A hormone deficiency in pregnancy raises a child's risk of attention deficit hyperactivity disorder by a quarter, according to new research.

An under-active thyroid which results in low levels of key, body-regulating chemicals in mums-to-be during the first three months can interfere with a baby's brain development, according to the study.

The vital hormones are produced in the thyroid gland in the neck and are known to influence foetal growth.

Investigators have suspected disruptions in their production - known as hypothyroidism - leads to ADHD.

The discovery could offer hope of a screening program to identify expectant mothers who require hormone supplements.

Their children could also potentially benefit from earlier surveillance for signs of inattention, hyperactivity and difficulty focusing on a task.

Swift intervention helps manage ADHD and make it easier for children to succeed in the classroom and learn social skills.

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ADHD the most common neurodevelopmental disorder in children - affecting almost one in ten across the world. It can last into adulthood.

The study found children whose mothers were diagnosed with hypothyroidism shortly before or during the early stages of pregnancy were 24 percent more prone.

And boys born to hypothyroid women were four times more vulnerable than girls whose mothers had the condition. ADHD rises threefold in males.

Lead author Professor Morgan Peltier, a gynaecologist at New York University, said: "Our findings make clear thyroid health likely has a much larger role in foetal brain development and behavioural disorders like ADHD than we previously understood."

It may also be associated with other brain development problems like epilepsy, cerebral palsy and difficulties with speech.

Prof Peltier, who is based at the city's Winthrop Hospital, said: "It increases the risk of other lifelong problems such as substance abuse, relationship problems, poorer job and academic performance, riskier driving, criminality and suicide."

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Interestingly, once a pregnancy had gone beyond three months, a hypothyroidism diagnosis had little effect on children.

By this point the foetus has begun to produce its own thyroid hormones and so is less susceptible to its mother's deficiencies, explained Prof Peltier.

Unlike previous research in Europe, it included people of diverse ethnic backgrounds and tracked the children for nearly two decades.

This enabled the team to better capture cases of ADHD as participants aged and progressed through school.

The team analysed their medical records and collected key information about the mothers - including age during pregnancy, race and household income.

All children were evaluated for ADHD using the same criteria - preventing inconsistencies in incidence. Overall, 16,696 children were diagnosed with ADHD.

The results are strong enough to warrant careful monitoring of pregnant women with low thyroid hormone levels, said Prof Peltier.

Symptoms of ADHD typically appear at an early age and become more noticeable as a child grows.

These can include constant fidgeting, poor concentration, excessive movement or talking, risk taking, careless mistakes and forgetfulness.

Most cases are diagnosed between six and 12 years old. The exact cause is unclear but is believed to involve genetic mutations that affect the brain.

There is no cure. A combination of medication and therapy is usually recommended to relieve symptoms and make day-to-day life easier.