The cervix, also known as the cervical canal, is part of the female reproductive system and is directly connected to the vagina1. It has a small passage, like a “tiny hole,” through which menstrual blood passes each month. It’s also the passageway that allows sperm to access the uterus and reach the eggs. But during pregnancy, does the cervix remain open?

Changes to the Cervix in Pregnancy

Many changes occur to the cervix during pregnancy, as it needs to stay tightly closed to support the baby’s weight. The body itself produces mucus that blocks this passage and only disappears when preparing for labor, which is when the well-known mucus plug is lost.

Function of the Cervix

The role of the cervix when a woman is not pregnant is to prevent the entry of infectious agents and disease-causing organisms, which can manifest in various ways2.

Function of the Cervix During Pregnancy

Normally, during pregnancy the cervix remains hard, firm, and well sealed with mucus to protect fetal development and prevent the baby from coming out3. When there are changes, a miscarriage can occur, so it is extremely important to notify your obstetrician about any unusual symptoms.

What is the Size of the Cervix During Pregnancy?

The size of the cervix during pregnancy varies slightly from woman to woman. In general, it is about 2 to 4 centimeters in length and 3 centimeters in depth. This is in a “normal” pregnancy; in the case of twins, these measurements change due to the extra weight that needs to be supported.
When it’s a twin pregnancy, the cervix may be shorter (less than 25 millimeters), thinning and shortening as labor approaches. During check-ups and internal examinations the cervical measurements are not checked, only the consistency and thickness.

Cervix Preparing for Labor

As we’ve mentioned, during pregnancy the cervix should remain hard, thick, and tightly closed to support the weight and prevent the baby from coming out. However, with the arrival of the third trimester, it gradually starts to thin in preparation for labor. During internal exams, the obstetrician checks the thickness to estimate how close labor is.

Thinning of the Cervix

There is no rule regarding cervical thinning, after all, each body reacts differently and each pregnancy is unique. However, it is usually possible to gauge how close labor is by this sign — the thinner the cervix, the closer labor tends to be.

Also read: Effaced Cervix – Is It Time for Labor?