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

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).

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.

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).

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.