Objective: This prospective clinical trial was designed to assess the ability of the mammary stimulation test to predict preterm birth in a private nulliparous population.
Study design: The mammary stimulation test was performed between 26 and 28 weeks' gestation by 267 nulliparous women with singleton pregnancies. Test results were blinded to both patient and referring physician. Pregnancy outcome data were collected from the perinatal database and medical records.
Results: The mammary stimulation test was positive in 45 of 266 (17%) patients. Delivery occurred at < 37 weeks in 27 patients (10.2%) and at < 34 weeks in five (1.9%). The mammary stimulation test demonstrated a sensitivity of 37%, a specificity of 84%, a positive predictive value of 20%, and a negative predictive value of 92% for delivery at < 37 weeks' gestation. For delivery at < 34 weeks' gestation the mammary stimulation test had a sensitivity of 60%, a specificity of 82%, a positive predictive value of 6%, and a negative predictive value 99%. The odds ratio for delivery at < 37 weeks was 3.0 (95% confidence interval 1.3, 7.1), and for delivery at < 34 weeks the odds ratio was 7.0 (95% confidence interval 1.1, 43.0). One third of preterm deliveries were secondary to idiopathic preterm labor, and the mammary stimulation test was positive in 77.8% (seven of nine) of these pregnancies. Patients with a positive test were more likely to require observation in labor and delivery for preterm uterine contractions (34% vs 4.3%, p < 0.01), and they were more likely to demonstrate change at cervical examination (14% vs 2%, p < 0.01).
Conclusion: In this population traditionally considered to be at low risk for preterm birth a positive mammary stimulation test was useful in identifying patients at risk for preterm uterine activity and preterm birth. Equally important was the identification of women who were at low risk for preterm birth.