Fetal Bladder Size by Gestational Age: Normal Ultrasound Measurements

Fetal bladder dimensions are assessed during routine obstetric ultrasound to evaluate urinary tract patency and renal function. Size varies predictably with gestation.

Typical Transverse Diameter Measurements

Gestational Age	Bladder Diameter (cm)	Notes
12–14 weeks	0.3–0.8	First visible as small fluid-filled sac
18–20 weeks	1.5–2.5	Anomaly scan; should fill and empty cyclically
24–28 weeks	2.5–3.5
32–36 weeks	3.5–5.0	Near term; may reach 5–6 cm when full
Term (38–40 wks)	4.0–6.0	Highly variable with fetal voiding cycle

Key Ultrasound Observations

• Bladder should be visualized filling and emptying every 30–60 minutes.
• Persistent distension (>7 cm, thickened wall, keyhole sign) suggests outlet obstruction (posterior urethral valves, urethral atresia).
• Non-visualization after 30–60 min scanning may indicate renal agenesis, severe oligohydramnios, or empty bladder—repeat scan.

Clinical Significance

• Normal cyclical filling/empty = intact renal function + patent outflow.
• Abnormal size or wall thickness prompts detailed urologic evaluation and postnatal follow-up.
• Megacystis in first trimester (≥7 mm at 10–14 wks) associated with chromosomal anomalies (trisomy 13, 18) and obstructive uropathy.

Measurements are guidelines; individual variation exists. Serial assessment trumps single measurements.