Smoking is a risk factor for heart disease

They studied 28 pre-menopausal white women in the Quebec area, ages 35-47, who were admitted to the hospital for heart attack or serious chest pain.

The women filled out a detailed questionnaire about their disease and medication history, symptoms, hormonal status and risk factors such as family history of heart disease, diabetes, high blood pressure, high cholesterol and smoking.

Twenty of the women reported heart-related problems within five days of the beginning of their period when oestrogen levels are at their lowest.

Lead researcher Dr Bettina Hamelin said: "We found that significantly more pre-menopausal women experienced their heart attacks during or after their periods when their natural oestrogen levels are low.

"However, these young women also had a number of other important risk factors such as smoking and diabetes."

Link established

The link between oestrogen levels and heart disease was first established by a US study in the late 1970s.

The well known risk factors of smoking, raised blood pressure and high cholesterol levels need to be highlighted over other less conclusive risks

British Heart Foundation

Researchers examined the residents of a town called Framingham in Massachusetts, US.

They found that the rate of heart attacks in women increased significantly after the age of 55.

This is the normal time of the menopause when oestrogen levels fall off significantly.

It is thought that high levels of oestrogen circulating in a women's bloodstream help to protect the heart.

The Quebec study will continue until at least 50 pre-menopausal women with heart attacks and known menstrual-cycle history have been recruited.

Dr Hamelin said more data is needed to confirm that there is a vulnerable time during the menstrual cycle which may constitute a trigger in women at risk for heart disease.

A spokeswoman for the British Heart Foundation said the study raised interesting questions about the effect of the sex hormones on the heart.

She said: "We understand that oestrogen may have a protective effect on the heart in pre-menopausal women so it does seem logical that when hormone levels are low, this protection may diminish temporarily.

"However, this study is not large or controlled enough to make any firm conclusions.

"Whether this hypothesis is true or not, it still has to be established conclusively so further research is important.

"The well known risk factors of smoking, raised blood pressure and high cholesterol levels need to be highlighted over other less conclusive risks."

A study published in July by a team at St Thomas' Hospital, London, found that young women with heart disease suffer worse symptoms at particular times during their monthly menstrual cycle.

They found that the women were most likely to suffer angina - a chest pain associated with heart disease - during or within a week of their period.

The Quebec research was presented at a conference of the American Heart Association.