If you’re wondering at what stage twins form, the answer is during the early stages of pregnancy. Identical twins are formed when a single fertilized egg splits into two embryos, and fraternal twins occur when two eggs are fertilized by separate sperm. Signs of twins can include a higher level of hCG (pregnancy hormone) in your blood, severe morning sickness, and a larger-than-usual uterus size. As for the position of twins in the womb, this can vary. Twins can be positioned side by side or one on top of the other. In some cases, one twin may be in a breech position (feet first) while the other is in a head-down position. Your doctor will monitor the position of your twins throughout your pregnancy to ensure a safe delivery.