Constipation is one of the most common pregnancy complaints and one of the least discussed, which is unfortunate, because it is also one of the most fixable. It is worth knowing that the largest single contributor is often something you were handed at a booking appointment.
The short answer: progesterone relaxes the bowel wall and slows transit; the uterus adds pressure later; and iron supplements make it substantially worse. What counts as constipated is a drop below your own normal with straining or hard stools — not a failure to go daily. Fibre, fluid and movement first, then bulk-forming laxatives, then lactulose or a macrogol. Call about no movement for several days, severe pain, vomiting, or blood in the stool.
Why it happens
Three mechanisms stack up over the pregnancy. Progesterone rises early and relaxes smooth muscle everywhere — it is the same effect that produces heartburn at the stomach valve and breathlessness in the airways. In the bowel it slows peristalsis, which gives the colon longer to reabsorb water, which is what makes stool hard.
Iron is the second. Oral iron is poorly absorbed and the unabsorbed portion irritates and slows the gut. This is why the timing so often lines up with a prescription rather than with a trimester.
The third is mechanical and arrives late: from the third trimester the uterus sits directly on the rectum, and there is only so much that fibre can do about anatomy.
What is normal
| Normal in pregnancy | Worth raising | |
|---|---|---|
| Frequency | Less often than before, but going | Nothing for several days despite treating it |
| Stool | Firmer, harder work | Blood beyond a streak on the paper, or black and tarry |
| Pain | Cramping that eases after opening the bowels | Severe, worsening, or not relieved at all |
| With it | Bloating, wind | Vomiting, fever, a rigid abdomen |
| Pattern | Steady, related to iron or trimester | Alternating with diarrhoea, or with weight loss |
One connected symptom worth naming: straining is how most pregnancy haemorrhoids start. Treating the constipation is what treats them, which is why the two pages point at each other.
When to call your midwife or doctor
Same-day call: severe or worsening abdominal pain, vomiting with the constipation, a swollen tender abdomen, or significant blood in the stool. Ring during working hours if you have had no bowel movement for several days despite laxatives, if constipation alternates with diarrhoea, or if you have lost weight without trying.
Do not stop prescribed iron on your own. Ring and ask about alternate-day dosing or a different preparation instead — the anaemia it treats is not a small thing to trade away.
Constipation in pregnancy — at a glance
Progesterone relaxes the bowel wall and slows transit, giving the colon longer to reabsorb water; iron supplements slow it further and are often the largest single contributor; and from the third trimester the uterus presses directly on the rectum. Constipation is defined against your own baseline rather than a daily rule — a meaningful drop in frequency with straining, hard stools or a sense of incomplete emptying. Fibre, fluid and walking come first, then bulk-forming laxatives such as ispaghula with plenty of water, then lactulose or a macrogol; castor oil and liquid paraffin are avoided, and anything new is worth checking with a pharmacist. Prescribed iron should not simply be stopped: ask about alternate-day dosing, which absorbs comparably and is easier on the gut. Straining is how most pregnancy haemorrhoids begin, so treating the constipation treats them too. Several days with no movement despite treatment, severe or worsening pain, vomiting, or blood in the stool all need a call rather than more fibre.
Inside Baby Novum: supplements are logged alongside the daily symptom check-in, so a change that started three days after a new iron prescription shows up as a sequence instead of a coincidence — which is the observation that changes the prescribing conversation. Encrypted on the device, no account, no server copy.
Frequently Asked Questions
Why does pregnancy cause constipation?
Progesterone relaxes smooth muscle throughout the body, and the bowel wall is smooth muscle. Transit slows, more water is reabsorbed from the stool, and what arrives is harder and less frequent. Later in pregnancy the uterus adds mechanical pressure on the rectum. Iron supplements, which many people are prescribed, slow things further — often the single largest contributor.
What counts as constipation in pregnancy?
Not a daily rule. Normal ranges from three times a day to three times a week, so the measure is your own baseline. Constipation is a meaningful drop below it, plus straining, hard or lumpy stools, or a sense of incomplete emptying. Someone who went twice a day and now goes every third day with difficulty is constipated even though "every third day" is inside the normal range.
Is it my iron tablets?
Very often, yes — it is the commonest reversible cause and the one people rarely connect, because the tablet was prescribed by the same service being asked about the constipation. Do not simply stop it: iron is prescribed for a reason. Ask about alternate-day dosing, which absorbs comparably well and is easier on the gut, or about a different preparation.
Which laxatives are safe in pregnancy?
The usual order starts with bulk-forming agents such as ispaghula, taken with plenty of water. If those are not enough, an osmotic laxative — lactulose or a macrogol — is commonly used. Stimulant laxatives may be used short term on advice. What is avoided: castor oil, and liquid paraffin. Ask a pharmacist or midwife before starting anything, and say how many weeks you are.
When should I call about constipation?
If you have had no bowel movement for several days despite treating it, severe or worsening abdominal pain, vomiting alongside it, or blood in the stool that is more than a streak on the paper. Also ring if constipation alternates with diarrhoea or comes with unexplained weight loss — that is a different question from pregnancy slowing the gut.