Ovarian cysts arise from ovarian follicles. The primary stimulus for follicular development is follicle-stimulating hormone (FSH) secreted by the fetal pituitary, but maternal estrogens and placental human chorionic gonadotropin (hCG) also contribute to follicle growth in utero (Pryse-Davies and Dewhurst, 1971). Primary follicles can be seen as early as the 20th week of gestation, and graafian follicles first appear after 28 weeks of gestation (Pryse-Davies and Dewhurst, 1971; Peters et al., 1978). At birth, maternal estrogen and hCG levels decrease sharply, and FSH production is decreased by the inhibitory mechanism of the hypothalamus–pituitary–ovarian axis (Grumbach and Kaplan, 1975). This decrease in circulating estrogen and hCG levels at birth usually precludes the formation of simple ovarian cysts during childhood.

Ovarian cysts develop from mature follicles in hormonally active ovaries and are therefore most often seen after puberty. The cause of fetal ovarian cysts is unclear but is most likely stimulation of the fetal ovaries by fetal gonadotropins, maternal estrogens, and placental chorionic gonadotropin. The increased incidence of ovarian cysts seen in infants of mothers with diabetes mellitus, rhesus sensitization, and preeclampsia—(conditions that are associated with excessive levels of serum chorionic gonadotropins (DeSa, 1975; Nussbaum et al., 1988)–-supports this pathogenesis. The association with fetal hypothyroidism has also been reported (Jafri et al., 1984). However, in most cases the cysts are detected in otherwise normal pregnancies (Sakala et al., 1991). It is most common for fetal ovarian cysts to be unilateral; however, bilateral fetal ovarian cysts have been reported. In the review by Sakala et al., (1991) of 65 cases of fetal ovarian cysts, 62 (95%) were unilateral, whereas 3 were bilateral.

Ovarian and genital abnormalities account for 20% of all newborn abdominal masses, second only to those of urinary-tract origin (Griscom, 1965; Wilson, 1982). The first report of prenatal diagnosis of a fetal ovarian cyst was by Valenti et al. (1975); subsequently, over 300 cases of prenatally diagnosed ovarian cysts have been reported (Holzgreve et al., 1989). Kirkinen and Jouppila (1985) reported 8 ovarian cysts detected antenatally in 21,000 pregnancies, for an incidence of 1 in 2625. DeSa, (1975) found small follicular cysts in 113 (34%) of 332 stillborn fetuses and neonatal deaths. The cysts were defined as cystic structures lined by recognizable granulosa epithelium, with the greatest diameter in excess of 1 mm on microscopical section.

The following criteria are used to identify fetal ovarian cysts during fetal ultrasound examination:

1. 
   

   The presence of a cystic structure, usually located on one side of the fetal abdomen.

   
   
2. 
   

   Identification of normal genitourinary tract (kidneys, ureter, and bladder).

   
   
3. 
   

   Identification of a normal gastrointestinal tract (stomach, small and large bowel).

   
   
4. 
   

   Female fetus.

Two types of cysts have been described: a simple cyst that is completely anechoic (Figure 68-1) and complicated cysts containing internal echoes and fluid levels, internal septations, or anechoic areas intermixed with echogenic foci (Figure 68-2). The antenatal diagnosis of an echogenic cyst is an accurate sonographic indicator of ovarian torsion. In several series of echogenic cysts detected prenatally, postnatal surgery has confirmed the diagnosis (Nussbaum et al., 1988; Brandt et al., 1991; Meizner et al., 1991; Giorlandino et al., 1994).

The distinction between a pathologic cyst and a physiologic, mature follicle is based on size alone, with fetal ovarian cysts > 2 cm considered pathologic (Grapin et al., 1987). Cysts may vary in size from 2 × 2 cm to 8 × 11 cm in diameter (Grapin et al., 1987).

The diagnosis of a fetal ovarian cyst is usually presumptive because mesenteric cysts, urachal cysts, or enteric duplications cannot be ruled out with absolute certainty prenatally. Factors more indicative of a fetal ovarian cyst include bilaterality, echogenicity, and cyst septations. Most cysts are benign; while ovarian neoplasms have been reported, they are extremely rare (Croitoru et al., 1991). Confirmation of normal kidneys and bladder will reduce the potential to confuse renal cysts or posterior urethral valves with persistent distention of the bladder as an ovarian cyst.

Most cases of fetal ovarian cysts have been reported after 28 weeks of gestation (Brandt et al., 1991). Cases have been reported during the second trimester, beginning as early as 19 weeks (Meizner et al., 1991; Foley et al., 2005). With the exception of a report of congenital hypothyroidism (Jafri et al., 1984), all other series report no associated congenital anomalies.

The size of most cysts remains unchanged in utero. A few cases have been reported to resolve in utero completely (McKeever and Andrews, 1988; Rizzo et al., 1989). Polyhydramnios is present in approximately 18% of cases and is more commonly seen with cysts >6 cm (Sakala et al., 1991). The mechanism of the polyhydramnios has been suggested to be due to partial bowel obstruction secondary to compression by a large cyst (Carlson and Griscom, 1972).