Urinary tract infection during pregnancy and time relation to preterm birth-a Swedish observational study

It is well known that urinary tract infection (UTI) during pregnancy is associated with preterm birth, but information about the risk of preterm birth depending on gestational week of infection is sparingly described in the literature.

An observational study based on data from Swedish registers including women giving birth 2014-2020 (n = 684 595). Pregnant women with UTI diagnosis were identified by the ICD-10 codes (O230-O239, N300, N309, and N390) from the national patient registry. Hazard ratios (HR) for preterm birth after UTI diagnosis were calculated, considering the time elapsed after infection and interactions with gestational week at UTI diagnosis. Adjustments were made for maternal age, parity, previous caesarean section, body mass index, diabetes, and smoking.

A diagnosis of UTI was detected in 2.8% of pregnant women. A significant association between UTI diagnosis and preterm birth was found. The aHR for preterm birth was most pronounced during the first week after a UTI diagnosis and was dependent on gestational week. The aHRs (with 95% CI) for preterm birth 0-6 days after UTI diagnosis were 18.5 (13.5-25.4), 13.5 (10.4-17.6), and 6.7 (5.6-8.0) for infections debuting week 22-27, 28-31, and 32-36, respectively. The corresponding aHRs for preterm birth 7-13 days after UTI diagnosis were 10.5 (7.2-15.2), 3.7 (2.4-5.7), and 2.9 (1.9-4.5). The increased risk for preterm birth was still significant 3 weeks or more after UTI diagnosis debuting at 22-27 weeks (aHR 2.5; 95% CI: 2.1-3.1) or at 28-31 weeks (aHR 3.2; 95% CI 2.0-4.6).

UTI diagnosis during pregnancy is an important risk factor for preterm birth. The magnitude of the increased risk is highest 0-6 days after diagnosis; thereafter, it declines but remains significant throughout pregnancy until 37 weeks. The increased risk is especially elevated (and especially concerning) if UTI diagnosis was confirmed before 28 weeks of gestation.