http://www.epinions.com/content_###-###-####
H.,
This is an article that talks about it. Basically, your doctor is indicating that since you place your baby on his/her back to sleep, the head is being distorted. This usually means the doctor wants to make sure you alternate the way the baby lays his/her head down each night. Also, they recommend more tummy time during the day. When my baby was 2 months old, they made mention of this and said that at that stage, I should just "watch it" by alternating the way I laid her down each nap time. But, if you don't "watch it" and the head continues to distort, you could end up having to put the baby in a helmut to control the shape of the head.
At this point, I would call your pediatrician back and get clear instructions as to how YOU are to proceed. Make sure you understand the instructions. I would think that they would just want you to alternate the head position when you lay the baby down to sleep (every time - not just at night). So,if someone else is watching her/him during the day, you need to convey the pediatrician's directions to your caregiver as well.
Good luck!
L.