The NOVEL cohort study conducted in Sweden found a strong positive association between severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection during pregnancy and the risk of preterm birth (PTB) within 2 weeks after infection. SARS-CoV-2 is a respiratory virus responsible for the COVID-19 pandemic that began in late 2019.
Scientists in Sweden conducted a nationwide population-based study that included all singleton pregnancies registered in the Swedish Pregnancy Register (n = 233,335) between March 2020 and May 2022. Supplementary data, including SARS-CoV-2 test results, vaccination status, and disease severity, were extracted from the Swedish vital health register. The association between SARS-CoV-2 positivity during pregnancy and PTB risk was analyzed using Cox regression with time-dependent covariates.
Risk peaks within two weeks of COVID-19 outbreak
Younger gestational age at onset of infection was associated with an increased risk of PTB within 2 weeks of a positive test (p = 0.003). Risk was highest when the infection occurred at 22 to 27 weeks of gestation (aHR 10.1, 95% CI 5.2 to 19.6), followed by 28 to 31 weeks (aHR 6.4, 95% CI 3.5 to 11.6) and 32 to 36 weeks (aHR 3.3, 95% CI 2.6 to 4.4). However, there was no increased risk of PTB. This was observed more than 2 weeks after the onset of infection (aHR 1.0, 95% CI 0.8–1.1).
Getting vaccinated reduces your risk
Vaccinated women infected with SARS-CoV-2 did not have a statistically significant increased risk of PTB (aHR 1.3; 95% CI, 0.7-2.4). The risk varies depending on the virus strain, with Alpha and Delta strains being associated with a higher risk of PTB compared to the Omicron strain.
The study found that pregnant women, especially unvaccinated women, were at higher risk of developing PTB within 14 days of SARS-CoV-2 infection. The authors emphasize the importance of counseling infected pregnant women about symptoms of preterm birth to reduce the risk of birth complications and allow for timely medical evaluation and intervention. The findings also support promoting COVID-19 vaccination during pregnancy as an effective measure to reduce infection-related complications.
reference
Berglin et al. SARS-CoV-2 infection during pregnancy and risk of preterm birth and stillbirth: a Swedish nationwide cohort study. Acta Obstet Gynecol Scand. 2025. DOI: 10.1002/pmf2.70136.
