Definately honest. This is not the time to put yourself under extra pressure, such as white you might encounter in changing companies and starting on bad terms at the company, combined with extra emotions due to your pregnancy homones. They might just still say yes, if not, that is life. Unless you have financial problems and can't survive given your current situation.
It is also possible you will start, and that the company will find out within 2 weeks, and within your probation period of ~3months and can in such a case try to push the limits of the law and work you out of the company, saving them money.
Best of luck with your decision, and hope they are the type of company you want to work for, the type of company that will take you on despite the circumstances of the near future.