Pregnant women who have a smoking habit are more than twice as likely to have children with a cleft palate or lip birth defect, a new study has found.

The findings were based on the measurement of cotinine level, a metabolite of nicotine, in the blood from about 500 pregnant women.

"This research is the first time we've been able to measure something – in this case cotinine – and determine the risk of smoking during pregnancy for oral-facial birth defects," Gary M Shaw, PhD, research director and senior epidemiologist of the March of Dimes California Research Division, Children's Hospital Oakland Research Institute in Oakland, California said.

Children with oral clefts often have difficulty feeding, frequent ear infections, hearing loss, speech difficulties, and dental problems. Surgery often can repair these birth defects, which typically occur by the seventh week of pregnancy.

"The message to women is simple and clear: Don't smoke during pregnancy or even if you are considering becoming pregnant," said Dr Jennifer L Howse, president of the March of Dimes said.

“If we can help mothers quit smoking, we can help give more babies a healthy start in life," Dr Howse added.

The study conducted by Mid-Pregnancy Cotinine and Risks of Orofacial Clefts and Neural Tube Defects" by Shaw's California group along with colleagues from Norway, The Netherlands, and Texas, will be published in The Journal of Paediatrics.