Two large British studies found that people who had two children were at the lowest risk of developing coronary heart disease (CHD).

Those who had more than two children appeared to have a significantly increased risk - and the risk became larger for each additional child.

And researchers found that with increasing numbers of children, both parents were more likely to be obese.

Parents of large families tend to be poorer and also have less healthy lifestyles Dr Debbie Lawlor

Among women with more than two children, the risk of CHD increased by 30% with each additional child. For men the figure was 12% for each additional child.

Researchers studied 4,286 women and 4,252 men aged 60 to 79 who had from zero to five or more children.

The risks were also higher for those people who had just one child, or who remained childless.

Mothers of more than two children were also more likely to have evidence of insulin resistance, elevated blood sugar and diabetes.

They were also more likely to have low levels of good cholesterol and increased triglycerides.

Lifestyle and biological factors

Lead researcher Dr Debbie Lawlor, of the University of Bristol, said: "Parents of large families tend to be poorer and also have less healthy lifestyles - which explains some of their increased CHD risk."

She said the different results for fathers and mothers suggest that multiple pregnancies also have a specific adverse metabolic effect on women.

During pregnancy women become temporarily insulin resistant allowing blood sugar levels to rise to feed the growing foetus.

It may be that multiple pregnancy raises the risk that this resistance - an important risk factor for CHD - might become permanent.

Dr Lawlor said: "These results reinforce the need to encourage mothers and fathers of several children to maintain a healthy weight and lifestyle which may reduce the chances of developing obesity and CHD in later life.

"In addition, careful screening and control of metabolic abnormalities during pregnancy should be undertaken in women receiving prenatal care, particularly among women who have had a number of earlier pregnancies."

Dr Lawlor said the most likely explanation for the apparent increased risk among people with less than two children was that they were likely to suffer from general ill health.

This could explain both increase their increased risk of CHD, and their reduced fertility.

She said: "It has to be remembered that these study participants are currently aged 60-79 years of age and so from a generation where voluntarily having no children or even just one child was relatively rare."

Pressures

The study was part funded by the British Heart Foundation.

A spokesperson said: "This study shows that parents from lower socioeconomic groups are at a greater risk of coronary heart disease (CHD) due to the pressures of looking after a large family.

"Mothers in particular need to reassess their lifestyles and make changes to avoid CHD - the single biggest killer of women in the UK.

"It is interesting that fathers were also found to be at increased risk, lending further evidence of the significance of lifestyle factors in the development of CHD.

"Avoiding obesity, eating a balanced diet rich in fruit and vegetables, stopping smoking, more physical activity and an awareness of blood pressure and blood cholesterol levels will all help reduce the chance of CHD."

The research is published in Circulation: Journal of the American Heart Association.