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Uterus (pregnancy)

The uterus undergoes remarkable structural, vascular, and functional changes during pregnancy. Normally a pear-shaped pelvic organ, it enlarges progressively to accommodate the growing fetus, placenta, and amniotic sac. By the end of pregnancy, the uterus extends into the abdominal cavity and can reach the xiphoid process.

The uterine wall becomes thinner but highly vascular, with hypertrophy of smooth muscle fibers and significant remodeling of blood supply. These changes are essential to support fetal growth, protect the fetus, and allow labor.

Synonyms

  • Gravid uterus

  • Uterus gestationis

  • Pregnant womb

Structure and Changes in Pregnancy

  • Size and shape: Enlarges from ~7.5 cm in length (non-pregnant) to >35 cm near term, becoming more spherical

  • Wall thickness: Thins as gestation advances, particularly in lower uterine segment

  • Myometrium: Hypertrophy of smooth muscle fibers; highly vascular with spiral artery remodeling

  • Endometrium: Becomes decidua, providing support and interface with placenta

  • Cavity: Expands to house fetus, placenta, and amniotic fluid

  • Lower uterine segment: Develops late in pregnancy, important in labor and cesarean delivery

Relations (Pregnancy State)

  • Anteriorly: Abdominal wall (after second trimester), bladder inferiorly

  • Posteriorly: Rectouterine pouch, loops of bowel

  • Superiorly: Abdominal viscera (small intestine, omentum) as uterus enlarges

  • Inferiorly: Cervix, vagina, pelvic floor

Function

  • Provides protective environment for developing fetus

  • Facilitates maternal-fetal exchange via placenta and decidua

  • Produces contractile forces for labor and delivery

  • Maintains pregnancy through hormonal and vascular adaptations

Clinical Significance

  • Placenta previa/accreta: Uterine wall and placental interface abnormalities

  • Uterine rupture: Risk in scarred uterus, particularly at lower segment

  • Fibroids in pregnancy: May cause complications (pain, malpresentation)

  • Preterm labor: Associated with cervical or uterine abnormalities

  • MRI relevance: Used to evaluate placenta accreta spectrum, uterine anomalies, or masses during pregnancy when ultrasound is inconclusive

MRI Appearance

T2 HASTE (T2 GRE):

  • Myometrium: intermediate signal

  • Junctional zone: often less distinct in pregnancy, appears as slightly darker band

  • Placental bed and venous lakes: bright hyperintense foci

  • Uterine wall: may show heterogeneous intermediate-to-bright signal depending on vascularity

  • Lower uterine segment: thinned, intermediate signal

T1 GRE:

  • Myometrium: isointense to mildly hypointense relative to muscle

  • Decidua and placenta: isointense to mildly hyperintense

  • Blood products (hemorrhage/hematoma): bright hyperintense

  • Amniotic fluid: low signal

MRI image

uterus (pregnancy) MRI coronal image