About one in five babies aged younger than 6 months has this problem, experts say. It has become more common in the wake of campaigns urging parents to place babies on their backs when they sleep, to reduce the risk of sudden infant death syndrome (SIDS).
The use of these expensive helmets to treat flattened heads is controversial and there has been little research into their use, the study authors noted.
The new study was published May 1 in the online edition of BMJ.
Researchers in the Netherlands looked at 84 babies who had moderate to severe flattening of the head. Starting when they were 6 months old, half of the babies wore a custom-made, rigid, closely fitting helmet for 23 hours a day over six months. The others received no treatment.
By the time the children were 2 years old, there was no significant difference in the degree of improvement in head shape between the two groups, nor in the number of them who made a full recovery to normal head shape -- 25.6 percent of those who wore helmets and 22.5 percent of those who did not, according to a journal news release.
The parents of babies who wore helmets reported numerous side effects, including skin irritation (96 percent), an unpleasant smell (76 percent), sweating (71 percent) and pain (33 percent). Also, 77 percent of the parents said the helmet interfered with them cuddling their baby.
When the children reached 2 years of age, parents in both groups reported that they were generally satisfied with the shape of their child's head. Among parents whose babies wore helmets, the average satisfaction score was 4.6 out of 5, and among those whose babies had not received the treatment the score was 4.4 out of 5, the study found.
"Based on the effectiveness of helmet therapy, and the high prevalence of side effects and high costs, we discourage the use of a helmet as a standard treatment for healthy infants with moderate to severe skull deformation," concluded researcher Renske van Wijk, at the University of Twente, in Enschede, and colleagues.
According to the American Academy of Pediatrics, the vast majority of these cases can be treated with physical therapy and noninvasive measures.
The American Academy of Pediatrics has more about flat head syndrome.