ECHOGENIC (HYPERECHOGENIC) BOWEL

• Echogenic bowel is a well-defined, homogenous, hyperechogenic, lesion in the lower abdomen, which does not shadow (i.e. has no calcific areas), and usually resolves within a few weeks with no long term sequelae to the fetus (1,2).
• Fetal bowel is considered hyperechoic if it is as echogenic as the adjacent bone (more echogenic than fetal liver).
• Although initially considered a normal variant in second-trimester fetuses, it has been described as a prenatal marker for cystic fibrosis, chromosomal aneuploidy and numerous other pathologies.
• Prevalence = 0.2% to 1.8% (3).
• Typically localized in the lower abdomen.
• Nyberg and associates 1993 (2):
• Hyperechoic bowel is present in 0.81% of all second trimester fetuses scanned, compared with 14.5% of fetuses with Down syndrome.
• Bernacerraf an co-workers (11):
• 0.6% incidence of hyperechoic bowel in the second trimester fetal population. Among 50 fetuses with hyperechoic bowel: 8(16%) were aneuploid (trisomy 21,6; Turners Syndrome,1; trisomy 13,1) and 8(16%) had severe IUGR
• 12.5% of all second trimester fetuses with Down syndrome had hyperechoic bowel.
• Hyperechogenic bowel therefore carried a 16-fold greater risk for Down syndrome.

GRADING SYSTEM FOR ECHOGENIC BOWEL

Link to Grading System for Echogenic Bowel

CAUSES OF ECHOGENIC BOWEL

• Fetal aneuploidy especially Trisomy 21 and less frequently trisomy 18 or 13, Turner's syndrome and triploidy. The cause of echogenic bowel in aneuploidy is less clear. It is thought to be due to decreased bowel motility with increased water absorption from the meconium. There appears to be decreased microvillar enzymes activity in the amniotic fluid of aneuploid fetuses (2).
• Cystic fibrosis associated with thick inspissated meconium (7-9). Meconium is more likely to be inspissated in the second trimester when the bowel lumen is narrow. This usually resolves as a result of enlarging small bowel and fetal swallowing of amniotic fluid.
• Small bowel obstruction proximally (especially duodenal atresia) can produce hyperechogenic bowel by reducing the meconium fluid content (2,5).
• Hirschsprung's disease (increased frequency in fetuses with Down syndrome) could produce hyperechogenic bowel due to hypoperistalsis.
• Bowel atresia (6-8). Echogenic bowel is thought to be due to decreased amniotic fluid content of the meconium.
• Intra-amniotic hemorrhage (1,11). Echogenic bowel is probably due to swallowed blood products resulting in a hypercellular meconium, probably with small clots within the bowel lumen.
• Oligohydramnios. Echogenic bowel is also thought to be due to decreased amniotic fluid content of meconium (2,5).
• IUGR - Most fetuses with IUGR do not have echogenic bowel. The suggested mechanism is bowel ischemia due to hemodynamic redistribution and subsequent mesenteric ischemia is therefore questionable.
• Poor perinatal outcome due to uteroplaental insufficiency, especially in cases in which the maternal serum alpha-fetoprotein concentration is elevated (2,5,6,9). As alpha-fetoprotein is a pure fetal product, cases with raised levels are likely to have experienced significant feto-maternal bleeding. This group may represent a subset of fetuses with severe placental damage.
• Other less common associations.
• Cytomegalovirus.
• Toxoplasmosis.
• Parvovirus.
• Thalassemia – The high frequency of echogenic bowel in first and second trimester fetuses suggests bowel wall edema, due to severe anemia and hypoxia may be the cause (10).

CMV infection
Trisomy 21

Table of Echogenic Bowel and Pregnancy Outcome
Ref	GA (wks)	Poor perinatal outcome (%)	Chromosomal defects (%)	Cystic fibrosis(%)	Infection(%)	IUGR (%)*	Perinatal death(%)**
1	15-34	40	3	13	0	21	14
2	15-24	47	25	0	1	18	8
3	14-24	42	16	0	0	19	12
4	15-21	34	6	0	6	13	17
5	14-34	39	4	1	4	7	13
6	16-20	14	6	5	0	0	0
7	14-23	46	6	3	6	7	8
8	16-22	28	5	5	0	11	11
9	15-23	29	11	0	10	7	8
* Excluding chromosomally abnormal fetuses. ** Excluding chromosomally abnormal fetuses and terminations.

It is still controversial whether isolated echogenic bowel justifies invasive testing irrespective of whether associated findings are present. Strocher and co-workers (9) found the prevalence of chromosome defects is at least five-fold higher than that expected on the basis of maternal age. They suggest that even for a 20-year old with a background prevalence of 1:1295 for trisomy 21, the adjusted risk is 1:260.

REFERENCES

1. Dickie LM, Crane JP. Sonographically detected hyperechoic fetal bowel: significance and implications for pregnancy management. Obstet Gynecol 1992;80:778-782
2. Nyberg DA, Dubinsky T, Resta RG et.al. Echogenic fetal bowel during the second trimester: clinical importance. Radiology 1993;188:527-531
3. Bromley B, Doubilet P, Frigoletto FD et.al. Is fetal hyperechogenic bowel on fetal sonogram an indication for amniocentesis. Obstet Gynecol 1994;83:647-651.
4. Hill LM, Fries J, Hecker J et.al Second trimester echogenic small bowel: an increased risk for adverse perinatal outcome. Prenata Diagn 1994;14:845-850.
5. Muller F, Dommergues M, Aubry MC et.al. Hyperechogenic fetal bowel: an Ultrasonographic marker for adverse fetal and neonatal outcome. Am J Obstet Gynecol 1995;173:508-513.
6. Slotnick RN, Abuhamed AZ. Prognostic implications of fetal echogenic bowel. Lancet 1996;347:85-87.
7. Yaron Y, Hassan S, Geva E et.al. Evaluation of fetal echogenic bowel in the second trimester. Fetal Diagn Ther 1999;14:176-180.
8. Ghose I, Mason GC, Martinez D et.al. Hyperechogenic fetal bowel: a prospective analysis of sixty consecutive cases. Br J Obstet Gynaecol 2000;107:426-429.
9. Strocker AM, Snijders RJ, Carlson DE et.al. Fetal echogenic bowel:parameters to be considered in differential diagnosis. Ultrasound Obstet Gynecol 2000;16:519-523.
10. Lam YH, Tang MHY, Lee CP et.al. Echogenic bowel in fetuses with homozygous alpha thalassemia –1 in the first and second trimesters. Ultrasound Obstet Gynecol 1999;14:180-182.

Benacerraf BR. The second trimester fetus with Down syndrome detection using sonographic features. Ultrasound Obstet Gynecol 1996;7:147-150.