Hips can start to widen during the first trimester of pregnancy as the hormone relaxin is released and causes the ligaments in the pelvis to relax and become more elastic. This can cause instability in the hips and lead to pain. Round ligament pain, which is pain in the muscles that support the uterus, is also common during this time. As the pregnancy progresses, the hips will continue to widen to accommodate the growing baby. This process can cause discomfort and pain for some women.