Progesterone and smooth muscle relaxation in pregnancy
In pregnancy, progesterone relaxes smooth muscle (the involuntary muscle in blood vessel and gut walls), which slows the gut and loosens the valve at the bottom of the gullet.
Remember
Progesterone is the main smooth muscle relaxant of pregnancy. A slower gut tends to leave drier, harder stool (constipation) and bloating, and a loosened lower oesophageal sphincter lets acid flow back up (reflux), felt as a burning feeling in the chest (heartburn).
Progesterone rises steeply in pregnancy, and its main action is to relax smooth muscle, the involuntary muscle found in the walls of blood vessels and the gut. One hormone acting on smooth muscle all over the body explains many everyday symptoms of pregnancy.
In the wall of the bowel, relaxed muscle weakens peristalsis, the wave-like squeezing that pushes contents along. Contents pass through more slowly, and the colon, which reclaims water as they go, tends to leave drier, harder stool. This can lead to constipation and bloating.
At the bottom of the oesophagus (the gullet) is the lower oesophageal sphincter, a ring of smooth muscle that works as a valve and normally stays closed. Progesterone lowers the pressure in it, so acid can move back up. That is reflux, and it is felt as the burning feeling in the chest called heartburn. The fault is mainly a weak valve rather than too much acid.
These gut changes are caused mainly by hormones rather than by the physical pressure of the uterus, which is why they can begin early in pregnancy, before the uterus has grown much, although the enlarging uterus can add to symptoms as pregnancy progresses. Oestrogen probably acts as a primer, meaning it prepares the tissue so that progesterone works better.
Gastric emptying (how quickly the stomach empties) is a separate process: evidence suggests it may be slower early in pregnancy and during labour, but it is not consistently delayed later in pregnancy, so constipation and reflux are the safer examples to give of these effects.
Key facts
- Hormone
- progesterone, whose main action is to relax smooth muscle.
- Gut wall
- weaker peristalsis means contents pass through more slowly, which can lead to constipation and bloating.
- Lower oesophageal sphincter
- lower pressure in this valve allows reflux and heartburn.
- Main cause of reflux in pregnancy
- a hormonal fall in sphincter pressure rather than extra acid.
- Hormonal, not mechanical
- the gut changes of pregnancy are caused mainly by hormones rather than by the uterus.
- Oestrogen
- it probably acts as a primer, preparing the tissue so that progesterone works better.
- Extra contributors
- iron supplements can make constipation worse.
Exam traps
- Extra acid from oestrogen does not explain heartburn in pregnancy: the predominant cause is a fall in the pressure of the lower oesophageal sphincter, not more acid.
- Compression of the colon by the enlarging uterus is not the main explanation for constipation in pregnancy, because the gut effects are caused primarily by hormonal changes and constipation can begin before the uterus has grown much.
- Progesterone does not enhance peristalsis in the oesophagus: it reduces peristalsis, so the direction of change is wrong.
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