My son's development was really slow as a baby/ toddler. He had physio to help him walk and he also had glue ear which delayed his speech. We are fairly confident there is nothing wrong neurologically with my son. He had orthopedic problems.
However at the age of seven he has now caught up and is enjoying school and has loads of friends. He can ride a bike and swim.
My five month old daughter development is quite advanced. She is sitting well without support and making attempts at crawling. She certainly wont be needing the child physio.
However my children both have the same father and have been parented in exactly the same way. It goes to show that development is luck and nothing to do with how good someone is at mothering.
It has been a surprise how different two babies are.