Standard Navy policy is that you are deployable until 20 weeks. Standard practice, however, is that you are sent back when you are determined to be pregnant. The caveat is that if you are in an operational billet, you will be returned to an operational billet after the 6 week post-partum period.
I have seen young women sent to sea immediately following delivery, leaving infants at home with dad or grandparents. Although this seems Draconian, it was done in response to the large number of women who became pregnant at the beginning of a deployment cycle in order to get out of it. Is it always done? No, but it is something keep in mind.