This was a prospective cohort study of women receiving care in 79 midwifery-led birth centers in 33 US states from 2007 to 2010. Data were entered into the American Association of Birth Centers Uniform Data Set after obtaining informed consent. Analysis was by intention to treat, with descriptive statistics calculated for maternal and neonatal outcomes for all women presenting to birth centers in labor including those requiring transfer to hospital care
labour and birth.