Association of COVID-19 mitigation measures with preterm birth and cervical length: A cohort study.

BACKGROUND: We have previously reported that, in metropolitan Melbourne, mitigation measures implemented to mitigate SARS-CoV-2 transmission were associated with a reduced preterm birth (PTB) rate, both in spontaneous and iatrogenic births <34 weeks. Here, we assess the impact of mitigation measures on PTB in relation to mid-pregnancy cervical length (CL). METHODS: We conducted a comparative cohort study in three maternity hospitals in Melbourne, Australia, between women with a previous PTB <32 weeks who were exposed to COVID-19 mitigation measures while pregnant (lockdown cohort, estimated conception November 1, 2019 to February 29, 2020) and women who were pregnant before mitigation measures (pre-lockdown cohort, estimated conception November 1, 2018 to February 28, 2019). In our clinics, women with a previous PTB <32 weeks routinely undergo targeted assessment of CL at around 20 weeks. Within each cohort, we assessed the relationship between CL and perinatal outcomes. RESULTS: We studied 113 women, of whom 60 were in the lockdown group and 53 in the pre-lockdown group. Baseline characteristics, including CL measurements taken on average at 20.4 and 20.3 weeks of gestation, respectively, were comparable. Mean CL was 38.4 ± 8.8 mm versus 35.8 ± 9.7 mm (p = 0.141). Progesterone usage occurred in 5% (lockdown cohort) versus 23% (pre-lockdown cohort) of women (p = 0.006). After adjusting for age, BMI, smoking status, and parity, the rates of PTB <37 weeks were 20% and 26%, respectively (adjusted odds ratio [aOR], 0.60; 95% confidence interval [CI], 0.27-1.35; p = 0.199), while rates of PTB <34 weeks were 5% and 19% (aOR, 0.21; 95% CI, 0.06-0.82; p = 0.038). In the pre-lockdown cohort, CL measurements predicted PTB (aOR, 0.79; 95% CI, 0.65-0.90; p = 0.009), but this association was not present in the lockdown cohort (aOR, 1.21; 95% CI, 0.95-1.73; p = 0.169). CONCLUSION: COVID-19 mitigation measures were associated with a reduction in PTB. The predictive capacity of CL disappeared during mitigation measures. This effect may be attributed to a reduction of PTB in women with a short CL. While the underlying mechanism remains unclear, these findings suggest a potentially modifiable risk factor in lifestyle in high-risk women with a short CL, warranting further investigation through larger prospective studies.