Hi, I know a fair bit about this one !!!!!
There was a time when pregnant women were encouraged to smoke, to result in smaller babies, and easier births- I am absolutely serious. Menthol cigarettes were recommended to people suffering from colds, or other respitary illness- i'll bet it helped alot.
Also- marijuana is actually a depressant- like alcohol, not an anti-depressent. This is a common misconception.
Either one will decrease the amount of oxegen in your blood and therefore reduce the amount of oxygen getting to your baby, and to the plancenta. It is not good for anyone during pregnancy, but the actual death rate of babies whose mother's smoke tapers off in babies where mother smokes less than 10 a day. There have also been links made with sids and cigarette smoke- even in cases where the mother does not smoke, but others in the house do.
My own personal experience......
I've been a cigarette smoker since I was 17, working full time, and earning my own money- before that I pinched the odd few from my parents, who were both smokers.
I started smoking pot very regularly when I was about 18 or 19, and fell pregnant (while on the pill) when I was 23. I didn't find out I was pregnant 'till I was about 8 weeks along-because I was on the pill, it wasn't 'till I missed a second period that I thought something might be wrong. I had been smoking pot the whole time, and not only that, but I had had a birthday, and a relocation, so i'd had 2 big parties (birthday then house leaving) I found out I was pregnant the morning of my housewarming party !!!! I already had all the party "supplies" drinks, pot etc, and I couldn't cancel the party on the day without an excuse- besides which, I didn't even have contact details for lots of the people coming, it was a word of mouth thing.
I spent the whole night holding the same glass of red wine so it would look like I was drinking, and rolling my pot into joints, lighting them and passing them to other people straight away- so it looked like I was smoking too.
I wasn't eeven sure at that point whether we'd have the baby or not, but once I knew it was in there, I just couldn't.
We did have the baby, and she was not only fine, but quite big, and is now a very intelligent 2 and a half year old. I never smoked once thoughout the rest of the pregnancy, and didn't even have a cigarette until after I was finished breastfeeding. I guess the first 8 weeks didn't harm her. This pregnancy was planned, and I gave up smoking again- this time in advance !!!!
I was told by a doctor onece, that if I was going crazy for a cigarete, and it was causing me alot of stress, that it was actually less ahrmfull to actually smoke the thing, and have done with it, instead of spending a lot of time in that state. I never did want one though. I was lucky.
If you have trouble quitting and you really want to, your doctor will help you, and he would be delighted to. He can atleast help you to cut down a lot.