It may not come as a surprise to parents, but having children really does impinge on marital bliss.

Nine in ten couples suffer a downturn in 'marital satisfaction' once the first child is born, according to an eight-year study.

And relationship and stress problems are worse for those who live together before getting married and starting a family.

The findings will ring true for millions of parents who found everything from their finances to their sex lives suddenly changed with a new arrival.

But the researchers insist the findings should not be seen as a reason to remain childless as bringing children into the world can bring a 'whole dimension of family happiness'.

Psychologists at the universities of Denver and Texas examined the marriages of around 220 couples over an eight-year period.

They found 90 per cent suffered what they called a 'decrease in marital satisfaction' after the birth of a child.

Childless couples suffered the same problems - only a little later, explained Denver University psychology professor Scott Stanley.

He said: 'Couples who do not have children also show diminished marital quality over time.

However, having a baby accelerates the deterioration, especially seen during periods of adjustment right after the birth of a child.'

The research, published in the Journal of Personality and Social Psychology, found relationship problems were worse for those who had lived together first - possibly because long-established shared routines are more disrupted by new arrivals.