Dilated rectum in anomaly scan
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Dilated Rectum on Fetal Anomaly Scan
Normal Fetal Rectal Dimensions
The fetal rectum progressively enlarges with gestational age. Any value exceeding the 95th percentile should prompt further evaluation. Bowel loops >7 mm are generally considered suggestive of obstruction.
Table: Normal Fetal Rectal Diameters (mm) — Creasy & Resnik's Maternal-Fetal Medicine
| GA (weeks) | Rectal Diameter Mean (mm) | 95% CI (mm) |
|---|---|---|
| 19–20 | 3.64 | 1.45–5.82 |
| 22 | 3.95 | 1.78–6.13 |
| 24 | 4.34 | 2.17–6.52 |
| 28 | 5.38 | 3.20–7.55 |
| 32 | 6.80 | 4.63–8.98 |
| 36 | 8.68 | 6.51–10.85 |
| 40 | 10.87 | 8.66–13.08 |
Key Point: Dilated Rectum Often Appears Late
Dilated bowel and/or colon may not be evident until the third trimester, even when significant pathology is present. A dilated rectum seen at the mid-trimester anomaly scan (18–22 weeks) is therefore a significant finding.
Differential Diagnosis
Most Important Cause: Imperforate Anus / Anorectal Atresia
The classic finding is a dilated, echogenic rectum due to accumulation of meconium proximal to the obstruction.
Fig. 24.15: Imperforate anus — dilated rectum at 20w2d filled with echogenic meconium, 4.27 cm × 1.36 cm. — Creasy & Resnik's Maternal-Fetal Medicine
Additional appearances include:
- V-shaped segment of dilated bowel (~10 mm diameter), suggestive of anorectal atresia
- "Bag of marbles" appearance — markedly dilated colon with echogenic meconium, suggesting rectourinary fistula with anal atresia
Other Causes to Consider
| Category | Differential |
|---|---|
| Obstructive (distal) | Anorectal/anal atresia, Hirschsprung disease (aganglionosis of distal colon/rectum), colonic atresia, meconium ileus (especially with CF) |
| Normal variant | Mildly dilated, normal colon (physiologic) |
| Mimics | Hydrocolpos (fluid-filled vagina in females), hydroureter, umbilical vein varix, ovarian/mesenteric/duplication cysts |
| Syndromic | Cloacal malformation (females only — rectum + vagina + urinary tract communication) |
Associated Anomalies — Critical to Search For
A dilated rectum from anorectal malformation is frequently non-isolated. Screen systematically for:
- VACTERL association: Vertebral defects, Anal atresia, Cardiac defects, Tracheo-Esophageal fistula, Renal agenesis/anomalies, Limb defects
- VATER syndrome: Vertebral defects, imperforate Anus, Tracheo-Esophageal fistula, Renal defects
- OEIS complex: Omphalocele, Exstrophy of bladder, Imperforate anus, Spinal deformities
- Caudal regression syndrome
- Congenital heart defects (ASD/VSD)
- GI malformations: malrotation, esophageal atresia, annular pancreas
- Genitourinary malformations
Antenatal Management
- Detailed anomaly survey — search for VACTERL components and other structural defects
- Fetal echocardiogram — cardiac defects are common associates
- Amniocentesis with chromosomal microarray or NIPT — assess karyotype; also test for cystic fibrosis (meconium ileus association)
- Fetal MRI — consider to determine the level of obstruction (high vs. low anorectal malformation)
- Serial ultrasound — monitor fetal growth, amniotic fluid volume, bowel distention, and signs of perforation → meconium peritonitis
- Third-trimester surveillance — non-stress test / biophysical profile
- Pediatric surgery consultation antenatally
- Delivery at a tertiary centre — neonatal surgical intervention required
- Therapeutic amnioreduction if polyhydramnios develops (reduces preterm labour risk)
- Caesarean delivery reserved for obstetric indications only
Neonatal Workup (After Delivery)
- Orogastric/nasogastric decompression
- IV fluid hydration
- Plain abdominal X-ray
- Upper GI contrast study
- Surgical correction (timing and approach depends on lesion level and associated anomalies)
Key Take-Away: A dilated rectum on anomaly scan should prompt a thorough search for anorectal malformation, with systematic evaluation for VACTERL/OEIS components, fetal echo, karyotyping, and MRI. Isolated low-type anorectal malformations carry a good prognosis; complex/high-type and syndromic forms require multidisciplinary planning.
— Creasy & Resnik's Maternal-Fetal Medicine: Principles and Practice, Chapter 24
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