A NOVEL cohort study carried out in Sweden has found a strong positive association between infection with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) during pregnancy and the risk of preterm birth (PTB) within two weeks following infection. SARS-CoV-2 is the respiratory virus responsible for the COVID-19 pandemic that began in late 2019.
Scientists in Sweden carried out a nationwide, population-based study that involved all registered singleton pregnancies in the Swedish Pregnancy Register between March 2020 and May 2022 (n = 233,335). Supplementary data, including SARS-CoV-2 test results, vaccination status, and disease severity, were extracted from mandatory Swedish health registers. The association between testing positive for SARS-CoV-2 during pregnancy and risk of PTB was analysed using Cox regression analyses with time-varying covariates.
Risk peaks within two weeks after COVID-19
A lower gestational age at infection onset was associated with an increase in the risk of PTB within 2 weeks of the positive test (p = 0.003). The risk was highest when infection occurred between 22–27 weeks’ gestation (aHR 10.1, 95% CI 5.2–19.6), followed by 28–31 weeks (aHR 6.4, 95% CI 3.5–11.6) and 32–36 weeks (aHR 3.3, 95% CI 2.6–4.4).However, no increased risk of PTB was observed (aHR 1.0, 95% CI 0.8–1.1) 2 weeks or more after the beginning of infection.
Vaccination reduces the danger
Vaccinated women who contracted SARS-CoV-2 did not demonstrate a statistically significant increase in PTB risk (aHR 1.3; 95% CI, 0.7-2.4). Risk also varied by viral variants, with the Alpha and Delta variants demonstrating a greater risk of PTB compared with the Omicron variant.
This study has shown that pregnant women, particularly those unvaccinated, are at a higher risk of PTB within 14 days following SARS-CoV-2 infection. The authors highlight the importance of counselling infected pregnant patients about preterm birth symptoms to reduce the risk of birth complications and allow timely medical evaluation and intervention. The findings also support promotion of COVID-19 vaccination during pregnancy as an effective measure to reduce infection-related complications.
Reference
Berglin L et al. Infection with SARS-CoV-2 during pregnancy and risk of preterm birth and stillbirth: a Swedish nationwide cohort study. Acta Obstet Gynecol Scand. 2025. DOI: 10.1002/pmf2.70136.
