In an article on The Scientist, it talks about how through genetic testing they determined that two year old twins were actually semi-identical twins. This means that they have the same genes from their mom, but have different genes from their dad. So during fertilization, two sperm cells fertilized in the same egg simultaneously. That means that there is another type of twin, that is not fraternal, nor identical. They are technically sesquizygotic. This is very rare, and the last known case was in 2007. The DNA from the egg, and the 2 sperm cells divides into 3 cells, but only 2 contain enough DNA to develop into viable embryos. They have not determined all the health risks of sesquizygosity, but they know that there is an increased risk of cancer in reproductive organs.
Monozygotic twins are identical twins, meaning that they develop in one zygote, and then they split and form two embryos. There's also Dizygotic twins(fraternal), which are twins that have developed from separate eggs, and each egg is fertilized by its own sperm cell. So because in a sesquizygotic pregnancy is different because 2 sperm fertilize in the same egg at the same time. So they're neither identical nor fraternal. The twins were found during a 6 week ultrasound. The doctors saw a single placenta in the mothers ultrasound and positions of amniotic sacs, that indicated she was having identical twins. However, at the 14 week ultrasound they saw that the twins were female and male, so they could not be identical twins. So that is when they genetically tested them, and found they were identical on their maternal side, but only 78% identical on their fathers side-making them sesquizygotic twins.
I read this article because I find twins so interesting, and I have twins in my family so I could possibly have twins in the future. I think it is so crazy that two people can have identical DNA, but still be so different.