In my first pregnancy I quit everything except for a very occasional beer and I smoked around 5 cigarettes a day. I had been regularly using drugs when I found out I was pregnant but just stopped after I missed my first period. When I met my baby and feel in love I felt terrible about smoking at all so when I got pregnant the second time I quit smoking too. I had stayed off drugs after my first child and wasn't drinking much anyways so it was easy to stay sober for my second. I did still have an occasional beer with my second pregnancy though. But never more than one and like maybe 4 through the whole 9 months. I started smoking right away after giving birth like a fool! I have since quit though.
My thoughts about pregnancy is that you only get one shot at growing a healthy baby and they will have to live with the consequences of your decisions for the rest of their lives.