Mothers who experience extreme stress during or after pregnancy may be putting their children at increased risk of autism, a new study suggests. 

Researchers in Japan found that the children of women who experienced perinatal depression – defined as psychological distress around the time of pregnancy – were more likely to show autistic traits. 

This effect was seen most strongly in girls, who were also more likely to be born underweight and had weaker bonds with their mothers.

However, the researchers noted, boys showed significantly more autistic–like traits than girls, regardless of their mother's mental status. 

The scientists, from Tohoku University, concluded that it was 'critical' that doctors identified mothers with perinatal depression early in order to reduce the risk of autism.

In order to reach their findings, the researchers analysed medical data of over 23,000 mother–child pairs, assessing depressive symptoms during early and mid–pregnancy and at one month after birth.

To explore the biological mechanisms underlying their findings, the team established a stress model in mice.

This was done by raising stress levels in pregnant mice.

These affected mice showed signs of depression and were less caring as mothers. 

Moreover, their female offspring exhibited autism-like behavioural patterns, including increased self-grooming and reduced social awareness.

Further tests found that stressed mouse mothers had lower levels of oxytocin – the so–called love hormone – in a key brain area. 

Similarly, their female offspring had fewer receptors for this hormone in the same brain region, inhibiting mother–infant bonding. 

Publishing their findings in the journal Molecular Psychiatry, the researchers said: 'Developmental delays appear to be more common in girls with autism spectrum disorder.  

'These preliminary observations underscore the importance of screening and supporting maternal mental heath.' 

However, the researchers acknowledged some limitations to their study including a relatively small sample size which could not account for cultural differences. 

A previous research paper, published in the British Medical Journal, has also highlighted a link between taking antidepressants during pregnancy and autism.

The study on more than 140,000 pregnant women found that taking antidepressant tablets during the second and third trimester, to treat depression, almost doubles the risk of children developing the spectrum disorder. 

According to the NHS, some mood changes are normal during pregnancy and antenatal depression is not uncommon. 

Signs and symptoms include feeling sad or tearful a lot of the time, feeling irritable, a change in eating habits, negative thoughts about your baby and having problems concentrating or making decisions. 

It comes as new research this week found that girls may be just as likely to be autistic as boys, but less likely to be diagnosed during childhood.

Dr Caroline Fyfe, lead author of the study, said: 'Our findings suggest that the gender difference in autism prevalence is much lower than previously thought, due to women and girls being underdiagnosed or diagnosed late.'

Experts said the findings are 'timely' and 'important', supporting the theory that autism is markedly undiagnosed in women. 

The findings come following Wes Streeting's call for an independent review into soaring demand for mental health, ADHD and autism services after a warning of an 'over diagnosis' of conditions, in December last year.

As of March last year, figures show there are around 91,000 people aged between 10 and 25–years–old currently awaiting a referral.

Some experts have repeatedly blamed growing awareness of the spectrum disorder, which was only widely diagnosed as its own condition this century.