Objective: Our goal was to determine whether the mammary stimulation test combined with a risk scoring system and cervical examination at 26 to 28 weeks' gestation could effectively identify private nulliparous patients at risk for spontaneous preterm birth. Study Design: The mammary stimulation test was performed by 267 nulliparous patients with singleton gestations at 26 to 28 weeks. Risk scores were determined by the method of Creasy et al. and cervical examinations were performed at the first prenatal visit and at 26 to 28 weeks. Summary predictive values were calculated for each test, and a stepwise discriminate analysis was performed. Results: Spontaneous preterm birth occurred at < 37 weeks in 26 of 265 (9.8%) patients. The following variables were independently associated with spontaneous preterm birth: positive result of mammary stimulation test, risk score ≤ 10, soft cervix at 26 to 28 weeks, bacteriuria, and current smoking. The best discriminate model included positive result of mammary stimulation test, soft cervix, bacteriuria, current smoking, and prior spontaneous abortion(s). This model identified 19 patients as being at risk for spontaneous preterm birth with sensitivity of 35%, specificity of 96%, and positive predictive value of 47%. Conclusion: Combining the mammary stimulation test with a cervical examination at 26 to 28 weeks' gestation and routinely obtained prenatal data correctly identified 35% of spontaneous preterm births in nulliparous patients with a positive predictive value of 47%. Prospective validation of this model is warranted. © 1994, Mosby-Year Book, Inc.. All rights reserved.