The progress of the descent of the head can be judged by watching the perineum. At first there is a slight general bulge as the woman strains.when the head stretches the perineum the anus will begin to open, and soon after this the caput will be seen at the vulva at the height of each contraction. Between contractions the elastic tone of the perineal muscles will push the head back into the cavity of the pelvis. ----- head visible on vulval gapping .
The perineal body and vulval outlet become and more stretched until eventually the head is low enough to pass forwards under the subpubic arch.