In prepubertal girls, the functional squamocolumnar junction is present just within the cervical canal. Upon entering puberty, due to hormonal influence, and during pregnancy, the columnar epithelium extends outward over the ectocervix as the cervix everts. Hence, this also causes the squamocolumnar junction to move outwards onto the vaginal portion of the cervix, where it is exposed to the acidic vaginal environment. The exposed columnar epithelium can undergo physiological metaplasia and change to tougher metaplastic squamous epithelium in days or weeks, which is very similar to the original squamous epithelium when mature. The new squamocolumnar junction is therefore internal to the original squamocolumnar junction, and the zone of unstable epithelium between the two junctions is called the "transformation zone" of the cervix. Histologically, the transformation zone is generally defined as surface squamous epithelium with surface columnar epithelium or stromal glands/crypts, or both.