Throughout pregnancy, hormones affect a person's muscles and joints. The hormones relaxin and progesterone relax muscles and loosen ligaments and joints, especially in the pelvic area. The extra weight and body changes in pregnancy along with these loosened joints and ligaments can cause discomfort and even lead to injury.
As your uterus grows and becomes heavier, your center of gravity changes. This can lead to problems with balance and the potential for falls. The weight of your baby and weakening of belly muscles pulls your lower spine forward, adding strain to back muscles. Many pregnant people respond by leaning back in an awkward posture. This increases back strain and pain.