Common in every trimesterStrong evidence
More than two-thirds of pregnant women have low back pain and almost one in five have pelvic pain, a Cochrane review says, and both typically increase as pregnancy goes on. Single studies range from 39.5% of 1,835 Australian women asked about back pain to 68.5% of 950 women in US antenatal clinics and 72% of 891 Swedish women asked about low back and pelvic pain together, because the questions differ. In 1,158 Spanish women at around 35 weeks, 71.3% had had low back pain and 64.7% pelvic girdle pain in the previous 4 weeks.
Most of it is moderate, but it matters: in the US survey, back pain disturbed sleep for 58% and daily life for 57%, nearly 30% stopped at least one daily activity, and only 32% told their antenatal team. Severe disability affects about 8%, a review estimates.
Sources 1, 2, 3, 4, 5, 6
When it starts: any trimester, often worse laterMixed evidence
The NHS says backache is very common in pregnancy, "especially in the early stages". The cohort studies show it at a similar level from about 12 weeks onwards: in 855 Swedish women followed every 2 weeks, 22% already had back pain at 12 weeks and 22% to 28% had it at any one time from then until the birth, and in a survey of 950 US women it was as common early in pregnancy as late.
What tends to rise later is how bad it gets. Pelvic girdle pain often starts around 18 weeks and is worst between 24 and 36 weeks, a review says, and back pain intensity increased through the second half of pregnancy in a Swedish trial of 258 women. So the honest answer to "which trimester is worst?" is that it can start at any point and often gets more intense later.
Sources 7, 8, 3, 6, 9
Low back pain, pelvic girdle pain, or bothStrong evidence
About half of pregnancy back and pelvic pain is pelvic girdle pain, a third is low back pain and a sixth is both, a review estimates; in 313 Swedish women, those with both had the worst health and function. Low back pain sits in the spine above the pelvis, often as a dull ache. Pelvic girdle pain sits at the back of the pelvis, in the buttock or over the pubic bone, sometimes between the vagina and anus or into the thighs, and is brought on by walking, stairs, standing on one leg, turning in bed and moving the legs apart.
Pelvic girdle pain affects up to 1 in 5 pregnant women and isn't harmful to the baby, the NHS and the Royal College of Obstetricians and Gynaecologists (RCOG) say; it has its own guide. True sciatica, pain down a leg in a nerve's pattern, affected 10 of 855 women (1%) in the Swedish cohort: most buttock and thigh pain in pregnancy comes from the pelvis instead.
Sources 6, 10, 11, 12, 8
Why it happens is only partly knownMixed evidence
The NHS says the ligaments soften and stretch to prepare for labour, which can strain the joints of the lower back and pelvis, and the UK's pelvic health physiotherapists (Pelvic, Obstetric and Gynaecological Physiotherapy, POGP) add the changes in posture and in your centre of gravity as the bump grows.
The evidence for the usual explanations is thinner than it sounds. The hormone relaxin didn't match who got pelvic girdle pain in 3 of 4 good studies, and in 35 women, joint looseness didn't track hormone levels or joint pain. Posture changes in pregnancy, but in different directions in different women: 11 of 15 pregnant women had a flatter, not more curved, low back, and in 12 women the curve and back pain both rose without being related. A review of 50 studies found back pain wasn't clearly linked with the spinal changes.
Sources 7, 13, 14, 15, 16, 17, 18
Who is more likely to get itEmerging evidence
Back pain before pregnancy, with periods or in an earlier pregnancy was the clearest predictor, along with younger age; more previous births and very flexible joints went with low back and pelvic pain in a Swedish study. Higher body weight went with low back and pelvic pain in one study, and with pelvic girdle pain but not low back pain in another. These are links in observational studies, not causes, and exercise helps at every size.
Work matters too. Physical and psychological work factors raised the risk in the Swedish cohort, and across 16 studies, frequently lifting heavy loads at work (often 10 kg or more) went with pelvic girdle pain that limits what you can do, with limited but consistent evidence.
Sources 3, 4, 5, 8, 19
The causes that aren't the back, and how rare they areStrong evidence
A few causes of back pain in pregnancy aren't the back, and each has its own clues. About 8 in 100 babies are born early, and before 37 weeks, regular tightenings, period-type pains, a gush or trickle of fluid or backache that isn't usual for you need a call; backache is also a normal sign that labour has begun at term. Pre-eclampsia is most likely from 20 weeks and can follow the birth; it complicates 2% to 8% of pregnancies worldwide, is not severe in most cases, and fewer than 1 in a million women giving birth in the UK now die from it.
A kidney infection brings pain in the back or side with a fever, shivering or pain when you pee, and complicated 0.5% of 543,430 US pregnancies. Placental abruption, where the placenta starts to come away from the wall of the womb, usually causes bleeding and pain that comes and goes like a contraction or is constant; it affects about 0.4% to 1% of pregnancies. Blood clots affect about 1 to 2 in 1,000 pregnancies, most often just after the birth.
Rarer still: a slipped disc pressing on a nerve was found in about 1 in 10,000 births in one hospital, and cauda equina syndrome in pregnancy is known from case reports. Pregnancy and lactation associated osteoporosis, thinning of the bones that can cause spinal fractures late in pregnancy or while breastfeeding, is rare, and a third of those affected have a family history of osteoporosis.
Sources 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33
When a scan or test helpsStrong evidence
Back pain and pelvic girdle pain in pregnancy are recognised from the story and an examination, and no scan is needed for them. Tests answer specific questions: a urine test for a suspected urine or kidney infection; blood pressure and pee checks at every antenatal appointment, and hospital tests if pre-eclampsia is suspected; monitoring of contractions and the baby's heartbeat for suspected labour; and an ultrasound scan of the leg within 24 hours for a suspected clot.
For the rare disc or nerve emergency, a magnetic resonance imaging (MRI) scan, which uses no X-rays, shows the problem, and surgery during pregnancy is possible when it's needed; reviews of about 50 reported cases each found good outcomes in most. After the birth, a scan can find a stress fracture at the base of the spine or a pregnancy-related spinal fracture.
Sources 25, 34, 22, 20, 35, 30, 31, 36, 32
How it usually goes, in pregnancy and after the birthMixed evidence
In pregnancy it often lasts weeks to months, and pelvic girdle pain tends to build towards 24 to 36 weeks; exercise makes it less severe. Most women improve after the birth, but how many still have pain depends on how it's counted: about 25% still have back or pelvic pain after the birth, about 7% seriously, in a review, while 43.1% of Swedish women who had pain in pregnancy still had some at 6 months. The definitions differ, so the figures aren't directly comparable.
In 341 Danish women with confirmed pelvic joint pain, 62.5% were pain-free within a month of the birth and 8.6% still had pain at 2 years. By 3 months after the birth, 72% with low back pain and 66% with pelvic girdle pain had recovered, against 33% of those with both. Of the Swedish women with pain at 6 months, most had pain that came and went at about 14 months. NHS inform and the RCOG say around 1 in 10 have ongoing pelvic pain. No one can promise a timescale.
Sources 6, 37, 38, 39, 40, 41, 29, 12
