Here's another "don't" for would be moms: don't smoke while pregnant or you could increase the chances of your unborn child suffering from heart attacks and strokes.

A recent study from the University of Sydney shows that mothers who expose their unborn child to smoking increases the chances of an eventual heart attack and strokes by 20%. This is caused by a decrease in the amount of good cholesterol, high- density lipoprotein (HDL), which protects against heart disease.

The study published in the European Heart Journal, studied children born to mothers who smoked during pregnancy. Researchers discovered that by the age of eight the children had lower levels of HDL cholesterol, at around 1.3 millimoles per liter (mmol/L) compared to children with non-smoking mothers who had about 1.5 mmol/L.

HDL is important to the human body as it protects against atherosclerosis, where fatty materials collect along the walls of arteries and block them leading to heart problems and heart attacks, a Reuters report explained.

Researchers led by Professor David Celermajer, analyzed data gathered from 405 eight year olds born between 1997 and 1999 who were part of a controlled trial that was investigating asthma and allergic diseases. The data included information on the mothers' smoking habits before and after pregnancy. The researchers then used ultrasound to measure arterial wall thickness and took blood to measure lipoprotein levels.

The lower levels of HDL could remain until adulthood. "Cholesterol levels tend to track from childhood to adulthood, and studies have shown that for every 0.025mmol/L increase in HDL levels, there is an approximately 2.0 to 3.0 percent reduction in the risk of coronary heart disease," Calermajer said in a statement about his research. If you do the math that means there is a 10 to 15 percent higher risk for children born from smoking mothers to develop heart attacks.

Although it is common knowledge that smoking during and after pregnancy is bad for the child it wasn't until this study that scientists were accurately able to describe how cigarette smoke might lead to future heart risks.