I have polar body twins. My son's fertilized egg was much smaller than my daughter's. They thought it had stopped developing very early on and told me I was going to miscarry. Two weeks later the embryo was larger and he had a heartbeat. It took a while for him to catch up in size.
They were born early, at 29 weeks, and so they under went a lot of testing including genetic. I was told that they were indeed polar! I had no clue that even existed until I was told!