Inflammation of the placental membranes may increase the risk of cerebral palsy (CP), research suggests.
The cause of cerebral palsy is not always known
The University of California found that the condition, chorioamnionitis, was four times more common in mothers who gave birth to a child with CP.
CP covers a wide range of physical problems in which poor brain development or damage has affected the ability to control muscle and movement.
The study is published in the Journal of the American Medical Association.
CP is the most common cause of severe physical disability in children in developed countries. It occurs in up to 2.4 per 1,000 live births.
The condition is associated with premature babies, babies with a low birth weight, and multiple birth.
But many cases occur in babies that have gone to full term, who doctors would not have considered to be at risk.
The researchers examined data on 327 babies, of which 109 were born with moderate to severe CP.
They found that chorioamnionitis was noted in 14% of the CP babies, but in only 4% of healthy babies.
Chorioamnionitis is a bacterial infection of the placental membranes which enclose the amniotic fluid bathing the developing foetus.
The condition can cause blood poisoning in the mother, and many lead to premature birth and serious infection in the baby.
It can be treated with antibiotics, but often it is necessary to deliver the baby to prevent further complications.
Hope for the future
Lead researcher Dr Yvonne Wu told BBC News Online that it was possible that the inflammation triggered by the infection sparked an immune response in the foetus which either directly damaged the brain, or rendered it more vulnerable to injury if complications developed in the pregnancy.
"There have been a lot of improvements in obstetrics and gynaecology, and in neonatal care in the past few decades, but we have not been able to reduce the prevalence of CP.
"If inflammation does play a role in CP then hopefully it should be possible to devise some sort of intervention to prevent it in future."
Richard Parnell, head of research for the cerebral palsy charity Scope, told BBC News Online that little was known about CP.
He said: "Cerebral palsy is an umbrella term for a number of complex conditions, and causation is a very, very difficult issue.
"One research project is not going to solve the problem, but it does seem that maternal infection is in some way linked to the condition."