Since it's a nerve pinch, there's not really a great *cure* per se ... at least not in the way that you can stretch out a muscle... but you can sometimes relieve the pressure and give yourself a little relief... and sometimes it just sort of relieves itself as the baby settles in a new position
When I had sciatica with my last pregnancy, I found two stretches that helped relieve the pressure on the nerve...
1- go on all fours, making sure that your body is squared... put one leg (on the side where you are experiencing the sciatic) straight back and then lift it and cross it over the other leg (again, make sure that you keep your hips and shoulders in alignment).
2- sit on a stability ball or a chair and cross one leg (on the side that hurts)over the other, making the shape of a number "4" ; then, keeping your back straight, hinge forward slightly from the hips so that you get a stretch in the buttocks/hip.
Other than that, I could function in my prenatal/postpartum low-impact aerobics classes if I left out mambos and cha-cha's (they were excruciating!)... knee lifts didn't bother me, but I've heard it said that they do bother some women who suffer from sciatica... I also found that swimming worked well for me, and sometimes kicking around in the pool somehow worked out the sciatica for a day or so and I'd get a little relief!
Definitely mention your sciatica to your OB... in some cases it may be better not to exercise or to modify your exercise when you have sciatica.
Susan