Unfortunately there is no short-term travel medical insurance that covers for maternity services or childbirth. It is highly recommened that you wait to travel until after your pregnancy as the cost for complications and regular check-ups necessary for the health of both you and the baby can be quite high. Here is an article that may provide you with more information: Visitors Insurance for Pregnancy
There are plans however that may cover for some complications during your pregnancy.
See plan here: Atlas America Insurance
Insurance rates are regulated by law. You can't find the same insurance plan for a lower price anywhere else