About one in six mothers in Guernsey were smokers at the time their babies were born last year, figures have revealed.
Theresa Carr, a stop smoking specialist advisor and midwife at the Princess Elizabeth Hospital, said the list of harmful effects it caused was endless.
She said: "Most women try to cut down but that isn't as good as stopping."
Ms Carr said giving up had an immediate effect as within 24 hours the amount of oxygen going to the baby increased.
During 2010 there were 136 women who were smokers when they became pregnant, 45 of which had given up before the birth.
Ms Carr said: "Babies born to women who smoke are often smaller, they are very irritable, they often have feeding problems and they're more prone to infections."
She said it was hard to stop the addiction "especially if you're in an environment where partners or the people around you smoke".
Ms Carr said free advice and products, such as patches, that can help people give up were available from Quitline.