BACKGROUND: Preterm birth (PTB) is a multifactorial pathology that raises feto-maternal morbidity. Infection was associated with higher PTB risk. Earlier studies discussed the inconsistent role of Chlamydia trachomatis (CT) with PTB. To examine the contribution of CT to the overall incidence of PL in our population and to verify the concurrent impact of bacterial vaginosis (BV) on PTB risk. METHODS: A prospective cohort study enrolled low-risk pregnant women attending an antenatal clinic at a gestational age of 24-28 weeks. They were screened for CT and BV using cervical smears and subdivided into two groups: positive CT cases (study group; N = 79) and negative CT cases (control group; N = 235). They were followed until delivery. For each, maternal parameters [Maternal age, parity, socioeconomic class, BV, preterm labor, tocolytic, progestogen drugs, and dexamethasone] and Fetal parameters [Gestational age at delivery, fetal birth weight, admission to intensive care unit for respiratory distress syndrome] were recorded. RESULTS: Positive CT cases had significantly higher PBT; 20/79 (25.3%) vs. 21/235 (9%) among negative CT cases. Stratifying PTB results according to infection status revealed that 6.3% (12/189) of PTB cases were negative for both CT and BV, 12.2% (5/41) had CT alone, and 19.6% (9/46) were positive for BV alone. 39.5% (15/38) of PTB cases screened positive for CT and BV, thus highlighting a potential synergistic effect between the infection and PTB risk. Positive CT cases had higher BV incidence; 38/79 (48.1%), and higher use of tocolytic; 21/79 (26.6%), progestogen; 20/79 (25.3%), and dexamethasone drugs; 24/79 (30.4%); (P = 0.03, 0.006, 0.02, 0.009, and 0.01, respectively). Multiple logistic regression showed that positive BV cases had an increased risk of adverse outcomes, OR = 3.78. Overall model fit confirmed CT contribution to overall PTB 3-7%. Cochran-Mantel-Haenszel test discussed a BV synergistic role in PTB; OR = 2.55; 95%CI:1.44 to 4.51; P = 0.002. CONCLUSION: The current findings suggest that CT infection, especially when combined with BV, may significantly increase the risk of PTB even when both infections are appropriately treated during pregnancy. The observed synergistic interaction highlights the potential role of screening for co-infection during pregnancy and reinforces an integrated management approach beyond treatment.