Eucalyptus, like any herb has many alkaloids that may effect the delicate chemistry needed during a pregnancy. Though I don't know what may specifically in eucalyptus. Check with your doctor, it may be fine, however if you over do anything it is not too good for a pregnancy. I suggest gum, or getting a hold of something of the similar size and suck on that (feed the habit but not the addiction), or hard candies. Also check with your doctor, it may be OK to use a cessation drug, or nicotine patch/gum. Cigarettes are so bad due to many other chemicals than nicotine, and that it cuts down on the amount of oxygen in the blood stream. You are breathing for two now.