In the case of pregnancy, a powerful motivation is almost immediately presented to the female smoker, thus skipping much of the 'stages of change' that usually occur before quitting. After birth, it's common to go back to whichever 'stage' of the process you were in when you quit, since the primary motivation (the welfare of the fetus) has now been removed. Also, post-partum depression/hormonal swings can make staying smoke-free quite difficult (especially with no support network in place). Talk to your OB about a plan to stay smoke-free after delivery. Remember, it is not only important for your health, but your baby's health.