The study authors wanted to find out whether the coexistence of smoking and preterm birth would combine to result in further increases in maternal cardiovascular disease risk.
The authors also found that the risk of cardiovascular disease (CVD) increased for mothers in a dose-response relationship with how many and how severe the preterm births were.
"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," says lead author Dr. Anh D. Ngo, a research fellow at the University of Sydney in Australia.
While smoking is recognized as a risk factor for CVD, women are particularly dissuaded against it if they become pregnant. In addition to the usual risks, smoking can expose babies to harmful chemicals while in the womb, hinder growth and increase the risk of preterm birth and certain postbirth health problems.
Preterm birth is also known to increase the risk of CVD, with previous studies revealing that women with a history of preterm birth have a 1.2-4 times higher risk of CVD than women who have term babies.
"Smoking and preterm birth are established risk factors for maternal CVD," states Dr. Ngo. "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."
To find out whether the two conditions would combine to increase the overall CVD risk faced by mothers, the researchers looked at the perinatal records of 902,008 mothers during 1994-2011 alongside records for any hospital admissions or deaths from CVD.
The researchers discovered that the two conditions - smoking and preterm birth - were both independently and significantly associated with an increased risk of CVD. Compared with nonsmokers who had term births, smoking mothers who gave birth preterm were 3.35 times more likely to develop CVD.
"Smoking and preterm birth synergistically increase maternal cardiovascular disease risk," Dr. Ngo reports. "When these two conditions coexist 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."
The impact of recurrent and severe preterms births
The risk of CVD for smoking mothers increased in comparison with nonsmoking mothers with term births according to the severity of the preterm births. Extremely preterm births (20-33 weeks gestation) increased CVD risk 3.83 times and moderately preterm (34-36 weeks gestation) births increased CVD risk 3.18 times.
Recurrent preterm births among smoking mothers also increased CVD risk. Smoking women with two or more preterm births were 4.47 times more likely to develop CVD, while smoking mothers with just one preterm birth were 3.2 times more likely - both in comparison with nonsmoking women with term births.
Dr. Ngo suggests one explanation for their findings:
"One explanation could be the stress of caring for a preterm infant which may lead to unhealthy behaviors such as smoking, which increases the likelihood of future preterm births. Stress itself causes metabolic disorders, atherosclerosis and ultimately CVD."
Unfortunately, as the study only categorized mothers as having never or ever smoked, the researchers were unable to analyze the effects of smoking severity or whether the women smoked throughout their pregnancy or quit.
Despite this limitation, Dr. Ngo believes the findings have implications for CVD prevention, suggesting that smoking women who are considering assisted reproduction technology should be counseled on their risk of preterm birth and future CVD.
"Doctors should take into account the joint effects of smoking and preterm birth when assessing a woman's CVD risk," he concludes. "Doctors should also consider the need for preventive therapy to timely control for biological disorders preceding CVD such as hypertension and dyslipidemia when these occur."
Recently, Medical News Today reported on a study revealing that drinking alcohol during pregnancy is a prevalent practice despite medical guidelines advising against it. In particular, the study found that smokers were 17% more likely to drink during pregnancy.