A mother's smoking habits has now been linked to an increased risk of asthma in preschool children, according to a new study. Also, smoking during the early stages of pregnancy is especially dangerous.

Researchers say that women who are planning to have a baby should give up smoking before getting pregnant as smoking during early pregnancy increases the chances of the baby developing asthma in later years.

Medical Daily had earlier reported that smoking during pregnancy increases the risk of the child developing autism.

Babies born to mothers who smoke are at an increased risk of having a cleft palate and ear infections. They are also more likely to die of Sudden Infant Death Syndrome (SIDS).

There have been previous studies in the past that have linked maternal smoking to development of asthma in children but researchers in the present study say that previous research hasn't looked at the effects of smoking during the different stages of pregnancy and smoking after delivery.

The present study was based on the analysis of data available on more than 21,000 children.

Researchers found that children of mothers who smoke were at an increased risk of developing wheeze and asthma.

"These children were at increased risk for wheeze and asthma at preschool age. Furthermore, the likelihood of developing wheeze and asthma increased in a significant dose-response pattern in relation to maternal cigarette consumption during the first trimester," said lead author Åsa Neuman, MD, the Institute of Environmental Medicine at the Karolinska Institutet in Stockholm, Sweden.

Also, maternal smoking during the first trimester affected the child most when compared to mothers who smoked during last trimester or the first year after delivery.

"These results indicate that the harmful effects of maternal smoking on the fetal respiratory system begin early in pregnancy, perhaps before the women is even aware that she is pregnant," said Neuman.

According to reports, in the U.S, more than a fifth of white women smoke during pregnancy.

"Our large pooled analysis confirms that maternal smoking during pregnancy, particularly during the first trimester, is associated with a greater risk of offspring developing wheeze and asthma when they reach preschool age. Teens and young women should be encouraged to quit smoking before getting pregnant," concluded Neuman.

Other studies have shown that women who smoke during a pregnancy are at risk of miscarriage, stillbirth or having a baby with low birth weight.

The study was published in the American Journal of Respiratory and Critical Care Medicine.