Twin
![Marian and Vivian Brown, identical twins, photographed by Christopher Michel](/Images/godic/202501/08/Marian_and_Vivian_Brown4615.jpg")
![Eight-month-old sororal twins napping](/Images/godic/202501/08/Being_a_twin_means_you_always_have_a_pillow_or_blanket_handy4615.jpg")
![Comparison of zygote development in monozygotic and dizygotic twins. In the uterus, a majority of monozygotic twins (60–70%) share the same placenta but have separate amniotic sacs. In 18–30% of monozygotic twins each fetus has a separate placenta and a separate amniotic sac. A small number (1–2%) of monozygotic twins share the same placenta and amniotic sac. Fraternal twins each have their own placenta and own amniotic sac.](/Images/godic/202501/08/Identical-fraternal-sperm-egg.svg4615.png")
![Various types of chorionicity and amniosity (how the baby's sac looks) in monozygotic (one egg/identical) twins as a result of when the fertilized egg divides](/Images/godic/202501/08/Placentation.svg4615.png")
Twins are two offspring produced by the same pregnancy. Twins can be either monozygotic ("identical"), meaning that they develop from one zygote, which splits and forms two embryos, or dizygotic ("fraternal"), meaning that they develop from two different eggs. In fraternal twins, each twin is fertilized by its own sperm cell.