Smoking during pregnancy may have even worse consequences than we originally thought.

It's no secret that smoking while pregnant has consequences including the development of asthma and allergies among young children, but researchers now are saying that the risk could continue into the children's teenage years.

The study, published in Pediatrics, followed almost 4,000 children in Sweden for 16 years.

Fetal exposure to second-hand smoke from smoking during pregnancy showed an increased risk of the child developing allergies even through adolescence. It was associated with a 45 percent higher risk of getting asthma up until age 16.  

For infants exposed to second hand smoke, the numbers were not much better. Their risk for developing asthma during childhood or adolescence was 23 percent. The risk for eczema was 26 percent greater.

For children developing asthma, researchers found the effects for second hand smoke were most pronounced from birth until age 4. For children developing rhinitis, the association was strongest in children ages 1 to 4. And for children who developed eczema, the risks were highest between ages 8 and 16.

"Increased risks for asthma and rhinitis were seen primarily in early childhood, whereas those for eczema occurred later in life," said Jesse Thacher, study researcher and doctoral student at the Karolinska Institute in Stockholm.

Thacher said that although the study was done on Swedish children, it would be reasonable to generalize the results to the US because of the similar lifestyles.

According to the American Academy of Pediatrics, second-hand smoke contains about 4,000 chemicals - 50 of which are known to cause cancer. This builds on previous evidence that smoking is dangerous whether it is direct smoking or second-hand.

One key strength of the study is its large sample size and extended follow-up as well as the information collected from the parents.

Researchers said one limitation to the study would be that smoking habits among parents are typically underreported, especially when phrased in the context of their child's exposure to second hand smoke.

Additionally, the study did not intend to find cause and effect, so more research will have to be done to confirm the results.  

Smoking during pregnancy has also been linked with a variety of other health issues during and after pregnancy such as premature birth, miscarriage and learning problems.