Before I got pregnant with twins, I too, thought there were only two types: 1- two sacks = fraternal and; 2 - one sack = identical. There are actually four types of twins and it all depends on when an egg splits or if there are two eggs and two sperm.
We have Dichorionic/Diamniotic twins - the safest kind of twins to carry, thank goodness, because I've been a nervous wreck the whole time already! Di/Di twins each have a sack and a placenta. Eight times out of nine, they are due to two eggs and two sperm meeting and making two embryos that both decide to hang out and grow into two non-identical babies, in which case, they are fraternal twins. One out of nine times, Di/Di twins are the result of one egg being fertilized by one sperm, and splitting before it implants, turning into two embryos. Again, the two embryos then decide to tuck in and grow into two identical babies.
I met some beautiful Di/Di twins this weekend who are four weeks old - so precious! And they look enough alike that the mom is getting their genetics tested to confirm whether they are Di/Di identical or Di/Di fraternal. We also have two other sets of Di/Di twins in our twin club who are confirmed identical.
Below are illustrations of mono/mono twins, mono/di twins, and two types of di/di twins. Ours are the bottom right example.
An example of Di/Di twins (ours!)
An example of mono/di twins (like my cousin's babies!)
An example of mono/mono twins
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