From the 4th week of pregnancy, under the effect of pregnancy hormones, cervical mucus coagulates at the level of the cervix to form the mucous plug. This mass of mucus seals the cervix and ensures its tightness throughout pregnancy, thus protecting the fetus from ascending infections. The mucous plug is in fact made up of mucins (large glycoproteins) which stop viral replication and block the passage of bacteria. It also has immunological properties leading to an inflammatory response in the presence of bacteria. Studies suggest that a mucous plug that plays poorly in its barrier function could increase the risk of preterm delivery (1).

Under the effect of contractions at the end of pregnancy (Braxton-Hicks contractions) then those of labor, the cervix matures. As the cervix moves, the mucous plug will then be released and be evacuated in the form of sticky, gelatinous, translucent, yellowish or brownish losses. Sometimes they are pink or contain small filaments of blood: this blood corresponds to the rupture of small blood vessels when the mucous plug detaches.

The loss of the mucous plug can be done gradually, as if it were crumbling, so that the mother-to-be does not always notice it, or all at once. It can take place several days before childbirth, the same day, or even during childbirth. It should also be noted that as the pregnancies progress, the cervix is ​​more elastic, the mucous plug is sometimes more abundant and therefore easier to spot.

The loss of the plug is not worrying: it is quite normal and shows that the cervix is ​​working. However, the loss of the mucous plug alone does not give the signal to leave the maternity hospital. This is an encouraging sign that labor is coming soon, but it will not necessarily start within an hour or days.

On the other hand, any vaginal bleeding of red blood or darker clots should prompt a consultation (2).

To announce the true onset of labor, other signs should accompany the loss of the mucous plug:

• regular, painful, rhythmic contractions of increasing intensity. If this is a first baby, it is advisable to go to the maternity ward when the contractions return every 10 minutes. For a second or third child, it is advisable to go to the maternity ward as soon as they become regular (3).
• the rupture of the water bag which manifests itself by the flow of a transparent and odorless liquid, comparable to water. This loss can be direct or continuous (there can then be a crack in the water pocket). In both cases, head to the maternity ward without delay because the baby is no longer protected from infections.