The Maternal Fetal Medicine Units Network focuses on clinical questions in maternal fetal medicine and obstetrics, particularly with respect to the continuing problem of preterm birth. The Network provides an infrastructure to conduct multiple large studies simultaneously, in both a cost-effective and timely manner. The 12 participating clinical centers cover over 160,000 deliveries a year and are racially, ethnically, and geographically diverse, allowing study results to be generalizable to the U.S. population. Results from MFMU Network studies have impacted clinical practice, both by finding treatments that prevent poor pregnancy outcomes and by stopping ineffective, costly, and potentially harmful therapies.