I thought I would post this since I had to deal with this with our 2nd baby. I had no idea about it before and if it helps someone, then great! DS#1 for his first 2 months, until the pediatrician caught it, was basically putting his head on one side when laying down, or being held, or sitting in the bouncer. He also didn't like tummy time so I didn't push it. I didn't notice that he was always laying his head on the one side. Because of this, he developed a flat spot on the side of his head and we had to get a helmet and go to physical therapy to strengthen his neck muscles. His head is better but not 100% perfect.
I am now paranoid about this for DS#2 and am always watching what side he is laying on. At night I switch where he is in the crib so he gets time on both sides of his head (they will typically face w head towards middle of the room and not towards the wall).
I've seen a fair amount of babies with flat spots so keep a watch out and ask your ped if you are concerned. You only have until about month 9 to correct any flat spots before the head hardens.
Hope this helps someone out there. I was sad when I found out that my son had a flat spot and I maybe could have prevented it if I knew to look for the signs.
Re: Flat spots on baby's head
BFP # 1 - 12/19/09 EDD 08/27/10 - D&C 1/26/10 @ 9w5d
BFP # 2 - 06/05/10 EDD 02/17/11, DS1 born on 2/14/11
BFP # 3 - 04/10/13 EDD 12/21/13 - D&C 05/15/13 @ 8w4d
BFP # 4 - 07/27/13 EDD 04/08/14 - CP 07/29/13
BFP # 5 - 09/14/13 EDD 05/28/14, DS2 born on 5/22/14