This protocol describes a seamless Phase II/III, randomized, double-blind clinical trial evaluating the efficacy and safety of daily low-dose aspirin (81 mg) plus lansoprazole (30 mg) in pregnant individuals at high risk for preterm birth when compared to existing standard of care, identified through biomarker-enriched screening. Participants will be enrolled between 12-16+6 weeks' gestation and followed through delivery and postpartum. The primary objective is to determine whether the investigational combination reduces the incidence of preterm birth before 37 weeks of gestation compared with placebo.