umbilical artery

**Fetal circulation (the high-yield core):** - The umbilical cord normally has 2 umbilical arteries and 1 umbilical vein ("AVA": 2 Arteries, 1 Vein; the vein carries oxygenated blood). - Umbilical arteries carry deoxygenated, waste-laden blood (PO2 ~ 15-25 mmHg) from the fetus to the placenta. The umbilical vein carries oxygenated blood (~80% saturated) from the placenta to the fetus, draining via the ductus venosus to the IVC. - Blood flow: descending aorta → common iliac → internal iliac → umbilical arteries → placenta. - The umbilical arteries are surrounded by Wharton jelly (mucoid connective tissue) within the cord.

**Embryology:** - Arise from the paired dorsal aortae/the internal iliac (hypogastric) arteries. - Postnatal fate: distal portions → **medial umbilical ligaments** (raise the medial umbilical folds of the anterior abdominal wall peritoneum). Proximal patent portions → **superior vesical arteries** (supply the bladder). - Related remnants: umbilical vein → ligamentum teres hepatis (round ligament of the liver, in the falciform ligament); ductus venosus → ligamentum venosum; urachus → median umbilical ligament; ductus arteriosus → ligamentum arteriosum. - Mnemonic: the umbilical arteries become the **medial** umbilical ligaments; the urachus becomes the **median** one.

**Clinical significance:** - **Single umbilical artery (SUA; "two-vessel cord")**: occurs in ~1% of singleton pregnancies. Associated with congenital anomalies (renal/genitourinary, cardiac, GI, and musculoskeletal), aneurysmal/chromosomal syndromes (trisomy 18, trisomy 13), and VACTERL, as well as IUGR, preterm birth, and low birth weight. Often isolated and benign, but a detailed anatomy scan and fetal echocardiography are generally warranted. - **Umbilical artery Doppler**: used to assess placental resistance in IUGR. Normally there is forward diastolic flow with a falling S/D ratio as gestation advances. Increased resistance → absent end-diastolic flow → **reversed end-diastolic flow** (ominous, signals impending fetal compromise and the need for delivery). - **Umbilical cord blood gas**: arterial sample reflects fetal acid-base status (metabolic acidosis, pH < 7.0 and base deficit ≥ 12, supports hypoxic-ischemic injury and is one criterion for neonatal encephalopathy). Venous sample reflects placental/maternal status. - **Umbilical artery catheterization (UAC)** in sick neonates: used for blood gas monitoring and BP. Complications: thromboembolism, vasospasm, limb or bowel ischemia, NEC, renal artery thrombosis/hypertension, infection. - **Cord clamping**: after birth, increased systemic vascular resistance and the loss of the low-resistance placenta, along with umbilical artery vasoconstriction (reaction to cold, O2, and mechanical stimulation), help close the arteries functionally. - **Hematologic**: the cord blood from the umbilical vein is the source for banking stem cells. - **Cord abnormalities**: velamentous insertion and vasa previa (fetal vessels unprotected by Wharton jelly, which risks fetal exsanguination when membranes rupture, classically with painless vaginal bleeding and fetal bradycardia).

**Board pearls:** - 2 arteries (deoxygenated) + 1 vein (oxygenated) = reverse of the usual systemic pattern (arteries carry oxygenated blood). - SUA → look for renal and cardiac anomalies. - Medial umbilical ligament = obliterated umbilical artery; Median = urachus; Lateral umbilical fold = inferior epigastric vessels.