Association between COVID-19 outbreak and preterm birth rates in a low-infection cohort from a tertiary Shanghai maternity center: a retrospective study.

OBJECTIVE: To evaluate the association between the COVID-19 pandemic and preterm birth (PTB) rates, stratified by gestational age, in a low-infection pregnant population delivering at a non-designated COVID-19 maternity center. DESIGN: A hospital-based retrospective cohort study was conducted, comparing deliveries during the pandemic period (January 2020-April 2022) with the pre-pandemic period (January 2017-April 2019). Logistic regression models were used to assess crude and adjusted associations, with singleton-only sensitivity analyses performed to address confounding by multiple gestation. RESULTS: A total of 67,238 deliveries were included, comprising 29,310 pandemic and 37,928 pre-pandemic births. Overall PTB rates were similar between the two periods (7.5% vs. 7.4%; adjusted odds ratio [AOR] = 1.0, 95% confidence interval [CI]: 1.0-1.1, P = 0.66). Preterm birth excluding extremely preterm birth (EPTB) (28+0-36⁺6 weeks) rates also remained stable (7.3% vs. 7.3%; AOR = 1.0, 95% CI: 0.9-1.1, P = 0.95). In contrast, the rate of EPTB (22+0-27⁺6 weeks) was significantly higher during the pandemic (0.2% vs. 0.1%), with a nearly threefold increased risk after adjustment (AOR = 2.8, 95%CI: 1.7-4.7, P < 0.001). This association persisted in the singleton-only cohort (AOR = 3.2, 95% CI: 1.8-5.9, P < 0.001), despite the rarity of this outcome. CONCLUSIONS: Among this low-infection cohort reflecting pandemic measures rather than viral effects, overall and preterm birth excluding EPTB rates remained stable during the pandemic, though a significant increase in extremely preterm birth (<28 weeks) risk was noted, indicating a subtype-specific effect requiring further validation.