To assess a screening program for sexually transmitted infections (STIs) in under-30-year-old pregnant women, focusing on Chlamydia trachomatis, Neisseria gonorrhoeae and Mycoplasma genitalium infections, which though often asymptomatic, may significantly affect women's health and can be vertically transmitted. Design: Prospective, descriptive, observational study. Setting: Gipuzkoa (Basque Country, Spain). Population: Under-30-year-old pregnant women. Methods: Between 2016 and 2020, cervical samples were taken at first prenatal appointments from 3051 women. STIs were detected by multiplex nucleic acid amplification. Main Outcome Measures: Prevalence of infections by age and geographical origin groups, and screening coverage. Results: The coverage rate was 86% between 2017 and 2020. At least one STI was detected in 5.2% of the under-30-year-olds screened (95% confidence interval [CI]: 4.5%, 6.1%): C. trachomatis in 4%, M. genitalium in 1.3% and N. gonorrhoeae in 0.1%. C. trachomatis and M. genitalium prevalence rates were higher among younger women (7.2% and 2.8%, respectively in under-25-year-olds), while C. trachomatis prevalence in 25-to 30-year-olds was 2.5%. C. trachomatis was more common among Latin American women (9.7% [95% CI: 7.4%, 12.3%] vs. 2.5% [95% CI: 2.0%, 3.2%] in other women). Additionally, analysing samples from 708 pregnant over-30-year-old women with STI risk factors, 1.6% had at least one of the STIs studied. Notably, 20% of all cases had STI-related symptoms or complications during pregnancy. Conclusions: The high coverage achieved, and prevalence, clinical and epidemiological results support the view that the implementation of a screening program across Spain for C. trachomatis in under-30-year-old pregnant women would be both feasible and appropriate.