Originally Posted by Bonnie
Two sacs = fraternal
Corrected: Okay, I just saw Amy's post. This is news to me but I'm going with her info because she's the pro and knows this stuff. I've always heard that fraternal twins come from two separate eggs/two separate sacs and identical twins are one egg that split but share the same placenta. But, I haven't read up on this in a while, so I'll go with Amy since this is her business and she's around all this up-to-the-minute....
If the embryo splits before implantation into the uterine wall, there will be 2 placentas and 2 amniotic sacs. Fraternal twins ALWAYS have separate placentas and sacs but the placentas can grow together and look like one at delivery. So DNA testing is required to know 100 % for sure if the twins are same sex, look alike, and have the same blood type.