Your situation sounds very similar to mine. I am 31 and partner of 5 years is 39 so we have been considering when is the best time to start a family. I have completed my first year of a science phd. How did you find out that your grant had the maternity funding? I am not sure about mine and don't want to ask my supervisor. I believe if he thinks this is my plan then he will not be very supportive. As I am lab based, I think the best for me would be to get my lab work nearly done and write up whilst pregnant. It might be worth checking if your partner can take paternity leave. These days often the father can get the time off that the mother usually would.