A double gestational sac, which is the presence of two gestational sacs in the uterine cavity, carries twins. Normally, a woman releases one egg each month during ovulation, which encounters sperm and is fertilized to form a single fertilized egg or to form identical twins. However, under the influence of certain factors, the ovaries may release two mature eggs and fertilize them at the same time, resulting in the formation of two different fertilized eggs. Since heterozygous twins come from parents who are genetically different, the two babies born are not the same. In identical twins, the genes from the parents are identical, so the twins have the same gender and external features. Pregnant women who are carrying twins need to be monitored more closely during pregnancy. Detection tests are scheduled earlier relative to single births, and the delivery of twins is usually scheduled earlier as well.