One Egg Only
There are two possible or known methods of how a mother can give birth to multiple babies at once. The first one is known as monozygotic pregnancy. This means that one fertilized egg will split and form 2 separate eggs. Then one egg will split again.
Dizygotic pregnancy means that multiple eggs will be fertilized at once. It is far more common of the two and most women bring twins in this way. Believe it or not, Olivia was able to bring triplets into this world thanks to the first method. However, this also means that they are not the same!