Hi Cynthia, glad to see you are still around

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They can't be 100% certain while you are still pregnant, because it can be hard to tell from ultrasounds whether there in fact 2 sacs or just one with a membrane between the babies (especially if you didn't have an ultrasound until 13 weeks like me LOL). Placentas often fuse together making them look like one, and ID twins can actually have seperate sacs and placenta if they split very early. In our case, we were told that our girls were most likely ID (93% likely to be precise!). The only way to tell for sure is with DNA testing. I guess the exception would be monoamniotic/monozygotic twins were there is no seperating membrane at all.
We had our girls tested because they are part of some twin research, and it's important for the researchers to know if they are ID or not. It doesn't really matter in every day life, it's just nice to know IYKWIM.