My wife had the same doubt before the labor. She was in the end of her PhD, she managed to end it on February and our little girl was born in March. She is getting help from both grandmothers, she likes to take care of the house, and she barelly has time to do that.
And she still feels she is not doing a good use of the time, because even not having much free time during the day, she feels she's loosing her time because she can't see any improve in her career, etc.
What has worked is reading: take the time to read things that you'll need later. Improve your knowledge.