Identical twins are very similar in sex, appearance, and genetic material. Heterozygotic twins may be of different sexes or of the same sex and differ in appearance, looks, and genetic material in all respects. Heterozygotic twins are different in gender and may be identical or heterozygous in the case of the same gender. It is very difficult to distinguish between them during pregnancy and genetic chromosomal testing is necessary to determine whether they are identical or dizygotic. It is not possible to distinguish between identical and heterozygous twins by ultrasound because about 1/3 of identical twins divide within 3 days after fertilization, and this type of twin is the same as heterozygous twins in that the blood supply to the two placentas is not connected, so it cannot be distinguished by ultrasound when the mother is in the abdomen.