Lying on Back During Pregnancy: Safety & Abdominal Exercises

Question
I work out everyday, and I am in excellent shape; however, I "do abs"--crunches and leg lifts (to work the lower ab muscles). I am 26 weeks and usually only lay flat on my back for 30 seconds to a minute. The rest of the ab workout I have both legs in the air, one leg in the air, one leg crossed over a knee, or my legs bent (with bottom feet on the ground). I never feel short of breath or light-headed . . . I just want to know if it is "ok" to lay on my back for short periods of time during exercise, or I have harmed the baby my cutting off blood flow and thus oxygen.

Kelly

Answer
In rare cases, some pregnant women experience Supine Hypotensive Disorder, now often called positional low blood pressure, a condition that causes quick onset dizziness and or fainting. It it presumed to happen when the weight of the uterus compresses the vena cava (the largest vein in the body) which then reduces blood flow back to the heart, causing dizziness/fainting.

The validity of the theory behind vena cava suppression is a hot topic among prenatal fitness and health experts. Many believe that a intrinsic "plumbing" problem that harms the mother/baby dyad, would not have stood the test of evolution. And in light of the fact that ALL other maternal adaptations during pregnancy are the opposite, i.e., fetal-protective, that the theory is bogus.

Some women do experience dizziness during pregnancy and this is caused by blood pooling in the extremities and is due to vasodilation during pregnancy. This is not at all the same as vena cava restriction.

Most of the exercises you've described all take advantage of the force of gravity, as well as muscle contraction in the legs, to increase blood return to the heart. Since you never feel any dizziness, then you have adequate blood return to your heart. Many prenatal exercise specialists allow women who experience no negative side effect to exercise for short periods (up to two minutes) on the back.

But in no case have you reduced blood flow to your developing baby. Theories about reduced blood flow to the placenta, due to maternal need, or position has NEVER been demonstrated. This common misrepresentation is repeated by many journalists who do not understand prenatal physiology, and is one of several myths surrounding maternal exercise.

Scientific studies have shown a very small reduction of arterial blood flow to the uterus when the mother moves from lying on her left side to either her back, or right side, regardless of whether she is exercising or not. But the change is VERY small, and viewed as insignificant.

If you are not doing so already, start doing core exercises that require you to stabilize a neutral spine, as well as Transverse Abdominis isolations and pelvic tilts that are performed solely by lower abdominal flexion.