A study has linked a small number of cases of cerebral palsy to antibiotics given to women in premature labour.
The UK study found 35 cases of cerebral palsy in 769 children of women without early broken waters given antibiotics.
This compared with 12 cases among 735 children of women not given the drugs. Advice is being sent to the study's 4,148 mothers and a helpline set up.
Medical experts stressed pregnant women should not feel concerned about taking antibiotics to treat infections.
These findings do not mean that antibiotics are unsafe for use in pregnancy
The Royal College of Obstetricians and Gynaecologists
The Oracle study was the largest trial in the world into premature labour and was set up to investigate whether giving antibiotics - which might tackle an underlying symptomless infection - to women with signs of premature labour would improve outcomes for babies.
One in eight babies in the UK is born prematurely and prematurity is the leading cause of disability and of infant death in the first month after birth.
In 2001, ORACLE found the antibiotic erythromycin had immediate benefits for women in premature labour (before 37 weeks gestation) whose waters had broken. It delayed onset of labour and reduced the risk of infections and breathing problems in babies.
Pregnant women should not feel concerned about taking antibiotics to treat infections
Chief Medical Officer Sir Liam Donaldson
Erythromycin and the other antibiotic studied - co-amoxiclav - showed no benefit or harm for the women whose waters were still intact, however, and doctors were advised not to routinely prescribe them in such circumstances.
To study the longer-term outcomes, the Medical Research Council-funded scientists followed up the children seven years later.
Unexpectedly, both antibiotics appeared to increase the risk of functional impairment - such as difficulty walking or problems with day to day problem solving - and treble the chance of cerebral palsy in the children of the women whose waters had not broken.
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Of the 769 children born to mothers without early broken waters and given both antibiotics, 35 had cerebral palsy, compared with 12 out of 735 whose mothers did not receive antibiotics in the same circumstances.
The reasons behind this link are unclear, particularly as there was no increased risk of cerebral palsy in women whose waters had broken.
The researchers believe cerebral palsy is unlikely to be a direct effect of the antibiotic but rather due to factors involved in prolonging a pregnancy that might otherwise have delivered early.
Researcher Professor Peter Brocklehurst of Oxford University said: "We have a suspicion that infection is implicated in premature labour.
"Antibiotics may merely suppress levels of infection to stop preterm labour, but the baby remains in a hostile environment."
Infections during pregnancy or infancy are known to cause cerebral palsy.
In a letter to doctors and midwives advising them about the findings, Chief Medical Officer Sir Liam Donaldson says: "Pregnant women should not feel concerned about taking antibiotics to treat infections.
"It is important to note that these women had no evidence of infection and would not routinely be given antibiotics."
Where there is obvious infection, antibiotics can be life-saving for both mother and baby, the CMO says.
The Royal College of Obstetricians and Gynaecologists said: "These findings do not mean that antibiotics are unsafe for use in pregnancy. Pregnant women showing signs of infection should be treated promptly with antibiotics."
Cerebral palsy can cause physical impairments and mobility problems.
It results from the failure of a part of the brain to develop before birth or in early childhood or brain damage and affects one in 400 births.
A helpline is available for study participants on 0800 085 2411 between 0930 and 1630 BST. NHS Direct has information available for other members of the public.
A spokeswoman from the special care baby charity Bliss said: "This highlights the importance of fully understanding both the immediate and long-term impact of the care and treatment that both mother and baby receive at this crucial time."