The umbilical cord is a narrow, tube-like structure that connects the developing baby (also referred to, in medical terms, as the fetus) to the placenta. The cord is sometimes called the baby’s “supply line” because it delivers the nutrients and oxygen the baby needs for normal growth and development and removes waste products.
The umbilical cord begins to form about five weeks after conception. It becomes progressively longer until about 28 weeks of pregnancy, reaching an average length of 22 inches. As it gets longer, the cord generally twists around itself and becomes coiled.
There are three blood vessels inside the umbilical cord—two arteries and one vein. The vein carries oxygen-rich blood and nutrients from the placenta to the baby, while the two arteries transport waste from the baby back to the placenta (where waste is transferred to the mother’s blood and disposed of by her kidneys). A gelatin-like tissue called Wharton’s jelly cushions and protects these blood vessels.
A number of abnormalities can affect the umbilical cord. Sometimes the cord is too long, too short, connects improperly to the placenta or becomes knotted or compressed. Cord abnormalities can lead to problems during pregnancy or during labor and delivery. In some cases, cord abnormalities are discovered after delivery when a doctor examines the cord and the placenta. Here are some of the most frequent cord problems and how they can affect mother and baby.
What is single umbilical artery?
About 1 percent of singleton and about 5 percent of multiple pregnancies have an umbilical cord that contains only two blood vessels, instead of the normal three, as one artery is missing. The cause of this abnormality is unknown. If an ultrasound examination shows that the baby appears to have no other abnormalities, the baby is likely to be born healthy.
However, studies suggest that about 25 percent of babies with single umbilical artery have birth defects, including chromosomal and/or other abnormalities. A woman whose baby is diagnosed with single umbilical artery during an ultrasound examination may be offered prenatal testing using ultrasound evaluation of the fetal heart and amniocentesis to diagnose or rule out chromosomal abnormalities. Even if the baby does not appear to have birth defects, the pregnant woman will probably be monitored carefully for the remainder of the pregnancy because of a somewhat increased risk of poor fetal growth, preterm delivery and stillbirth.
What is umbilical cord prolapse?
Umbilical cord prolapse occurs when the cord slips into the vagina after the membranes have ruptured, before the baby descends into the birth canal. This complication affects about 1 in 300 births. The baby can then put pressure on the cord as he passes through the cervix and vagina during labor and delivery, reducing or cutting off his oxygen supply. Umbilical cord prolapse can result in stillbirth unless the baby is delivered promptly, usually by cesarean section. Babies who are delivered promptly are usually unharmed.
If a pregnant woman’s membranes rupture outside of the hospital, and she feels something in her vagina, she should have someone take her to the hospital immediately or call 911. A health care provider may suspect that a woman in labor in the hospital has umbilical cord prolapse if her unborn baby develops heart rate abnormalities after the membranes have ruptured. The provider can confirm that the cord has prolapsed by doing a pelvic examination. This is an emergency situation, and the provider will take steps to relieve pressure on the umbilical cord by lifting the presenting fetal part away from the cord while preparing the woman for prompt cesarean delivery. Occasionally, if a woman’s cervix is fully dilated, she may be able to deliver vaginally.
The risk of umbilical cord prolapse is increased if the baby is in a breech (foot-first) position or if the baby is premature. In these cases, the baby’s presenting part (the foot or a smaller than-normal head) does not fill the pelvis and allows the cord to slip. Prolapse is more common when the umbilical cord is too long, when there is too much amniotic fluid or when the membranes are ruptured artificially to start or speed up labor. Umbilical cord prolapse also is frequent in vaginal twin deliveries, with the second twin most commonly affected.
What is vasa previa?
Vasa previa is an uncommon cord abnormality (occurring in about 1 in 3,000 births) that can be life-threatening for the unborn baby. This complication occurs when the umbilical cord inserts abnormally in the fetal membranes of the placenta, instead of in the center of the placenta (which may be abnormally shaped or positioned). The abnormal cord placement results, in a minority of cases, in fetal blood vessels that run through the membranes being unprotected by the umbilical cord. Vasa previa occurs when these unprotected fetal blood vessels cross the cervix, sometimes rupturing when the membranes do, causing life-threatening bleeding in the baby. Even if the fetal blood vessels don't rupture, the baby may suffer from lack of oxygen due to pressure on the blood vessels.
When vasa previa occurs unexpectedly at delivery, more than half of affected babies are stillborn. However, recent studies show that vasa previa can be diagnosed with ultrasound by mid-pregnancy. Fetal deaths generally can be prevented when the baby is delivered somewhat early by cesarean section, prior to the onset of labor, once the lungs are mature. Pregnant women with vasa previa may have painless vaginal bleeding in the second or third trimester. A pregnant woman who experiences vaginal bleeding should always report it to her health care provider so that the cause can be determined, and her provider can take any necessary steps to protect her baby.
A pregnant woman may be at increased risk of vasa previa if she has certain placental abnormalities (such as a low-lying placenta that covers part or all of the cervix [placenta previa] or an abnormally shaped placenta) or if she is expecting more than one baby. Studies also suggest that women whose pregnancies result from in vitro fertilization may also be at increased risk.
What are nuchal loops?
Up to 25 percent of babies are born with their umbilical cords wrapped one or more times around their necks. This rarely causes any problems, and babies with nuchal loops, also called “nuchal cords,” are generally healthy.
Sometimes fetal monitoring shows heart rate abnormalities during labor and delivery in babies with nuchal loops. This may reflect pressure on the cord. However, the pressure is rarely serious enough to cause death or any lasting problems, though occasionally a cesarean delivery may be needed.
Less frequently, the umbilical cord becomes wrapped around other parts of the baby’s body, such as a foot or hand. Generally, this doesn’t harm the baby.
What are umbilical cord knots?
About 1 percent of babies are born with one or more knots in the umbilical cord. Some knots form during delivery when a baby with a nuchal loop is pulled through the loop. Others form during pregnancy when the baby moves around. Knots occur most often when the umbilical cord is too long, and in identical-twin pregnancies that share a single amniotic sac, when the babies’ cords become entangled.
As long as the knot remains loose, it generally does not harm the baby. However, sometimes the knot or knots can be pulled tight, cutting off the baby’s oxygen supply. Cord knots result in miscarriage or stillbirth in 5 percent to 10 percent of cases. During labor and delivery, a tightening knot can cause the baby to have heart rate abnormalities that are detected by fetal monitoring. In some cases, a cesarean delivery may be necessary.
Are there other umbilical cord abnormalities?
Occasionally, the baby moves around in a way that causes the cord to become too tightly coiled, reducing blood flow. This is sometimes referred to as a “cord accident.” Sometimes babies with tightly coiled cords begin to move less. Pregnant women should report decreased fetal movement to their health care provider immediately.
Pregnancies with too little amniotic fluid, called oligohydramnios, can experience umbilical cord compression leading to decreased oxygen supply, poor growth or fetal death. This condition can be followed with ultrasound and fetal testing.
An extremely short cord can make a safe vaginal delivery impossible and can contribute to placental abruption, in which the placenta peels away (partially or completely) from the uterine wall prior to delivery, endangering mother and baby.
Sometimes an ultrasound examination will show bulging in a cord blood vessel. This may be a “false cyst,” which is actually a varicose vein, or a “true cyst,” a fluid-filled sac. Neither form of cyst is proven to pose a risk to the pregnancy. However, some studies suggest true cysts may be associated with birth defects, including chromosomal abnormalities, so the health care provider may recommend some additional tests to diagnose or rule out these problems.
Does the March of Dimes support research on umbilical cord abnormalities?
March of Dimes grantees are seeking ways to prevent umbilical cord abnormalities and the complications they cause. One grantee is studying the role of a gene in causing single umbilical artery to prevent it and the birth defects that sometimes accompany it. Another is developing drug treatment that can boost amniotic fluid levels in women who have too little fluid to help prevent the umbilical cord compression (sometimes resulting in fetal death) that sometimes goes along with this condition.
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