Life Inside the Womb: Implications for Newborn and Infant Nurses

Kim LaMar, ND, RNC, CNNP; Cheryl Hamernik, MSN, RNC, CNNP

Disclosures

NAINR. 2003;3(4) 

In This Article

Fetal Membranes

The fetal membranes consist of the chorion, amnion, yolk sac, and allantois. The chorion has three layers: cytotrophoblastic, chorionic connective tissue, and basement membrane. The chorion lies closest to the placenta. The chorionic sac encompasses the fetus and the amnion and is covered with chorionic villi until the 8th week. After the 8th week, these villi are constricted due to a decreased blood supply until a smooth chorion is formed in the portion of the sac that is not forming the placenta.[12] The chorion is thicker than the amnion.

The amnion has five distinct layers: amniotic epithelium, basement membrane, compact layer, fibroblast layer, and intermediate layer. There are no blood vessels or nerves in the amnion. This membrane lies closest to the fetus and the amnion is supplied with nutrients by the amniotic fluid.[20] In the 3rd and 4th weeks' gestation, the amnion will somewhat fold over and engulf the yolk sac and body stalk that attaches the conceptus to the uterus. The amnion eventually forms an epithelial covering of the umbilical cord, which is further assisted by its own formation of Wharton's jelly as a protective substance for the precious umbilical vessels within. As the amnion expands, it moves into the chorionic sac until it completely fills the chorionic sac at around 10 weeks' gestation.[11]

The yolk sac is a primitive structure that usually disappears after 20 weeks' gestation. A remnant of the yolk sac merges in the forming embryo into the primitive gut. The role of the yolk sac in early gestation is to supply the embryo with oxygen and nutrients until the formation of the placenta is accomplished. The yolk sac is also the first site of hematopoeisis, with embryonic red blood cells and macrophages visible in the yolk sac as early as 2 weeks' gestation. Neutrophils, platelets, and lymphocytes are not produced in the yolk sac.[21] The allantois develops into a fibrous cord, the urachus, which runs from the fetal bladder to the umbilicus. This urachus can remain patent in the neonate with urine secreted from the umbilicus. The yolk sac and allantois are the only portions of the fetal membranes that develop to form a part of the fetus itself. The chorion and amnion are discarded after birth when their usefulness has ended.

The purposes of the fetal membranes include protection of the fetus from ascending infection by forming a physical barrier, containment of the amniotic fluid, and secretion of substances into the amniotic fluid.[20]

Fetal membranes may develop abnormally into amniotic sheets or bands occurring in 0.8% of all pregnancies. Previous scars of the uterus either from cesarean section or dilation and curettage (D&C) can result in amniotic sheets forming around the scar tissue and may have an association with early labor. Amniotic bands are less common than amniotic sheets, occurring in 0.08% of all pregnancies. Amniotic bands are fibrous strands that stretch through the amniotic cavity as a result of rupture to the amnion without compromise of the chorion. Amniotic bands usually occur spontaneously. The danger to the fetus occurs when the bands compromise limb development but they have been known to entrap the fetal trunk and head as well. Up to 70% of all amniotic bands seen on prenatal ultrasound disappear upon repeat ultrasound. If the ultrasound demonstrates amniotic bands with free fetal movement and lack of fetal attachment to the band, they are termed innocent amniotic bands and pose little risk to the fetus.[22]

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