rest,
working from home, reduced exposure to other respiratory pathogens),
might have contributed to PTB reduction. Other possible pandemic-related
changes, which are known to impact on PTB, might have been the adoption
of a different and healthier diet,22 and diminished
exposure to air pollution.23
Finally, the large reduction in activity of Medical Assisted
Reproduction services at the beginning of the pandemic period (end of
February-April 2020)24 could be responsible of the
marked drop in PTB prevalence we observed in the last months of 2020.
In univariable and multivariable logistic analyses we found an inverse
association between being born in the pandemic period and being late
PTB, but also very PTB and extremely PTB. With the interrupted time
series regression analysis we were able to demonstrate a reduction in
the frequency of PTB after the lockdown only for the subgroup of late
PTB, and not for the other subgroups which had much lower frequencies
and thus more scattered data. Similar results were reported with this
type of analysis by Been et al.16 in the Netherlands
and by Bian et al.25 in China, while in Canada Shah et
al.20 did not find any significant change either in
the rates of all PTB or in the subgroups.
We also analyzed separately singletons and multiples; as expected for
singletons, who constituted 72.9% of all PTB, we found results very
similar to those of the total population, while for multiples we
probably had a lower power to reach conclusive results. In detail, for
multiples, all point estimates were very similar to those of singletons
and below 1, indicating a reduction in PTB during the pandemic, but the
CIs in the logistic analyses were wide, and encompassing the null value,
especially for very preterm and extremely preterm infants. No data are
available in the literature on the pattern of preterm multiples during
the COVID-19 pandemic in comparison to previous years. These data would
have been interesting, considering that PTB is largely represented in
multiple pregnancies (in our data 61.7% of all multiples are born
before 37 weeks’ GA) which are in turn associated with assisted
conception which possibly decreased during the COVID-19 period. A sub
analysis on multiples pregnancies would probably require an
international collaboration to have larger sample sizes.
Finally, though we elected as the main analysis the unadjusted one,
because we wanted to abstain to correct for variables which could not be
considered “true” confounder, our results were unchanged after
adjustment for many covariates considered in previous studies like
maternal ethnicity /country of birth, socioeconomic background/income/
education, maternal age, parity, and pregnancy conceived with
ART.15,17,19,