Statistics: Probability. Identical Twins

Sexes of Twins
bb b/g g/b g/g
5 0 0 5 Identical Twins
5 5 5 5 Fraternal Twins

After having a Sonogram, a pregnant woman learns that she will have twins. What is the probability that she will have identical twins?

*I know the answer is 1/3 or 0.3333, but I cannot seem to figure this out! I don't see the concept of getting this answer. I could find other answers easily concerning conditional probability, but I'm stuck on this one, if anyone can help me, i would really appreciate it =) Thank you