When placed in the closed cervix, a osmotic dilator absorbs moisture from the tissues surrounding the cervix and swells, opening the cervix slowly and with little discomfort. Two common types of osmotic dilators are a laminaria, a small tube made of dried seaweed, and synthetic dilator, a man-made sterile, dry sponge.

Unless a woman is in labour before childbirth, the cervical opening is very narrow. An osmotic dilator is commonly used to gently open the cervix before a gynecologic procedure that requires the cervix to be open, allowing access to the uterus and fallopian tubes. Cervical dilation reduces the risk of injury to the cervix during such a procedure.

Most of the cervical dilation with laminaria occurs in the first 6 hours. But maximum dilation usually occurs 12 to 24 hours after the laminaria is placed. This means that laminaria placement may be done the day before a procedure. Osmotic dilators may be sequentially added to or replaced to increase the cervical opening.

A synthetic dilator opens the cervix in less time and can be used several hours before a procedure.