Of course you can't help comparing your baby's accomplishments to those of other tots, but when the one who's leaping ahead happens to be his twin, you're bound to have double the anxiety. Why is one twin not like the other? Because no two babies are the same, even though they were womb-mates. So don't worry if your twins aren't hitting their early developmental markers at the same time.
Fraternal twins are just like any brother and sister pair. Did you and your sibling roll over or walk at exactly the same age? Probably not. Though identical twins may have a similar rate of development, there are many reasons why there could be disparities.
Twins with very different birth weights often have different development schedules. And since multiples are more likely to be born preterm, there can be medical conditions present that affect their early development.
Keep in mind that slight differences don't mean much. With preemie and low-birth weight babies, expect to see some delays in milestones like holding up their heads, smiling, rolling over, and grasping objects. But if one twin is significantly lagging behind the other, let your pediatrician know.