Fraternal twins is hereditary, since your body is releasing more than one egg at a time. Identical twins are spontaneous, and no one knows why the zygote splits. My grandmother had 17 children total, and in that 17, she had three sets of fraternal twins. I am currently 30 weeks pregnant with identical twins, and am fearful of having another set of fraternal twins, since it runs in my family.
Thanks for your reply! Just curious, aside from family history, do you think your diet played a role as well? I've been reading up on this, and i'm very curious as to whether the specific things we eat during conception, ie. dairy, yams, bananas,etc specific foods - as well as a certain age group - do they really influence spontaneous twins conceptions? Were you eating any particular foods? And if this is a second pregnancy, was your diet any different with the first?
I do not believe that diet plays a role in twin conception. This was my second pregnancy, and I did not change my eating habits. I have read that age plays a role, however. I am 36 years old. :-)