Hi H.,
I was in a similar situation just over a year ago. I received a job offer which also involved a relocation that the company was paying for and then found out I was pregnant 1 week after I accpeted the job. I too was nervous about what they would think especially because I had to fill out a medical questionare before starting the position which asked if I was pregnant. I told them the truth knowing that legally they could not do anything to take the offer away. The company was very good about keeping my info confidential within the human resources dept and just wanted my medical info to make sure I wasn't exposed to anything hazardous and that I wasn't lifting too much. Everything worked out fine. I did tell my boss the news on my first day of work (even though I was only 2 mo) just because I knew he suspected something because he was told I had "temporary" lifting restrictions. I slowly told other people at work after I started to show a bit.
I did take maternity leave for 9 weeks and am now back at work. The company was great and actually gave me partial pay during my leave. However, please note that most companies won't pay during your leave if you have less than 1 yr with the company. I am also not sure if they legally have to give you the 12 weeks FMLA if it is under 1 year. You may want to look into that. Either way, I don't think you should worry about taking the job in good faith. From what you say it sounds like you plan on returning to work after your leave and most people/companies are very understanding. If they aren't understanding then you probably don't want to work for that kind of company anyways.
Good luck. I know how stressful this is when starting a new job. You want to make a great impression right off the bat and I am sure you will. If you have any questions, email me.