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The yolk sac, or umbilical vesicle, is a small membranous structure outside the embryo with various functions during embryonic development (see Image. Human Embryo at 14 Days With Yolk Sac). The yolk sac reduces in size, communicates ventrally with the developing embryo via the yolk stalk, and later regresses. The yolk stalk is a term that may be used interchangeably with the vitelline duct, omphaloenteric duct, or omphalomesenteric duct. The yolk stalk serves to connect the yolk sac to the midgut, which is an early derivative of the gastrointestinal system (see Image. The Digestive Apparatus). Both the midgut and yolk sac are endodermal in origin. See Image. Embryology. Although it contains no yolk in humans, the yolk sac has several critical biological functions during early gestation, including primitive hematopoiesis and germ cell production. Clinically, the yolk sac can be visualized on transvaginal sonography by 5 weeks post-fertilization, and its presence is a significant predictor of adverse pregnancy outcomes until its disappearance on sonography in the second trimester.

During the sixth to tenth weeks post-fertilization, physiologic herniation and rotation of the embryonic bowel occur at the midgut. Herniation of the midgut involves the projection of the embryologic bowel into the developing umbilical cord within the yolk stalk, or vitelline, omphaloenteric, or omphalomesenteric duct. At the base of the umbilical cord, the midgut undergoes 2 episodes of physiologic rotation. Initially, the midgut rotates 90 degrees counterclockwise using the superior mesenteric artery as an axis. The midgut then returns to the abdomen as the abdominal cavity has enlarged. The second rotation is 180 degrees counterclockwise and occurs approximately 10 weeks post-fertilization. In total, 270 degrees of rotation occur. This herniation occurs due to a lack of space inside the abdominal cavity. The fetal midgut, kidneys, and liver are large at this time. The abdominal cavity, however, grows at a much slower rate than the midgut. Herniation allows the midgut enough space to grow rapidly outside of the peritoneum in the extra-embryonic coelom. Upon completion of herniation and rotation, the yolk stalk degenerates. Degeneration commonly occurs in the seventh week of gestation. The yolk stalk is connected to the midgut prior to herniation and is commonly referred to as the vitelline duct by clinicians. In approximately 2% of people, the vitelline duct fails to degenerate and persists. This outcome results in a gastrointestinal outpouching called Meckel's diverticulum, characterized by a complete or partial opening between the umbilicus and the bowel. Meckel's diverticulum is the most common congenital abnormality of the gastrointestinal tract. In its classically described form, it sits within 2 feet of the ileocecal valve and measures approximately 2 inches long. However, Meckel's diverticulum is highly variable in appearance and may be asymptomatic or present with symptoms of heterotopic gastric or pancreatic mucosa, which are often present. For example, as heterotopic gastric mucosa produces acid in the bowel, it may cause gastrointestinal bleeding, ulcers, and eventual perforation if left untreated.[10]

The yolk stalk is connected to the midgut prior to herniation and is commonly referred to as the vitelline duct by clinicians. In approximately 2% of people, the vitelline duct fails to degenerate and persists. This outcome results in a gastrointestinal outpouching called Meckel's diverticulum, characterized by a complete or partial opening between the umbilicus and the bowel. Meckel's diverticulum is the most common congenital abnormality of the gastrointestinal tract. In its classically described form, it sits within 2 feet of the ileocecal valve and measures approximately 2 inches long. However, Meckel's diverticulum is highly variable in appearance and may be asymptomatic or present with symptoms of heterotopic gastric or pancreatic mucosa, which are often present. For example, as heterotopic gastric mucosa produces acid in the bowel, it may cause gastrointestinal bleeding, ulcers, and eventual perforation if left untreated.[10] A patent urachus is a rare congenital anomaly in which the urinary bladder remains connected to the outside world via the umbilicus. This results from a failed involution of the urachus. It often presents in neonates with urine leakage from the umbilicus after birth. If the urachus only partially involutes, a fluid-filled urachal cyst is formed. Complications of urachal anomalies include infection, urachal neoplasms, stone formation, and umbilical granulomas.[6]