Babies born after more than 42 weeks of pregnancy are at much higher risk of behavioural and emotional problems such as attention deficit hyperactivity disorder in early childhood, according to new research.
"Post-term children have a considerably higher risk of clinically relevant problem behaviour. They are more than twice as likely as term-born children to have clinical ADHD," concluded a study of 5,145 Dutch babies published in the International Journal of Epidemiology.
Births are classified as pre-term (before 37 weeks), term (37-42 weeks) or post-term (after 42 weeks).
Post-term babies are at just over twice as great a risk as term-born children to have overall problems behaviour, and almost two and a half times as likely as them to have ADHD, the research found.
Researchers led by Dr Hanan El Marroun of the Erasmus medical centre in Rotterdam examined 5,145 babies born in the city between 2001 and 2005, of whom 382 were post-term. They based their conclusions on two questionnaires filled in by the babies' parents when they were toddlers. "Both post-term and pre-term babies were at higher risk of behavioural and emotional problems at 18 and 36 months", the study says.
The results were not explained by the mother's weight and height, ethnic origin, family income, alcohol consumption, smoking, level of education or the mother's psychapothology in mid-pregnancy.
The existing evidence on this issue is mixed. One previous study suggested that post-term birth raised the risk of neonatal encephalopathy and death in the baby's first year of life, but another found that, when examined when they were two, such babies were no different from other children in terms of intelligence, physical milestones and illness rates. But another, more recent paper found that, by the age of five, 13% of post-term babies had a neurological or developmental problem.
Internationally some 5%-10% of births are post-term, even in industrialised countries. England has one of the lowest rates, at 3.6%.
A total of 668,195 babies were born in NHS hospitals in England in 2010-11 and 24,167 (3.6%) of those were born at or after 42 weeks, NHS statistics show.
Post-term babies are most common among women who are induced. Of the 114,783 women whose labour started with them being induced and where the length of gestation was known, 13,643 (11.9%) of the babies were post-term. By comparison, though, just 8,188 (2.3%) of the babies born to 361,323 women whose labour began spontaneously were post-term, while just 1,010 (1.6%) of the children of mothers whose onset of labour was by Caesarean section had been in the womb for at least 42 weeks.
The Royal College of Obstetricians and Gynaecologists recommends that "women with uncomplicated pregnancies should be offered induction of labour between 41 weeks (0 days) and 42 weeks (0 days) to avoid the risks of prolonged pregnancy."
Neena Modi, a professor of neonatal medicine at Imperial College London and the Royal College of Paediatrics and Child Health's vice-president for science and research, said the study's raised important questions about how prolonged pregnancy might adversely affect a baby.
Modi said its findings were "perfectly plausible [because] we know that birth after your due date is associated with a whole range of problems. If your baby stays in the womb for too long they are more likely to be a stillbirth, or weigh too little or be more likely to have a neurological disorder, because the longer a baby stays in the womb the more likely the placenta is to stop functioning normally."
But the NHS does not need to rethink when it induces women who are overdue, Modi added. "This research is interesting and important, and reinforces that the current guidance is well-based. It should, though, perhaps lead to a greater awareness among women about their risk of prolonged pregnancy. If they are offered to be induced, they should take it. If that's what their obstetrician is saying, that's good advice."
El Marroun said that an internet-based group of women, who call themselves the "10-month mammas", advocate natural birth, even when the child is past its due date. "These pregnant women should be educated about the potential harmful effects for their child", she said.