maidezmoi, I copied and pasted this for you from Safe Kids UK. It indicates that flat head is nothing to worry about, and most doctors think the helmets are completely unnecessary.
"In most cases, a baby's head will return to a more natural, rounded shape once the child begins crawling and standing, but some parents prefer to purchase a helmet to help reshape their baby's head. Most effective when a baby is between the ages of 4-12 months, these helmets are worn almost continuously for a number of months in order to help remold the baby's head. Their use is a bit controversial, however, since many doctors feel that they are unnecessary.
There are some recommendations to help babies retain a rounded shape to their heads while they are developing.
THIS BIT IS IMPORTANT: Although it is vital to continue placing infants on their backs to sleep, it is safe to adopt a "tummy to play" habit during the baby's supervised waking hours.
Another idea is to place the baby to sleep with his head at alternating sides of his cot, encouraging him to tilt his head in various directions to view the room. By finding ways for the baby to take the pressure off the flattened area, the head will grow in a uniform manner. As babies continue to grow and develop, their increased mobility and physical capabilities will ensure that their heads do not regress to the earlier flattened shape.
Flat head syndrome is a purely cosmetic issue; it in no way impacts the child's brain or intellectual development. Nonetheless, parents worldwide are expressing their concern and seeking the advice of specialists. Craniofacial physicians have noticed a dramatic increase in inquiries from worried parents over the past few years. While most cases of flat head syndrome correct themselves, if you are concerned, be sure to consult with your child's pediatrician.