Identical twins, which result from a fertilized egg dividing into two egg cells, can be detected as early as around 6 weeks of pregnancy and as late as 14 weeks of pregnancy. Identical twins can be detected at different times due to the timing of the fertilized egg division. There are 4 types of twins depending on how early or late they divide. If they divide within 3 days of fertilization, they are called double chorionic bilamellar twins; if they divide between 4 and 8 days of fertilization, they are called monochorionic bilamellar twins; if they occur between 9 and 13 days of fertilization, they are called monochorionic monoamniotic twins; and if they occur after 13 days of fertilization, they are called conjoined twins. The later the fertilized egg divides, the later it will be detected, usually at 6-14 weeks of gestation by ultrasound. If a conjoined twin is detected, the fetus will usually need to be surgically separated after birth, preferably as early as possible during pregnancy, and abortion will be induced if necessary.