Abdominopelvic cysts encompass a broad spectrum of neoplasms and malformations that are usually classified according to fetal gender, gestational age at the time of the diagnosis, and anatomic location. The most common pathologies arise from the urinary tract, the ovaries, the gastrointestinal tract, the liver, and the mesentery. The rare causes of fetal cysts are: adrenal cysts, splenic cysts, pancreatic cysts, choledochal cysts, and umbilical vein „cysts” (varices). 

• Hepatic cysts:  This malformation is rare, usually unilocular. They could be intra- or sub-hepatic, and attached to the liver capsule. Generally, they tend to remain stable in size or regress spontaneously. Cysts follow fluid signal on MRI with hyperintense T2-weighted (T2w) signal and hypointense T1-weighted (T1w) signal.
  

  Fig 1: 32-week-old female fetus with cystic lesion detected near the liver hilum. Possible differential diagnoses: liver cyst, choledochal cyst, enteric duplication cyst. Postnatal ultrasound was suggestive of liver cyst, no surgical intervention was performed. A: T2 HASTE coronal image, B: T2 HASTE transversal image, C: T2 HASTE sagittal imageL:liver, K:kidney, B:bladder, S:spleenCR: cranial, CA:caudal, D: dorsal, V: ventral, R: right, L: left

• Choledochal cysts:  These are one of the most frequent hepatobiliary anomalies. Usually, a cyst is present in the right upper quadrant near the porta hepatis and separate from the surrounding structures. They are fusiform or saccular dilatations of the bile ducts and the Todani classification system helps to categorize these anomalies into five groups. The most common form is type I. But they can also be associated with extrahepatic biliary obstruction. Choledochal cysts - as said above - also follow fluid signal on MRI.
  

  Fig 2: The Todani Classification of choledochal cysts.

• Enteric Duplication Cysts:  Typically unilocular structures, arising at any point along the gastrointestinal tract from the mouth to the anus, but most commonly affect the ileocoecal region. There are two types: the spherical which does not communicate with the lumen and the tubular one which communicates. Ectopic gastric mucosa can be present in some cases. They follow fluid signal on MRI, except when meconium or hemorrhage is present. 

• Ovarian cysts:  Fetal ovarian cysts are the most common abdominopelvic cysts in female neonates. They arise from ovarian follicles, typically unilaterally, rarely bilaterally, and categorized as simple or complicated. The presence of small cysts (≤ 2 cm) are normal, and they usually spontaneously resolve postnatally. On the other hand large cysts (≥ 4-5 cm) are associated with an increased risk of torsion. Simple cysts are hyperintense on T2w images and hypointense on T1w images, but complicated cysts with hemorrhage are hyperintense on T1w.  
  

  Fig 3: 36-week-old female fetus with an ovarian cyst (yellow arrow) on the right side. The cystic lesion dislocates the liver cranially, and the right kidney caudally and extends to the vertex of the bladder. During postnatal ultrasound a cystic lesion was seen under the right kidney, and then surgery was performed confirming the fetal MRI diagnosis. A: T2 HASTE sagittal image, B: T2 HASTE transversal image L: liver, B: bladder, D: dorsal, V: ventral, R: right, L: left
   
  

  Fig 4: 36-week-old female fetus with simple ovarium cysts (yellow arrows) on both sides. Postnatal ultrasound showed reduction in size, therefore surgery was unnecessary. K: kidney, D: dorsal, V: ventral, R: right, L: left, A: T2 HASTE coronal image, B: T2 HASTE transversal image

• Renal cysts:  Cystic lesions arising from the urinary tract are relatively common. They could be simple unilateral and unilocular renal cysts. However multiple cystic abnormalities could appear in the renal cortex or medulla uni- or bilaterally. Disorders without genetic transmission could be both uni- or bilaterally like: multicystic dysplastic kidney (MCDK) and obstructive cystic dysplasia. However bilateral diseases often have a genetic transmission like autosomal recessive and dominant kidney disease (ARPKD, ADPKD). Common anomalies that could mimic renal cysts are: renal tract duplication, and PUJ obstruction.
  

  Fig 5: 22-week-old male fetus, left solo kidney with multiple cysts (yellow arrows) on the caudal side suggesting multicystic dysplastic kidney disease. But a parenchymal bridge was also present in the middle alluding to a duplex collecting system. The pregnancy was terminated on week 22. A: T2 TRUFI coronal image, B: T2 TRUFI sagittal image, C: T2 TRUFI transversal image, L:liver, UC:umbilical cord CR: cranial, CA:caudal, D: dorsal, V: ventral, R: right, L: left

• Splenic cysts:  Uncommon findings, usually simple serosus cysts in the upper pole of the spleen. They arise in the left upper quadrant and often decrease in size or frequently resolve spontaneously. Splenic cysts also follow fluid signal on MRI.

• Pancreatic cysts:  Exceedingly rare, arising from the ductal system of the body or the tail of the pancreas.

• Lymphatic Malformations (including mesenteric and omental cysts):  Mesenteric cysts are lymphatic malformations (also called lymphangiomas). They could be uni- or multilocular along the mesentery of the bowels.
  

  Fig 6: 38-week-old female fetus with a type IV. SCT (sacrococcygeal teratoma) at the level of the coccyx, in a mainly intrapelvic localization. The cystic lesion has some inner septums. After birth, the SCT and the coccyx were eliminated with surgery. T2 HASTE coronal image, L: liver, K: kidneyCR: cranial, CA:caudal, D: dorsal, V: ventral
  

  Fig 7: 22-week-old fetus with a lobulated sacrococcygeal mass (yellow arrow) which is predominantly cystic and predominantly located external to the pelvis.T2 sagittal TRUFI image, D: dorsal, V: ventral

• Meconium pseudocysts:  After fetal intestinal perforation the meconium becomes inflamed.

• Cystic sacrococcygeal teratoma (SCT):  SCT is the most common germ cell tumor of the fetus. They comprise all 3 germ cell layers and can occur from the coccyx to the pineal gland, but most frequently originate from the presacral region. They are more common in females. They are classified by the extent of spread to the presacral and external components.
  

  Table 1: The American Academy of Pediatric Surgery Staging Classification of sacrococcygeal teratomas.
  Although most SCTs are benign, there are malignant teratomas that are often solid. Immature or mature teratomas are often cystic. Firsthand they are diagnosed prenatally by ultrasound, but it may be difficult to visualize precisely, though fetal MRI is useful to assess the extent of the mass and its characteristics which is critical for emergent operative planning.
  

  Fig 8: 26-week-old female fetus with lymphangioma (yellow arrows) in the right abdominopelvic region. Postnatal ultrasound revealed reduction is size, therefore surgery was not performed. T2 HASTE sagittal image, K: kidney
  

  Fig 9: 22-week-old fetus with a cystic lesion in the left lower quadrant of the abdomen with irregular margins. Appearance is most consistent with a lymphangioma, though the differential includes a meconium cyst. A mesenteric or duplication cyst is considered less likely given the irregular contour. T2 coronal HASTE image, R: right, L: left, CR: cranial, CA: caudal

• Rare anomalies worth mentioning include: Urachus cyst, Mesenchymal Hamartoma, Cystic Biliary Atresia, Gallbladder duplication

And just to mention some cyst-like abdominopelvic abnormalities:

• Urinary tract dilatation: the dilation of the renal pelvis, calyces, ureters and/or bladder.  The most important thing is to find the origin of the dilation and to find the associated anomalies. The most common causes: ureteropelvic junction obstruction, ureterovesical junction obstruction, vesicoureteral reflux and posterior urethral valves.
• Duodenal atresia: differential diagnosis includes mostly enteric duplication or choledochal cyst, but the duodenum is a fluid-filled structure, that is continuous with the stomach
  

  Fig 10: 26-week-old male fetus with distended bladder (yellow arrow) and ascites (blue star). Bilateral renal pelvis dilatation /hydronephrosis/ (blue arrow) can mimic multicystic kidney disease. Postnatally a posterior urethral valve was found and a vesicostoma was implanted.L: liver, K: kidney, R: right, L: left , T2 TRUFI transversal and coronal images
  

  Fig 11: 25-week-old male fetus with a slightly dilated stomach and proximal duodenum in the "double bubble" (yellow arrows) configuration. T2 coronal HASTE and transversal TRUFI images, R: right, L: left, K:kidney