Dilated rectum in anomaly scan
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Dilated Rectum on Fetal Anomaly Scan
Normal Reference Values
The fetal rectum gradually enlarges with gestation. A useful rule of thumb from Creasy & Resnik's is that bowel loops >7 mm are suggestive of obstruction. The table below gives normal colon and rectal diameters:
| GA (weeks) | Descending Colon mean (mm) | 95% CI | Rectal mean (mm) | 95% CI |
|---|---|---|---|---|
| 19–20 | 3.52 | 0.79–6.26 | 3.64 | 1.45–5.82 |
| 22 | 3.69 | 0.96–6.41 | 3.95 | 1.78–6.13 |
| 25 | 4.18 | 1.46–6.90 | 4.57 | 2.40–6.74 |
| 28 | 5.04 | 2.32–7.76 | 5.38 | 3.20–7.55 |
| 32 | 6.86 | 4.13–9.58 | 6.80 | 4.63–8.98 |
| 36 | 9.44 | 6.71–12.17 | 8.73 | 6.50–10.96 |
| 38–40 | 10.84 | 8.12–13.56 | 9.58 | 7.35–11.81 |
Importantly, dilated colon/rectum may not become apparent until the third trimester, even when obstruction is present earlier.
— Creasy & Resnik's Maternal-Fetal Medicine, Table 24-1
Sonographic Appearances & Differential Diagnosis
Key Pattern: Dilated Rectum with Echogenic Meconium
Imperforate anus / anorectal atresia is the most important cause:
- Produces a dilated rectum filled with echogenic meconium (meconium cannot exit → accumulates and becomes progressively echogenic)
- At 20 weeks: diameter of ≥1.3 cm (4.3 cm length) raises strong suspicion
- May appear as a V-shaped segment of dilated bowel measuring ~10 mm in diameter
Fig. 24.15: Dilated rectum in a 20-week fetus with imperforate anus — echogenic meconium contents, 1.3 cm diameter, 4.3 cm length. — Creasy & Resnik's
Fig. 24.14: V-shaped segment of dilated bowel measuring 10 mm in diameter, suggestive of anorectal atresia. — Creasy & Resnik's
Differential Diagnoses
When a dilated rectum is seen on anomaly scan, consider:
| Diagnosis | Key Features |
|---|---|
| Imperforate anus / anal atresia | Dilated rectum with echogenic meconium; most common cause |
| Anorectal atresia | V-shaped dilated bowel; echogenic contents |
| Cloacal anomaly / rectourinary fistula | Dilated colon with echogenic "bag of marbles" pattern; meconium in urinary tract |
| Hirschsprung disease | Aganglionosis of distal colon/rectum; may not be visible prenatally |
| Colonic atresia | More proximal distension; rectum may appear dilated secondarily |
| Meconium ileus (cystic fibrosis) | Echogenic bowel; multiple dilated loops |
| Meconium plug syndrome | Transient; may resolve postnatally |
| Mildly dilated normal colon | Within 95th centile for GA; no associated features |
| Intestinal malrotation | Variable appearance |
| Congenital chloride diarrhea | Rare; polyhydramnios |
| Mimics: hydrocolpos, ovarian/mesenteric/duplication cysts | Location, walls, no echogenic meconium contents |
Associated Anomalies to Search For
Anorectal malformations are strongly associated with several syndromes — a thorough anatomic survey is mandatory:
- VACTERL association: Vertebral defects, Anal atresia, Cardiac defect, Tracheo-Esophageal fistula, Renal agenesis, Limb defects
- VATER syndrome: Vertebral, imperforate Anus, Tracheo-Esophageal fistula, Renal defects
- OEIS complex: Omphalocele, Exstrophy of bladder, Imperforate anus, Spinal deformities
- Caudal regression syndrome
- Trisomy 21 (especially in duodenal atresia — present in ~30%)
- Congenital heart defects (ASD, VSD)
- Genitourinary anomalies, ventral wall defects, skeletal defects
Antenatal Management
Once a dilated rectum / lower bowel obstruction is suspected:
- Amniocentesis for chromosomal microarray ± NIPT — assess karyotype and cystic fibrosis gene status
- Fetal echocardiogram — cardiac defects common in VACTERL
- Targeted anatomic survey — spine, kidneys, limbs, sacrum
- Fetal MRI — can better define the level of obstruction and anatomy of the anorectal region
- Serial sonographic surveillance — monitor:
- Fetal growth
- Amniotic fluid (polyhydramnios risk with proximal obstruction)
- Progressive bowel distention
- Meconium peritonitis (bowel perforation)
- Third-trimester BPP / NST
- Pediatric surgery consultation antenatally
- Delivery at a tertiary centre — neonatal surgical team must be present
- Therapeutic amnioreduction if marked polyhydramnios threatens preterm labour
- Caesarean delivery only for obstetric indications (not indicated for bowel anomaly alone)
Prognosis
- Depends on level of obstruction and associated anomalies
- Isolated imperforate anus (low type): generally good prognosis with surgery
- Complex anorectal malformations with sacral agenesis or VACTERL: worse functional outcomes (continence, renal function)
- Survival for surgically repaired intestinal atresia has improved to 90–100% in modern series
- Long-term risk: short bowel syndrome (jejunoileal atresia), fecal continence issues (high anorectal malformations)
Bottom line: A dilated rectum on anomaly scan — especially containing echogenic meconium — is the hallmark sonographic sign of imperforate anus/anorectal atresia. It mandates a complete anatomic survey for VACTERL/OEIS features, fetal karyotyping, CF gene testing, fetal echo, and MRI for anatomical delineation. Delivery should be planned at a centre with neonatal surgery capability.
— Creasy & Resnik's Maternal-Fetal Medicine, Principles and Practice, Chapter 24
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