Twins, while becoming much more common, are still rare and people who don't have them probably don't know much about the biology behind twins. I know I didn't! So here is a short lesson.
Twins are either monozygotic (come from one zygote, or egg) or dizygotic (come from two different eggs). Monozygotes are identical, and fraternal twins are dizygotic. And just so everyone knows, twins can't be identical if one is a boy and one is a girl. The sperm that fertilizes the egg determines the gender so you would need two different sperm and two different eggs in order to have one of each. (Yes, I have been asked that question.)
Most of the time it is easy to tell whether a woman is expecting identical or fraternal twins. About 30% are identical while the remaining 70% are fraternal (with about 1/2 of fraternal twins being boy/girl). Fraternal twins will each have their own amniotic sac and their own placenta (termed diamniotic and dichorionic, or Di/Di). Identical twins, however, come in three different combinations. They can be Di/Di if the egg split within the first 3 days of fertilization. If the egg splits between 4 and 7 days the twins will have two different amniotic sacs but one fused placenta (diamniotic/monochorionic, or Di/Mo). The last combo is if the egg splits 8 days or after. In this case the babies will me monoamniotic and monochorionic, or Mo/Mo, with one amniotic sac and one placenta.
Raising Twins or Wikipedia for some good information if you want to read more into that.
My babies are Di/Di, but this doesn't necessarily mean that they are fraternal. I think I read somewhere that I have a 30% chance that they are identical and unless they are different sexes we won't know until they are born whether they are identical or not. And even then it may be difficult to tell and a DNA test may be needed. I've been asked if I would do the DNA test to find out and I don't know that it really matters whether they are identical or not, just as long as they are healthy. Though in the long run it may be good to know because if one has some sort of genetic disease and they are identical then the other will have it too and we can do what we need to do for both of them.
I am currently 16 1/2 weeks along and this is what fraternal twins tend to look like in utero at 16 weeks:
I have to say that I look much bigger than the drawing!
If you guys have any questions please ask away! I am an open book and not scared to share my experiences no matter how embarrassing. =)