The mucus plug is a jelly-like material that seals the uterus, protecting fetal kittens from outside contaminants during pregnancy.
Loss of the mucus plug occurs during the first stage of labor. If you are very attentive to your cat around the time that labor is starting, you might see it around the vulva. However, the queen often grooms excessively as part of early labor as well, so the mucus plug might be ingested as part of the grooming process and not be seen.
It is more important to be aware of other signs of early labor. These include restlessness and nesting behavior where the queen prepares a place to have her kittens. This first stage of labor can last up to 12 hours in most cats, with some a little longer.
If you’re not sure how close your cat is to going into labor, this pregnancy calculator might help. It also reviews stages of labor and what to look for.