Why do some doctors

Tell pregnant women it's okay to still smoke cigarettes because quitting cold turkey can put a lot of stress on the baby? Is it true? My doctor says smoking is a big no no and applaud me for quitting cold turkey. But a couple told me that it's dangerous to do that as told by they're doctor so she continued to smoke. I've never been to a doctor who encouraged me to keep smoking. They're always against it. I just don't get it, has there been research on this or something?