Listen up, oh reader: getting stressed or anxious during late pregnancy is not just bad for you, it is also bad for your baby. There is overwhelming evidence that attention deficit hyperactivity disorder (ADHD) and behaviour problems are caused by maternal stress or anxiety during pregnancy.

The story starts with the relationship between the hypothalamus in the brain, the pituitary gland and adrenaline, known as the HPA axis. As every GCSE pupil knows, it prepares us for fight or flight when confronted by threat. By measuring the hormone cortisol, we can identify how much the HPA axis has been activated.

If you are constantly placed in threatening circumstances, your HPA is permanently on. The result is high levels of cortisol which not only reduces your life expectancy but turns you into a gibbering wreck - anxious, depressed, irritable, prone to inattention and frantic fire-fighting in all departments of life. Your heart races, your palms sweat and your eyeballs start extending.

Alternatively, in some cases, having the HPA jammed on means your system closes down and cortisol levels are abnormally low. You are so used to feeling threatened that a mad axeman could run into the classroom and you would say: “Hey dude, how’s it going?”

A large body of evidence now proves that your baseline cortisol level, the thermostatic position to which you return when a threat has passed, is set by early childhood care. Having a depressed mother who does not respond to your needs, or being in a dodgy daycare set-up as a baby or toddler, creates a permanent insecurity which becomes electro-chemically enshrined as abnormally high or low baseline cortisol levels.

However, the latest evidence shows that the thermostat is also set pre-natally. Several groups of children have been followed from before birth into late childhood. At seven to 10, children whose mothers were stressed or anxious in pregnancy, were significantly more likely to have ADHD, behavioural problems and anxiety.

All these disorders correlate with abnormal cortisol levels. Pre-natal stress activates the mother’s HPA, jacking up cortisol levels which are passed via the placenta to the foetus - maternal and foetal levels correlate. In 2005, the clinching study proved that offspring of mothers who had been anxious or depressed had abnormal cortisol levels by the time they were 10.

The main implication is that, if the Government really does give a damn about rising crime, disorder in the classroom and so on, maternity leave needs to be greatly extended.

As regards the hyperactive nippers in your classroom, the message is not to blame their genes but militate for therapy for them. Or you can recommend they join the 340,000 children on Ritalin-like medication Oliver James is a child clinical psychologist and the author of They F*** You Up - How to survive family life. His latest book Affluenza will be published in January