Probably most of these cysts are follicular in origin and although bilateral cysts are described in the neonate they nearly always appear unilateral in the fetus. They may arise from stimulation by the maternal hormones and tend to disappear spontaneously following delivery. Most are simple follicular or theca-lutein cysts; very few are neoplastic. The primordial germ cells which originate in the endoderm of the yolk sac during the eighth week migrate to the genital ridge to form the indifferent gonad. In the absence of anti-mullerian hormone (AMH) and testosterone internal genitalia develops along female lines. In absence of the Y- chromosome the indifferent gonad will develop into an ovary at approximately 50 to 55 days of embryonic life. The ovarian cortex develops toward the end of the embryonic period and involves rearrangement within the gonadal blastema. The cortex is thought to arise from coelomic epithelium, containing many germ cells. The medulla is formed from a dense central core of cells. The chief cells forming the developing ovary are germ and supporting cells and temporary interstitial cells.
The strongest clue is that the fetus is female. The fetal bladder should always be visualised and this gives the impression of two cysts, one the ovarian cyst, the other, the bladder. The cyst may contain septa although the majority are simple unilocular structures. The presence of septation indicates a possible cyst accident. Although virtually all the cysts are unilateral they can appear central and may be solid, cystic or mixed. Malignancy has not been described in the fetus. If very large they may lead to obstruction in labour and prelabour aspiration may be considered. Torsion may occur pre- or post-delivery. Very large cysts (> 10 cm) may cause respiratory embarrassment in the neonate and may need prenatal aspiration. The majority are benign theca lutein or corpus luteum cysts.
Care is needed to distinguish hydronephrosis in which the cyst” is usually in the upper abdomen. A distended bladder is usually low and central and should be considered if a second cyst cannot be seen; such distension may produce renal pelvic dilatation which would not usually be seen with an ovarian cyst. Other intra-abdominal cysts such as mesenteric and reduplication cysts should always be considered equally when the fetus is male. Urachal cysts are midline and occur in either sex. Hydrometrocolpos presents as an oval mass in a female fetus and the perineum may be bulging. Persistent cloaca usually presents as a septated cystic mass in females although other anomalies may be present.”
Echo-free mass within the abdomen, usually unilateral
Mass may reach large dimensions but not usually greater than 10 cms
Septa may develop or already exist – these probably represent haemorrhage
No ascites unless cyst accident has occurred. e.g. Rupture Cyst may have cellular debris within it
Polyhydramnios is seen in only 10%
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