I would recommend you get yourself checked out for a spinal deformity called Lordosis.
It's an accentuated curve in the Lumbar region (lower back) caused by increased amount of weight being carried in the abdomen area. Commonly caused by pregnancy or obesity.
Also if you're within 20 weeks of giving birth then I'd advise you to go easy with weights and not go too heavy as the pubic symphysis (the joint where your hips meet in the middle) is still a little unstable during this time due to specific hormonal levels.
Once you're ready to exercise though, as Boss says, deadlifts and good mornings will be the way to go, along with back extensions and reverse hypers.