Mirror-image twins occur only in identical twins. In approximately 23 percent of identical twins the egg splits later than usual, most often day seven or beyond. The original right half of the egg becomes one individual and the original left half becomes the other. These twins will often have "mirror images" of their features, such as hair whorls that run clockwise in one and counter clockwise in the other, a birthmark on the right shoulder of one and the left shoulder of the other, etc. There is no specific test for determining if twins are mirror-image. The determination is made by observation only, and the twins must be monozygotic, or identical.