Standing work has the advantage of working multiple muscles at the same time, saving time, working stabilizer muscles that are not used when your body is supported as it is on the floor, being more functional, and allowing you to lift heavier weights. It also has more of an effect on your metabolism, because you burn (at least a few) more calories when doing it than when doing isolation work, which is what most floor work is, and it has a higher "afterburn" than isolation work.
You can work the lower body with floor work (and I love to do a floor-work only workout at least once a week, to target the glutes and hamstrings), but you have to be sure to hit all the muscles:
First off, let me say I'm not sure which exercises or positions may be contraindicated for pregnancy (did you post this in the pregnancy forum? they may have some ideas there), so you'll have to check on that.
QUADS (with something called "quad sets" : sit on the floor with legs bent and feet flat on floor. Straighten one leg on the floor and tighten the quad as much as possible. Actual "quad set" exercises are used in rehab, and don't involve any movement, just the intense contraction. You can also add a lift to the move, lifting the leg so that the knee is at about the same level as the other knee. This is often more comfortable when you go down to your elbows.
HAMSTRINGS: Ball work for the hamstrings (like Cathe does in PLB, for example). Or for a really tough hamstring exercise, lie on your back with a paper plate under each heel, legs bent. Push the paper plates away from you on the floor, either one leg at a time, or both at once. Then pull the plates back in towards you. You can either do this one leg at a time (doable) or both legs at once (yeowza! The hottest ham burn I've ever felt with an at-home exercise!). You can also do this with the new Gliding discs that are at some Walmarts, but two paper plates work just as well and are cheaper!
GLUTES: On-all-four kickbacks and raises (I especially like these moves, as they seem to raise the glutes nicely).
ADDUCTORS (inner thighs): lying leg lifts. I recommend NOT using ankle weights at the ankle, but holding a weight (or fastening the weght) above the knee for these, as it puts less stress on the knee.
ABDUCTORS (outer thighs): leg lifts. Again, with weight above the knees.
CALVES: these pretty much get worked automatically when you do standing work, so doing a few sets of standing calf raises (holding dumbbells at your side) can hit this area.
Incorporating a stability ball into floor work when you can (outer thigh lifts, hamstring work, even lying over one---though I guess you can't while pregnant!-- adds some stability challenge and incorporates more muscles.
You could also do some standing work with lighter weights held at your sides, supplementing floor work with lunges and squats, or doing wall squats, or wall squats with a stability ball behind your back. These can challenge the muscles without using heavy weights, especially if you do a long hold at the bottom. (Again, I don't know how far along your pregnancy is, and if this exercise would be appropriate---depending on how big you are, you may not be able to push back up!)
Hope this helps!
