There is no DEFINITE risk, of course, but the chances of your baby being born at a low weight, smaller size, premature, with decreased lung capacity and respiratory problems are MUCH HIGHER than if you didn't smoke. I think it has also been linked to asthma. Think about it, it isn't worth the risk. The poor mite hasn't asked to be conceived, the least you can do is make sure you give him/her the best chances.
Speak to your GP, they can help you. Loads of women give up smoking when pregnant, you can do it.