Mothers who smoke and give birth prematurely have a significantly higher risk of developing cardiovascular disease (CVD) later in life, a new study has found.

Smoking and giving birth prematurely are already known to be risk factors for CVD in women. However, this marks the first study to assess the impact of these two risk factors together.

"Fertility treatment is pushing up rates of preterm birth and smoking in pregnant women remains high, so knowledge of the impact of these conditions on CVD is important for prevention efforts. This is the first study to evaluate whether smoking and preterm birth synergistically increase mothers' CVD risk,' explained the study's lead author, Dr Anh Ngo, of the University of Sydney in Australia.

The study involved over 902,000 mothers and found, as expected, that smoking and giving birth prematurely independently increased the risk of CVD in women. However, when these two factors were assessed together, the risk was even greater.

Overall, women who smoked and gave birth prematurely were 3.3 times more likely to develop CVD compared to non-smokers who gave birth at full term. This risk was 29% higher than if the women only smoked or only gave birth prematurely.

"Smoking and preterm birth synergistically increase maternal CVD risk. When these two conditions co-exist in the same individual, they interact to produce a joint effect on maternal CVD risk that is 29% greater than the sum of effects when they act alone," Dr Ngo said.

The study also found that the risk of CVD was even greater in women who smoked and gave birth to very premature babies or who had recurrent preterm births. For example, among smokers, the risk of CVD was 3.8 times higher if the woman gave birth between 20 and 33 weeks gestation compared to non-smoking women who gave birth at full-term.

Furthermore, smokers who gave birth prematurely at least two times were 4.4 times more likely to develop CVD than non-smokers who gave birth at full term.

"Our research shows for the first time that smoking and preterm birth interact to create a greater CVD risk than either risk factor on its own. One explanation could be the stress of caring for a preterm infant which may lead to unhealthy behaviours such as smoking, which increases the likelihood of future preterm births. Stress itself causes metabolic disorders, atherosclerosis (hardening of the arteries) and ultimately CVD," Dr Ngo noted.

She said that smokers who seek assisted reproduction ‘should be counselled about their risk for preterm birth and CVD in later life so that they can make an informed decision'.

"Women who smoke and have a preterm birth more than triple their risk of CVD. More efforts are needed to persuade and support women to kick the habit. These efforts will have positive effects for both mothers and their babies," she concluded.

Details of these findings are published in the European Journal of Preventive Cardiology.

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