During pregnancy, a hormone is present, called relaxin, which helps to increase laxity of all the muscles, joints, ligaments and joint capsules. The purpose of this hormone is to increase mobility of the tissues so that the body is able to accommodate the rapid fetal growth and prepare the body for delivery. The abdominal muscles are most directly affected by the growing fetus and have the ability to stretch significantly as the fetus grows.
The abdominal wall is stretched taut by the last half of pregnancy and therefore cannot function efficiently during exercises that flex the upper spine, (exercises like crunches) against the force of gravity. Most traditional abdominal exercises such as crunches and oblique twists, either on the floor or on an apparatus, such as a large exercise ball, as well as Pilates mat exercises that flex the upper spine, should not be performed once the belly has expanded to the point where functioning is reduced. For some women, this could occur as early as the fourth month, particularly if they have had previous pregnancies.
At about 20 weeks and often sooner for a second pregnancy, the rectus abdominis muscle will begin to separate along the linea alba (at the body’s midline) into a right and left side, with the two rectus halves moving laterally. This is called a diastasis recti, or abdominal separation. This is a normal occurrence during pregnancy, and will happen to almost all pregnant women. When diastasis recti occurs there is less support for the lower back, which often results in an increase in low back pain or other discomforts. Women who do not control the size of the diastasis recti may have difficulty closing it postpartum and may be at risk for an umbilical hernia, especially if there is a subsequent pregnancy without proper closure of the separation.
To self-test for abdominal muscle separation, lie on your back with your knees bent and your feet flat on the floor. Place your fingertips just above your belly button. Lift your head and shoulder off the floor and at the same time press your finger firmly against your stomach, feeling for any separation between the bands of your abdominal muscles (the bands that run vertically). If the separation is greater than two fingers width, you should be careful not to strain your abdominal muscles as you exercise.
Certain abdominal exercises can be performed safely throughout pregnancy, including when a diastasis recti is present. Most transverse abdominus isolation exercises (either side-lying, standing, or seated) are extremely beneficial and should be included in every workout session. Focusing on the TVA is especially useful because this muscle directly supports the uterus, and if it is toned it helps keep the right and left rectus abdominis muscles closer together. This can help prevent diastasis recti from opening any further. Another great benefit of toning the TVA is that it prepares women for labor. The TVA is the muscle used to assist during the pushing phase of labor.
Pelvic tilt exercises that isolate and flex the lower spine (either standing, leaning against a wall, or seated on a large exercise ball) are also valuable to perform on a regular basis throughout your pregnancy. To read about more specific pregnancy safe abdominal exercises see my article, Safe and Effective Abdominal Exercises During Pregnancy.