How wide the vagina stretches during pregnancy is called dilation of the cervix.
Dilation of the Cervix
The dilation of the cervix is measured in centimeters. During the exam, the doctor basically is trying to decide how dilated (or open) the cervix is. The doctor will place his hand gently (we hope) inside your vagina all the way up to the cervix to feel the presentation of the baby's body part. He should feel the head if all is well. Occasionally, the baby will be breech – that is, the doctor will feel the baby's butt or feet. If this condition is diagnosed, the patient will probably be advised to have a c-section, or the doctor may try to turn the baby to the head-down position.
the vagina can stretch up to 10 centimeters during pregnancy