Say what you want about pregnancy, it's a time of tremendous change for the woman undergoing the various stages of it. These changes include physical and physiological, from hormones to moods and everything in between. For those women undergoing pregnancy, yet still wanting to engage in physical exercise, there's a useful article from CBS News' The Early show called "Pregnancy Workouts for Every Stage" that details some exercises that women at any stage of pregnancy can undertake.
In the first trimester, apparently ab lifts are recommended, and the article details two of them -- an attitude ab lift and a frog pull alternating passé ab lift -- wherein you lean back on your elbows and lift your legs up at various angles, in order to strengthen your abs. In the second trimester, the article suggests leg and butt exercises, and in the third trimester, they suggest arm exercises.
This makes sense, since stress needs to be kept away from the stomach region where the baby is growing, and keeps other areas of the body, which can be exercised while avoiding this tender spot, in shape and fit. There are, assuredly, many more exercises one can do to keep fit while pregnant, but to read descriptions and see video of these exercises, head over to cbs.com.