Umbilical Cord Abnormalities
At birth, the umbilical cord is approximately 2 cm in diameter and 50 to 60 cm long. It is tortuous, causing false knots. An extremely long cord may encircle
the neck of the fetus, usually without increased risk, whereas a short one may cause difficulties during delivery by pulling the placenta from its attachment in the uterus.
Normally there are two arteries and one vein in the umbilical cord. In 1 in 200 newborns, however, only one artery is present, and these babies have approximately a 20% chance of having cardiac and other vascular defects. The missing artery either fails to form (agenesis) or degenerates early in development.
Occasionally, tears in the amnion result in amniotic bands that may encircle part of the fetus, particularly the limbs and digits. Amputations, ring constrictions, and other abnormalities, including craniofacial deformations, may result (Fig. 6.16). Origin of the bands is probably from infection or toxic insults that involve either the fetus, fetal membranes, or both. Bands then form from the amnion, like scar tissue, constricting fetal structures.
At the end of pregnancy, a number of changes that occur in the placenta may indicate reduced exchange between the two circulations. These changes include (a) an increase in fibrous tissue in the core of the villus, (b) thickening of basement membranes in fetal capillaries, (c) obliterative changes in small capillaries of the villi, and (d) deposition of fibrinoid on the surface of the villi in the junctional zone and in the chorionic plate. Excessive fibrinoid formation frequently causes infarction of an intervillous lake or sometimes of an entire cotyledon. The cotyledon then assumes a whitish appearance.
The amniotic cavity is filled with a clear, watery fluid that is produced in part by amniotic cells but is derived primarily from maternal blood. The amount of fluid increases from approximately 30 ml at 10 weeks of gestation to 450 ml at 20 weeks to 800 to 1000 ml at 37 weeks. During the early months of pregnancy, the embryo is suspended by its umbilical cord in this fluid, which serves as a protective cushion. The fluid (a) absorbs jolts, (b) prevents adherence of the embryo to the amnion, and (c) allows for fetal movements. The volume of amniotic fluid is replaced every 3 hours. From the beginning of the fifth month, the fetus swallows its own amniotic fluid and it is estimated that it drinks about 400 ml a day, about half of the total amount. Fetal urine is added daily to the amniotic fluid in the fifth month, but this urine is mostly water, since the placenta is functioning as an exchange for metabolic wastes. During childbirth, the amnio-chorionic membrane forms a hydrostatic wedge that helps to dilate the cervical canal.
Hydramnios or polyhydramnios is the term used to describe an excess of amniotic fluid (1500-2000 ml), whereas oligohydramnios refers to a decreased amount (less than 400 ml). Both conditions are associated with an increase in the incidence of birth defects. Primary causes of hydramnios include idiopathic causes (35%), maternal diabetes (25%), and congenital malformations, including central nervous system disorders (e.g., anencephaly) and gastrointestinal defects (atresias, e.g., esophageal) that prevent the infant from swallowing the fluid. Oligohydramnios is a rare occurrence that may result from renal agenesis.
Premature rupture of the amnion, the most common cause of preterm labor, occurs in 10% of pregnancies. Furthermore, clubfoot and lung hypoplasia may be caused by oligohydramnios following amnion rupture. Causes of rupture are largely unknown, but in some cases trauma plays a role.
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