The following brief summary of how to modify your routine is from Colleen Porter.
During pregnancy, abdominal exercises can help preserve muscle tone and take strain off the lower back. However, you might need to learn new routines, since most experts have counseled against lying on your back after the fourth month due to pressure on the vena cava, the blood vessel that returns blood from the lower body to the heart. The books "Pregnancy and Exercise" by Raul Artal (currently out of print) and "Essential Exercises for the Childbearing Year" by Elizabeth Noble offer many suggestions for safely strengthening the abdominals during pregnancy. One exercise is the Rocking Back Arch: kneel on all fours and count to five as you rock back and forth, then return to the original position and arch your back. Repeat five times, several times a day.
Postpartum moms should check their abdominal muscles for separation before starting any abdominal exercise program, because damage can be exacerbated by exercise if there is separation. Test this by pressing your fingers into the area by your belly button as you attempt to do an abdominal crunch. If you can put more than one or two fingers in between the muscles, they have separated and you will need to modify your crunches. Place your feet the same way, but cross your arms across the abdomen and squeezing the muscles together as you exhale and contract the abdominals, lifting only your head (not the shoulders). You may also use a length of material (such as old sheeting) wrapped around the abdomen and pulled across to achieve the same effect.
The following ab training tip for pregnant women comes from Robin Burton:
"My midwife cautioned against crunches after the belly rose above the pubic bone, saying that the stress this caused was a factor in abdominal separation. I found that an excellent way of exercising the abdominals during pregnancy was belly dancing! The dancing strengthens the muscles of the abdomen with very little strain and the movements help during labor, too. Of course it isn't going to give anybody a washboard stomach, but no pregnant woman is going to have one of those anyway!"