Sonographic diagnosis of premature cervical dilatation. Potential pitfall due to lower uterine segment contractions.

Antenatal sonography is an accepted method of evaluating the cervix and lower uterine segment. Dilatation of the endocervical canal can be missed due to compression by an overdistended urinary bladder, intermittent changes in configuration of the cervix, and suboptimal visualization. However, the potential for overdiagnosis of this entity has not been previously emphasized. We present 14 cases demonstrating how contractions in the lower uterine segment can result in an appearance that falsely simulates preterm dilatation of the endocervical canal. The overdiagnosis of this entity can be avoided by distinguishing the following features typical of "pseudodilatation" due to a contraction: (1) apparent cervical length greater than 5 cm; (2) demonstration of normal-appearing cervical tissue distal to the apparently dilated region; and (3) rounding of the myometrium surrounding the apparently dilated area. If any of these features is detected in a patient with sonographic characteristics suggesting dilatation of the cervix, the possibility of pseudodilatation should be considered, and the lower uterine segment should be rescanned later in the examination.