Prior to the onset of labour, the junction between the lower and upper uterine segments is a slightly thickened ring.

"Scanzoni's second os": (hourglass uterine constriction) A constriction at the junction of the thinned lower uterine segment with the thick retracted upper uterine segment.

In abnormal and obstructed labours, after the cervix has reached full dilatation further contractions cause the upper uterine segment muscle fibres myometrium to shorten, so that the actively contracting upper segment becomes thicker and shorter.