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Topic

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Fetal buttock

The fetal buttock forms the posterior region of the pelvis and proximal thigh, consisting primarily of the gluteal muscles, subcutaneous tissue, developing fat, and overlying skin. It is shaped by the growth of the ilium, sacrum, and proximal femur.

In prenatal imaging, the buttock is an important anatomical landmark for fetal positioning (e.g., breech presentation), growth assessment, and the evaluation of soft tissue development. It reflects both muscular and fat deposition, which increase significantly in the third trimester, contributing to fetal body contour.

Synonyms

  • Fetal gluteal region

  • Prenatal gluteal muscles

  • Posterior pelvic–thigh region

Structure and Development

  • Bones: Supported by posterior pelvis (ilium, sacrum) and proximal femur

  • Muscles: Gluteus maximus, medius, and minimus (develop early, bulk increases with gestation); deeper rotators form beneath

  • Fat: Minimal early in gestation; increases substantially in the third trimester, contributing to contour and volume

  • Skin and connective tissue: Enveloping layers that thicken as gestation advances

  • Landmark: The intergluteal cleft forms progressively with fetal growth

Relations

  • Superiorly: Lower lumbar and sacral spine

  • Inferiorly: Posterior thigh (hamstring muscles)

  • Anteriorly: Pelvis and hip joint

  • Laterally: Greater trochanter and thigh muscles

Function

  • Provides cushioning and contour to the fetal pelvis

  • Contributes to intrauterine movement and posture via gluteal muscles

  • Stores fat reserves late in gestation, supporting neonatal thermoregulation and energy balance

  • Serves as a key landmark for breech vs. cephalic presentation during obstetric evaluation

Clinical Significance

  • Breech presentation: Buttock position relative to uterus/cervix is critical for delivery planning

  • Growth restriction (IUGR): Reduced soft tissue and fat deposition visible in buttocks

  • Skeletal dysplasias: May alter pelvic–buttock contour

  • Neuromuscular disorders: Reduced muscle bulk or abnormal signal may affect buttock morphology

  • Tumors/masses: Rare congenital lesions (e.g., sacrococcygeal teratoma) often arise in buttock/pelvic region and are best evaluated with MRI

MRI Appearance

T2 HASTE (T2 GRE):

  • Gluteal muscles: Low-to-intermediate signal intensity (darker than amniotic fluid, higher than cortical bone)

  • Developing fat: Appears hyperintense; minimal early, increases markedly in late gestation

  • Skin/subcutaneous tissue: Thin hypointense rim with surrounding bright amniotic fluid providing contrast

  • Pathology (edema, mass): Appears hyperintense relative to muscle

T1 GRE:

  • Gluteal muscles: Intermediate-to-low signal intensity

  • Fat: Bright hyperintense signal, increasing with gestation

  • Bone (pelvis, proximal femur): Very low signal cortex, intermediate marrow

  • Hemorrhage, calcification, or fat-containing mass: Appears focally hyperintense

MRI image

Fetal buttock MRI axial anatomy image-img-00000-00000