An attempt was made to verify the hypothesis that a low-lying placenta may be a factor leading to the dynamic form of cervical incompetence. A vaginal evaluation of the cervix was made in 143 pregnant women who by means of ultrasonographic method were found to have low-lying placentas. In as many as 68 out of the 143 studied cases (47.5%), the low-lying placenta coincided with a shortening of the cervix about half its normal length or in both shortening and patency of the uterocervical canal, measuring up to 2 cm in diameter (39 cases). The statistical calculations indicate that a low-lying placenta can influence cervical behaviour and that it appears to be a risk factor of cervical incompetence.