Call now if

One-sided pain has a wide range of causes, from entirely ordinary to genuinely urgent, and the sorting is done by what comes with it rather than by which side it is on.

Call 999 if you are pregnant and bleeding and you have severe pain in your tummy that stops you focusing on anything, pain in your shoulder, feel sick, faint, dizzy or lose consciousness, or the bleeding is heavy. Those are the NHS's own words, and pain in the tip of the shoulder alongside early-pregnancy tummy pain is the classic presentation of a ruptured ectopic pregnancy.

Call your maternity unit immediately if you have stomach pain and:

vaginal bleeding or spotting
regular cramping or tightenings
lower back pain
unusual vaginal discharge, or fluid leaking from your vagina
pain when you pee, or needing to pee suddenly or more often than usual
pee that looks cloudy, pink, red or brown
pain that is severe, or does not go away after you have rested for 30 to 60 minutes

If you are under 20 weeks, the NHS says you may need to contact your GP or an early pregnancy unit instead, depending on what is available in your area.

The ordinary explanations

The NHS names three harmless causes of stomach pain in pregnancy, and two of them are typically one-sided.

Ligament pain. Often called growing pains, as the ligaments stretch to support the bump. The NHS describes it as feeling like "a sharp cramp on one side of your lower tummy". It is provoked by standing up quickly, coughing, sneezing, rolling over in bed or a sudden movement, and it passes within seconds to minutes.
Trapped wind. Gas caught in the colon can produce genuinely sharp, one-sided pain, high or low, that shifts with position.
Constipation. Common in pregnancy from the earliest weeks, and classically felt on the left.

The test that covers all three is the NHS's: mild pain that eases when you change position, rest, pass wind or open your bowels is almost certainly one of them.

Does the side mean anything?

Less than people hope. There is no side that is reassuring and no side that is ominous. What sits where is worth knowing only as background: the appendix on the right, the liver and gallbladder on the upper right, the sigmoid colon on the lower left, and an ovary and fallopian tube on each side. But pregnancy displaces organs upwards and outwards as it goes on, and the appendix in particular moves, which is precisely why appendicitis is harder to diagnose in pregnancy.

Early pregnancy: the one that must be excluded

Ectopic pregnancy is when a fertilised egg implants outside the womb, usually in a fallopian tube. The NHS says symptoms typically appear between 4 and 12 weeks and can include:

tummy pain and bleeding
pain in the tip of your shoulder
discomfort when pooing or peeing

Shoulder tip pain deserves its own sentence because it is so easily dismissed. It is referred pain from blood irritating the diaphragm, and in the context of early pregnancy it means internal bleeding until proved otherwise. It does not feel like a pulled muscle; it is felt at the very top of the shoulder, often worse lying down.

RCOG's patient information on bleeding and pain in early pregnancy is built around getting this assessed quickly, usually through an early pregnancy unit with an ultrasound scan and blood tests. The NHS notes that miscarriage and threatened miscarriage can also cause cramping and bleeding before 24 weeks, and that you may be able to go straight to an early pregnancy unit for assessment.

Later pregnancy: the four to keep in mind

Urine infection. The NHS says UTIs are common in pregnancy and "can cause tummy pain and sometimes, but not always, pain when you pee". A one-sided ache in the back, just under the ribs, with a temperature, suggests it has reached a kidney and needs same-day assessment.
Premature labour. The NHS is unambiguous: if you are less than 37 weeks and having regular abdominal cramps or tightenings, call your maternity unit immediately. Early contractions are often felt more on one side or as backache.
Placental abruption. The placenta starting to come away from the wall of the womb, usually causing bleeding and pain, which the NHS says "can come and go like a contraction, or it can be constant". If the pain or bleeding is severe, call 999.
Pre-eclampsia. The NHS explains that pain just under the ribs is common in later pregnancy from the uterus pushing upwards, "but if this pain is bad or persistent, particularly on the right side, it can be a sign of pre-eclampsia". Severe, persistent, upper-right pain is a different symptom from a sharp low twinge.

The HSE adds that itching, severe nausea or vomiting, or the whites of the eyes turning yellow alongside third-trimester pain could indicate a liver or gallbladder problem and should be reported.

What to do with a pain that is probably nothing

Change position and give it up to an hour. The 30 to 60 minute rest test is the NHS's own threshold.
Empty your bladder and, if you can, your bowels.
Drink some water. Dehydration provokes tightenings.
Check your pants and your pee. Bleeding and cloudy or pink pee both change the answer.
From 24 weeks, check your baby's movements. Any change is its own reason to ring.

If it settles with those, it was almost certainly ligament or gut. If it does not, or if anything on the urgent list is present, the phone call is the correct next step and no maternity unit will mind.

Pain on One Side in Pregnancy