Now, this obviously depends on where you're setting up your startup. However, if I lived in a place where a long[ish] (6+months) maternity leave was the law, I'd be pretty worried about newly hired women going on maternity leave. For a large company, it isn't a big problem, but for a small cash-strapped company, losing a significant % of your workforce for an extended period of time sucks.
I've known women who went back to work, or switched to a job with better benefits, because they planned on getting pregnant. And I've known some who just happened to get pregnant shortly after getting a new job.
I strongly believe that the length of maternity leave should be proportional to the length of time at the company.