One of the first known creatures with teeth is a 400 million year old fish called Romundina stellina1 .Rather than having individual teeth, it had a tough, bumpy plate that it used to crush its food, and for this reason was a rather lucky fish because it never had to put up with an eruption cyst.

An eruption cyst is a soft tissue sac filled with fluid that can develop in the oral cavity of a nascent tooth, appearing before the tooth erupts. They are often blue or clear, dome-shaped, and most commonly found in children whose adult teeth are coming through. We aren’t sure why they appear, but it may be to do with early childhood caries (baby bottle tooth decay), trauma to the affected area caused by tough food or brushing, lack of space for the tooth to erupt, or genetic predisposition2.

Eruption Cyst

An eruption cyst. Image from J House Vlogs

Eruption cysts are nothing to worry about. They usually appear around four days before the tooth develops, are ruptured when it bursts through, and then heal after a few more days. A small study has shown that most eruption cysts appear in the maxilla—the bone that forms your upper jaw—where your primary molars are located.

If an eruption cyst is painful, it may be infected—a dentist will be able to diagnose whether the cyst is infected, and surgically remove it if necessary. This isn’t common through—a 2017 study of 53 patients with eruption cysts showed that only 7 of them required surgical treatment for the cyst, with the remaining 46 needing no intervention.2 Regardless, if your child’s eruption cyst is bleeding, or if they are finding it to be painful, take them to the dentist. You shouldn’t try to pop the cyst yourself, as this could be extremely painful for your child, and cause infection.

Eruption cysts can be confused with another type of dental cyst—the dentigerous cyst. These are also fluid-filled sacs that develop over unerupted teeth. But while an eruption cyst is popped and heals after the tooth comes through, a dentigerous cyst often appears when the tooth can’t erupt properly, which may require dental treatment to fix. A dentigerous cyst is often removed surgically, or through a draining technique called masupialization.


  1. Sid Perkins, 2015, Ancient fish sheds light on how teeth evolved, Science Mag
  2. Emine Şen-Tunç, Hatice Açikel, Işıl Şaroğlu-Sönmez, Şule Bayrak, and Nuray Tüloğlu, 2017, Eruption cysts: A series of 66 cases with clinical features, Med Oral Patol Oral Cir Bucal