Our 26 Year Old Daughter Is Pregnant With Twins. It

Our 26 year old daughter is pregnant with twins. It appears they are fraternal as there are two separate placentas very close together. She is in extremely great health and does everything exactly as she should however baby b is smaller than baby A. The doctors refer to it as significantly smaller as it is about one week difference in growth, it has been this way since the first ultrasound at 6 weeks. They do follow the same growth pattern, just a week apart. So it's not like one keeps growing and the other doesn't as on a scale they are just at different heights. Same heart beat or very close. Our daughter has been told. Because of the significant size difference it could be a chromosome abnormality. The Dr suggests an amnio. It has added a lot of worry and stress but our daughter looks and feels great and we pray for two healthy babies. My question: is it normal that fraternal twins measure different, and is there a need to worry? Thanks for your response