What is relaxin?

Relaxin is a vital pregnancy hormone that also can be the source of some decidedly unrelaxing aches and pains. Before I talk about its side effects, you first need to understand the hormone's purpose. Relaxin does just what you'd think it does - it relaxes. Most importantly, it relaxes the intrauterine ligaments. Your body needs flexibility so the uterus and pelvis can accommodate a baby that will grow from a couple of cells to the size of a mid-sized watermelon.

That relaxation effect isn't limited to your uterus. Relaxin also relaxes other parts of your body, like your arteries, which have to handle your increased blood volume without sending your blood pressure through the roof. If you open up the arteries, that also increases blood flow to the uterus so more nutrients can get to baby.

What could be wrong with being more flexible, staying loose, and letting your body expand and dilate all around you? Take that argument one step further. Sometimes, your body needs to be tight and inflexible. For instance, what happens when the muscle that prevents stomach acid from creeping back into your throat starts to let down its guard? You got it - acid goes the wrong way up a one-way street.

In addition, consider the fact that two hormones that influence sex drive - estrogen and progesterone - come with similar side effects to relaxin. Estrogen, for example, also opens up the esophageal sphincter and thus lets more acid spill back up. Progesterone contributes to loosening up the ligaments and joints throughout your body - good for growing and delivering a baby, not so good when you trip over a curb and tear the anterior cruciate ligament in your knee. (That's partly why menstruating women are 8 times more likely than men to tear a ligament.)