no offense, but i think i have to disagree with chixchix here. if you get hired, imagine what your employer's reaction would be if they find out you intentionally withheld that information? i don't think that's a good start and you definitely won't make a good impression.
i think you should take time out to really think what you want to do first -- continue working despite motherhood, or opt to put career on hold and stay with your child during the first year(s). if by "positions where they definitely want you to grow", you mean that it's going to be quite demanding, you have to do some serious thinking. talk it out with your hubby-- how you want your child to grow, what kind of set-up you both find acceptable.
i'd say go with job hunting, but do tell them you're pregnant. if you get hired, than you know they really want you. if you don't get hired, well perhaps the job simply doesn't fit your current situation. perhaps in a year, when the child is born, you can better assess what you can/want to do: job, parenthood, or both at the same time.
good luck with the decision making!