How far along are you? It's normal for babies legs to be short in the beginning. They will grow longer by the end of your pregnancy. Some Dr do ultrasounds every visit. Alot depends on weather or not insurance covers it. If you are really concerned, ask your Dr at your next visit. Best of luck.
I am 21 weeks and using the same doctor as with my first but the pregnancy and doctor visits are so different.
At 21 weeks, the legs should be just starting to grow longer. I would definitely talk to him on your next appointment.