Relaxin, progesterone and pelvic ligament laxity in pregnancy
In pregnancy, relaxin (a peptide hormone) and progesterone loosen the ligaments (strong fibrous bands that hold bones together) of the pelvic joints.
Remember
Relaxin, with progesterone, loosens the ligaments around the pelvic joints. The joints then move more, which helps the pelvis accommodate vaginal birth, but they can also move unevenly and become painful.
During pregnancy the body prepares the pelvis for birth by loosening it. Relaxin and progesterone loosen the ligaments around the sacroiliac joints (where the sacrum meets the hip bones) and around the pubic symphysis (the joint where the pubic bones meet, at the front).
These joints are normally very strong and allow only a little movement. When the ligaments become slightly lax (loose), a state called ligamentous laxity, the joints can move a little more and the pelvis has extra give. That helps it accommodate a vaginal birth, and the pelvic outlet (the lower opening of the pelvis) can enlarge. For this purpose the hormones act on the ligaments rather than on bone density or muscle bulk, so the change adds some movement, not rigidity or strength.
The loosening has a price. Many women develop a waddling gait (a waddling way of walking) and pain over the pubic symphysis. Pain over the pubic symphysis is part of pelvic girdle pain (pain from stiffness of the pelvic joints or the joints moving unevenly at the back or front of the pelvis). The pain typically gets worse when walking, standing on one leg or turning over in bed, and it is more common later in pregnancy.
Hormones are only part of the explanation for the pain. Extra weight, a change in posture, earlier back or pelvic problems and hypermobility (joints that stretch more than normal) add to the risk, and the evidence linking relaxin blood levels to symptoms is weak. The pain is not harmful to the baby.
Key facts
- Hormones
- relaxin and progesterone loosen the pelvic ligaments in pregnancy.
- Where
- the ligaments of the sacroiliac joints and of the pubic symphysis.
- Target
- for the pelvic loosening, the ligaments, not bone density or muscle bulk.
- Effect
- more movement at the pelvic joints, which helps the pelvis accommodate vaginal birth.
- Cost
- a waddling gait and pain at the pubic symphysis (part of pelvic girdle pain) can follow.
Exam traps
- Oestrogen matters in pregnancy, but its main actions include breast growth and fluid retention, and pelvic ligament laxity is primarily caused by relaxin and progesterone, not by oestrogen.
- Progesterone levels rise in pregnancy, and it is one of the hormones that contributes to pelvic ligament laxity, so reduced progesterone would tend to reduce laxity rather than produce it.
- Oxytocin is the best-known hormone of labour, and its major roles include the contractions of the uterus in labour, but it is relaxin and progesterone, not oxytocin, that loosen the pelvic ligaments.
Read the full notes
Tested in 6 SimpleMed+ questions
Practice it in Reproductive System, with the answer and a full explanation after every question.
Practice these questions