The length of the uterine cervix and its significance.

Sonographic measurement of 150 pregnant and nonpregnant uteri indicate that the uterine cervix is about 2.5 cm in length in the nongravid and is seldom or never more than 6 cm in length during pregnancy. This information is used to better identify the level of the internal cervical os, especially in evaluating early placenta previa. If careful sonography is used to identify the lowest attachment of the placenta and the cervical tip, placenta previa could be excluded when the placenta is more than 6 cm from the cervical tip. This seems reliable in clinical practice and reduces the unreasonably high level of early placenta previa predictions and the attendant anxiety of mother and obstetrician.