My son is not yet, 3 years old, but I do have the same problem!! For us, it usually happens when he's going through some kind of developmental change, physical change, emotional experience or something in his day has changed and/or effected him.
It usually passes in a week or two, when I remain consistent with the schedule. Being pregnant, you do need your rest, and I totally understand how sleep is precious. I'm thinking the best thing to do it to try to create a wake up routine for him, that keeps him in his room. Something he can have ownership over while Mommy gets a few extra winks. Like a video, reading a book, coloring special pictures.
My friend created the wake up box for her little girl, and it was the box she went to her closeet and got when Mommy and Daddy were still asleep. They decorated it together, and picked it out and the whole bit. She put all kinds of stuff in it, and rotated the things in it so it was always a surprise. Mommy would set their alarm clock for whatever time, they were going to wake up for the day and then she told her when you here the alarm it's okay to wake up Mommy and Daddy.
It worked really well, and it's something I plan to implement with my son. Eventually, she got so tired of waiting around for them to wake up, she just slept to her usual time...
Hope that helps and good luck with your future addition and current little one!