In singleton pregnancies the rate of spontaneous premature birth before 34 weeks is about 1% and the risk of spontaneous early delivery is inversely related to cervical length. The group with cervix of 1-15 mm accounted for 28% of all spontaneous deliveries before 34 weeks and those with cervix of 16-25 mm accounted for 21%. The rate of spontaneous premature birth before 34 weeks is about 13% in twin pregnancies.