Background and Aims: Urinary tract infections (UTIs) are common among pregnant women and major predisposing factors for pyelonephritis linked to obstetrical complications including preterm labour and low infants’ birth weights. This study sought to determine the relationship(s) between pregnancy trimesters, UTIs and changes in progesterone levels among pregnant women. Materials and Methods: The study was conducted in 2016 at Moi Teaching and Referral Hospital (MTRH) antenatal clinic which is a referral facility that attends to patients from most Counties in western region of Kenya. A cross-sectional study design was used to collect blood and urine specimens from 78 participants. Blood was used to determine progesterone levels using ELISA technique and urine cultures with bacterial colony counts ≥ 105 were appropriately identified to species level. Trimester periods and participants’ demographic information were obtained using a structured questionnaire. Results: Culture results showed that the most abundant bacterial species isolated in urine from the pregnant women was Escherechia coli (63.7%). The more affected age-group was women between 30 - 39 years during trimester three, suggesting that bacterial colonization of genital track occurred more frequently in older compared to the younger women. There was an exponential increase in progesterone levels among the pregnant women during trimester three compared to other trimesters, although these increases occurred independent of age. However, high levels of progesterone among pregnant women in third trimester corresponded with increased number of E. coli causing UTI. Conclusion: The results showed that progesterone levels increase with trimester and the most prevalent bacteria associated with this was E. coli even though age and increase in progesterone levels had no significant impact on E. coli infection.