Thank you-that was my 1st thought as well-around 30 years ago, a co-worker of mine had a small ovarian tumor removed that proved to be tissue and bone from a twin absorbed before any significant development. Interestingly, she had twin siblings, and her 3d pregnancy produced twin boys...
Twins definitely run in families.
My great aunt had 17 children but only 12 pregnancies. Triplets, 3 sets of twins, and the rest were single births. She was a busy woman.