The iron from your prenatal vitamins can cause constipation as well as dairy products, which you're probably taking more of since becoming pregnant. Watermelon is in season and is like 90% water. My sisters and I eat a lot of it during the summer months especially when we're dehydrated and become constipated as a result. It won't take long to get through your system. There are also the p fruits- peaches, pears, plums, and prunes, to get your bowels moving. The high amounts of water in the fruits should help to soften what needs to come out. Coffee is a good stimulant if you're not caffeine free.