It seems like there are too many variables to give generic advice on this issue. I'm in nearly the same boat and this is some advice I've received from associates and law students that worked as paralegals while having kids.
1. A BIG factor is how much the father will be involved in baby care and everything that has to be done at home. I pretty much told my MIL that if my (slightly lazy) husband doesn't get on board with being a co-parent, I won't be making her a grandmother. This gets her off my back and also puts her in the position of "reminding" him of his responsibilities as a future dad.
2. Another factor is whether you're working on litigation or transactional work. Transactional work seems like it can accommodate pregnancy and mat leave better because you may be able to get extensions on some types of projects, or the projects might be small and discrete so you aren't giving up all your clients to coworkers, just a few contracts or applications or whatever. One of my friends was in litigation and the other associates preyed on her hard-earned clients and matters when she was on mat leave and she can't get them back. Even though she's back to work and has her mom around to care for the baby, she can't find enough work to meet her billables so she's finding it extra hard to ramp back up. If you like transactional work and can join such a group, that might be a good option.
3. The vibe of your particular practice group is also very important. Working in a "boys-club" will make it much harder for your pregnancy and mat leave to be accepted and accommodated. Try to align yourself with a more family friendly partner because that person will hopefully be an ally when you're at doctor's appointments, or on mat leave.
4. My neighbor had two kids after age 35, but decided to stop working after the first one was born. She was burned out from years working as a lawyer and couldn't imagine going back as a 35 year-old sleep-deprived first time mom. She said energy levels and the ability to survive pregnancy decline rapidly in our 30's, so keep in mind that you might ride out pregnancy at age 31 better than when you're 33 or 35 (in addition to fertility issues). So you might not look like as much of a slacker if you're less burned out and can bounce back better in an earlier year.
For what it's worth I think I'm going to be 32 and a second year when I have my first. If I get fired for being a mom I don't really want to work there anyway. I'll take some time off, then interview for jobs without my wedding ring on.