This was actually a huge concern of mine when I found out I was pregnant. I'm a physical therapist, and on my feet most of the day at work, and my sister was completely debilitated by the end of her pregnancy with sciatica. I took some pro-active steps and avoided sciatica. No one is to say that I would have gotten it if I didn't do these things, but I think it helped.
Before you try any of this, check with you OB, some women have conditions that may be made worse from exercises.
1. You can get a pelvic stabilization belt. It's an elastic belt that you put around your pelvis, literally like a rubber band to hold your pelvis more stable. When you're pregnant, hormones make your joints much more lax. The excessive movement of the hypermobile joints can cause trauma/agitation to the surrounding soft tissue (gluteal muscles, sciatic nerve, lower back, etc.) I was able to wear it until about 30 weeks, then the pressure on my bladder along with baby pressure on my bladder was too much for me.
They make maternity support belts as well that essentially "lift" the weight of baby thought a harness system that goes over your shoulders. I didn't do that, but it's an option. My advice would be to start with the pelvic stabilization belt and move on to the maternity support.
2. Keep your hips and back strong (abs are important too for this condition, however, lucky us, we don't get that option as baby gets bigger). Again, the hormones make your joints lose, so you want to keep your muscles strong to combat that. Simple things like mini-squats, and straight leg raises in each direction help with that. Another good exercise is something called "bird-dog" You probably know it: get on your hands and knees, slowly lift your R arm and L leg at the same time, hold a few seconds, and then repeat on the other side. This is great for strengthening your back. I've also been a big fan of prenatal yoga though the whole thing. It's great for stretching and building the gentle strength that you need to support a changing body. Again, check with your OB before starting exercises.
3. WEAR SUPPORTIVE SHOES a lot of women don't do this, especially if you work in an office. But it really is the easiest thing you can do that will make a HUGE difference. You need a shoe with a lot of cushion and very good arch support. The body is all physics, your foot is the first thing to land on the ground, and all the force of your body landing is first controlled by the function of the foot. If you're walking around on 2 inch, pointy toe heels when you're carrying an extra 30 lbs and a big belly, you aren't setting yourself up for success.
Supportive shoes won't be as stylish as regular shoes, but for a few months, it's worth it. And if you have a special event, it's ok to pull out the heels for a few hours, just not all day every day. There is a store called Comfort One with multiple locations around DC. There are 3 in Dupont Circle and one in Clarendon... those are the ones I use the most, but I know there are more, just look it up online. Their staff is well trained, and they have a vast collection of supportive shoes that are a little more formal than sneakers.
I hope that helps. Good luck with #2!