Sacral dimple in babies: what it is, causes and when it needs a scan

A sacral dimple, also called a sacral pit, is an indentation in the skin of the lower back. The BNSSG neonatal guidance says it is present at birth but sometimes not noticed until the 8-week check. Most are blind-ending and sit just above or within the crease of the buttocks.

Because they are common, doctors see many. NHS Lothian guidance says they occur in 3 to 8% of children. As an isolated finding, a simple dimple or pit is not a useful marker of occult spinal dysraphism (OSD), which is a range of skin-covered abnormalities of the spinal column.

The practical question for clinicians is therefore not whether a dimple exists but whether it looks simple or has features that make a hidden spinal problem more likely.

The guidance used here does not explain why a dimple forms. NHS Lothian classes sacral dimples as minor congenital anomalies, meaning they are present from birth, seen in 3 to 8% of children, and BNSSG says they are present at birth but sometimes not noticed until the 8-week check.

What the guidance does explain is the concern behind the checks. As an isolated finding, NHS Lothian says a simple dimple or pit is not a useful marker of occult spinal dysraphism, which is a range of skin-covered abnormalities of the spinal column. The practical aim is to spot the small number of babies whose dimple has features that make a hidden problem more likely.

BNSSG guidance defines a typical dimple as one that is under 5mm, where the base can be seen, that is no more than 2.5cm from the anus, in the midline, and in a baby with a normal neurological examination. These need no imaging and no referral.

An atypical dimple may be associated with an underlying neurological problem such as spinal dysraphism and needs further imaging or referral. NHS Lothian adds that dimples are more likely to be linked with OSD when they lie above the gluteal cleft, the natal cleft between the buttocks.

NHS Lothian says a baby should be referred for an outpatient ultrasound if two or more of these are present: multiple dimples, over 5mm diameter, base not visible, a dimple pointing towards the head rather than straight down, other skin abnormalities or masses, a distance of more than 2.5cm from the anus, or an abnormal neurological examination.

It also lists combinations. A dimple together with one other feature should prompt a scan: excessive hair, a lipoma or other mass, pigmented patches, skin tags or tail-like appendages, vascular marks such as haemangioma, or signs of infection; anorectal abnormalities; aplasia cutis; or orthopaedic findings such as hip dislocation, leg length difference, scoliosis or talipes.

Local pathways differ in detail. BNSSG guidance says one or more of its criteria is enough to request imaging, so a clinician in a different area may act on a slightly different threshold. The GP or paediatrician explains the local approach.

BNSSG guidance explains that an ultrasound scan can be requested up to a maximum of 6 weeks of age, ideally at the initial neonatal check. After about 6 weeks, ultrasound cannot see deep enough to exclude a spinal cord abnormality because the bones at the back of the spine have started to harden.

That has a practical consequence. If a dimple that meets the criteria is picked up later, for example at the 8-week check, a general paediatric referral is made so that further imaging can be considered. GPs cannot refer directly for MRI.

NHS Lothian guidance says that where OSD is suspected, the scan should be done within 4 weeks, and that the person requesting it is responsible for chasing the result and telling the parents, rather than asking the baby to come back to clinic.

The concern with an atypical dimple is a hidden spinal condition under intact skin. NHS Lothian says most OSD is picked up on the antenatal anomaly scan. The NHS spina bifida page describes closed forms where there is a small gap in the spine covered by skin, and says fatty lumps, a hairy patch or deep dimples on the back at birth can be signs.

Closed spina bifida may cause no symptoms. When it does, the NHS lists pain or numbness in the legs or back, weakness in the legs, and difficulty controlling urine or stools. Milder types such as spina bifida occulta can be diagnosed by a spine scan such as X-ray, MRI or CT.

This is why the information doctors collect before referral includes whether the baby moves their legs normally and whether there are any problems with urination or stooling.

Mention any dimple, hair tuft, lump or birthmark near the base of the spine at the next baby check, or to your GP, health visitor or paediatrician, rather than waiting. NHS Lothian lists signs of infection such as redness or hardness near a dimple among the features that make a scan more likely, so tell a doctor if you see these.

The NHS spina bifida page lists, for closed forms, pain or numbness in the legs or back, weakness in the legs and difficulty controlling urine or stools. If your baby or child develops any of these, or you are worried about leg movement, ask for an urgent assessment through your GP or NHS 111, and call 999 if your baby is seriously unwell.

If you notice a dimple, mention it at the next baby check. Clinicians will look at its size, whether the base can be seen, how far it is from the anus, whether there is any hair, lump or skin change, how many dimples there are, and how the baby moves their legs.

BNSSG guidance suggests that if a photograph is being sent, a tape measure placed beside the dimple and in line with the anus helps the reader judge the distance. Parents may be asked to take such a photo.

Most families are told that the dimple is simple and no further tests are needed. If a scan is arranged, it is an ultrasound. Results are usually phoned through to the parents under the NHS Lothian approach.

A dimple present from birth is a different matter from a pilonidal sinus, which has its own guide on this site. If a lump, discharge or redness appears near the base of the spine at an older age, raise it with a GP.

Signs of infection in or around a dimple, such as redness, hardness or discharge, are listed by NHS Lothian among skin changes that make a scan more likely, so tell a doctor if you see them in your baby.