What Causes Increased Heart Rate In Pregnancy?

A woman’s body changes so much during pregnancy that it can be hard to keep up with the differences, even from day to day. Most women know to expect nausea and sore breasts early on, and back pain as a growing baby stresses the spine. Few anticipate the tremendous changes that take place with regard to organ function, however, and the cardiovascular system is no exception. Even early in pregnancy, many women notice a greatly increased heart rate, for which there are several possible explanations.

Lower Blood Pressure

Early in pregnancy, a woman’s blood vessels proliferate tremendously. Vessels become particularly concentrated in the breasts and the uterus, in both cases sending additional nutrients to cells. The uterus, in particular, needs more blood flow, as the mother’s blood provides for the needs of the developing baby. As the number of blood vessels in her body increases, a woman’s vascular space increases as well. In the same manner water flows with lower pressure through a large diameter hose than it does through one of small diameter, increased vascular space means her blood pressure drops. To compensate and make sure the mother’s organs, particularly the brain, receive enough blood, heart rate increases, explains Dr. Raymond Poliakin in his book, “What You Didn’t Think To Ask Your Obstetrician.” A faster-pumping heart ensures that low-pressure blood reaches body cells in sufficient quantities.

Increased Metabolism

While she’s growing a baby, a woman’s body works very hard. Authors Heidi Murkoff and Sharon Mazel, in their book “What To Expect When You’re Expecting,” liken pregnancy to the effort of running a race, even while sitting down. The harder cells work to produce products and grow tissue, the more nutrients and oxygen they need. Just as a woman’s heart beats faster when she’s working out, so it beats faster during pregnancy, even if she’s just lying on the couch. Blood volume increases during her second trimester, which takes the pressure off the heart a little bit and typically decreases heart rate, but a pregnant woman’s heart always beats faster than it would if she weren’t pregnant.

Body Strain

Late in pregnancy, there’s yet another reason that her heart has to work harder: She’s simply carrying more weight around. With the extra weight of a baby, a placenta, amniotic fluid, a bit more fat, and some extra blood, a woman late in her third trimester is up to 30 pounds heavier than she was pre-pregnancy, note Murkoff and Mazel. It’s as though she’s hiking and carrying a 30-lb. backpack each and every day. This strains the body, works the muscles harder, and requires the heart to beat faster to provide for additional oxygen and nutritional needs of cells.