Relaxin: You may be cursing it now for your aching hips and horrible heartburn (plus all those other gastrointestinal discomforts), but during baby's birth you can thank relaxin for doing its job. As the name implies, relaxin relaxes the body's muscles, joints, and ligaments. Not surprisingly, the effect centers on the joints of the pelvis, allowing them to stretch during delivery. It also softens and lengthens the cervix and helps relax smooth muscles in the uterus and elsewhere throughout the body. If your walk starts to get a little wobbly, it's not just a shifted center of gravity from your bulging belly, but relaxin's fault again. Limber joints, particularly in the pelvis and lower back, are unsteady joints — if your house were held together with rubber bands instead of nails, it'd be shaky too. Which means your balance and gait is thrown off-kilter, so be particularly careful to watch your step!