Attached on one end to the mother’s placenta and on the other to the growing baby, the umbilical cord is a baby’s lifeline during pregnancy and the physical connection between mom and baby.
What is the umbilical cord?
The umbilical cord contains blood vessels that carry nutrients and oxygen to your baby and remove your baby’s waste products, like carbon dioxide, so they can be processed by your body. The umbilical cord grows along with your baby, eventually reaching around 22–24 inches long, and the amount of blood the umbilical cord delivers to your baby increases with their weight. At term, a seven-pound baby would be receiving approximately 12 fluid ounces of blood through their umbilical cord every minute!
When you have routine ultrasounds as part of your prenatal care, the sonographer will look at your baby’s umbilical cord and examine a number of its aspects as your baby grows, including the number of vessels, insertion location, blood flow, and evaluation for any anomalies.
Anatomy of the umbilical cord
The umbilical cord typically contains three vessels: two arteries (which carry deoxygenated blood from the baby to the placenta to circulate back through the mom’s bloodstream) and one vein (which carries oxygenated blood from the placenta to the baby). Surrounding these three vessels is a squishy, gelatinous mixture called Wharton’s jelly that protects the umbilical blood vessels throughout the pregnancy so that the baby can move around without compromising its blood supply. The vessels and Wharton’s jelly are surrounded by a layer of amnion (a type of membrane).
At term, a seven-pound baby would be receiving approximately 12 fluid ounces of blood through their umbilical cord every minute!
When the umbilical cord is developing, there are initially four vessels, with the right umbilical vein typically disappearing around six to seven weeks.
Where does the umbilical cord attach?
Usually, the umbilical cord connects to the placenta near the center.
Sometimes, however, the umbilical cord connects near the edge of the placenta, in what’s called a marginal cord insertion. Marginal cord insertion happens in about 9 percent of single pregnancies, but increases to 24–33 percent in twin pregnancies. While not necessarily an immediate cause for concern, marginal cord insertion is associated with increased risk for intrauterine growth restriction, preterm labor, and fetal distress.
What are some umbilical cord complications?
As your baby wiggles and moves throughout your pregnancy, the way the umbilical cord coils in the womb helps protect it and the baby, but sometimes complications can arise.
Usually, the umbilical cord connects to the placenta near the center.
Cord Length. If your baby’s umbilical cord is too long or too short, it can be of concern. More often, the umbilical cord is too long, which is associated with an increased risk of nuchal cord, cord knots, and cord prolapse, which are discussed next.
Nuchal cord. A nuchal cord happens when the umbilical cord gets looped around the baby’s neck. This is actually quite common, happening in about one-quarter of births. Most of the time, a nuchal cord does not cause major issues for the baby, but in certain cases (and especially if there are multiple loops), a C-section delivery may be needed.
Knots. You know how it feels like your baby is doing somersaults in your belly? Well, sometimes they manage to tie a knot in their umbilical cord with all of those acrobatics. If a knot remains loose, it does not tend to cause issues, but a tightening knot (during labor, for example) could result in fetal distress. A C-section may be necessary to make sure your baby continues to get enough oxygen.
Prolapse. Umbilical cord prolapse is when the umbilical cord slips into the vagina before the baby has started to descend into the birth canal. According to the March of Dimes, cord prolapse occurs in about 1 in 300 births. If this happens, it is a medical emergency. Your providers may need to deliver your baby quickly, as the pressure on the cord during labor and delivery can deprive your baby of oxygen. If your water breaks and you feel something in your vagina, call 911 immediately.
A number of factors can increase your risk for umbilical cord prolapse, including:
- Too-long umbilical cord
- Too much amniotic fluid
- Preterm labor
- Low birth weight
- Baby in breech position
- Rupture of the membranes by a provider (often to speed up labor)
- Carrying multiples
Abnormal number of vessels. In very rare circumstances, the umbilical cord will continue to contain four vessels, requiring further evaluation. Sometimes, the umbilical cord only contains two vessels: one artery and one vein. Most often this is an isolated anomaly, but the presence of a single umbilical artery is sometimes associated with structural or genetic abnormalities, so its discovery often results in additional tests (like amniocentesis, further ultrasounds, or an echocardiogram to check on the baby’s heart) to evaluate for anything concerning.
Cysts. In about 3 percent of pregnancies, umbilical cords can also sometimes develop cysts. True cysts (where the cyst contains cells and embryologic remnants) and false cysts (which are fluid-filled sacs) can both occur, but false cysts (also called pseudocysts) are more common. If a cyst is identified during an ultrasound, follow up tests will usually be recommended, because they can be associated with chromosomal anomalies.
Velamentous cord insertion. In a velamentous cord insertion, which only happens in approximately 1–2 percent of pregnancies, the umbilical vessels (the two arteries and one vein) insert into the amniotic sac instead of directly into the placenta. This leaves part of the vessels exposed and unprotected by Wharton’s jelly, which can put the baby at serious risk of bleeding. If you are diagnosed with a velamentous cord insertion, your providers will likely monitor you and your baby more closely throughout the pregnancy.
Providers will often ask if your partner (or you!) would like to cut the umbilical cord, so be sure to let your providers know if that is something you are interested in.
Vasa previa. Vasa previa, which is very rare (occurring in only .04 percent of pregnancies), happens when vessels from the umbilical cord cross the cervix under the baby. Having a velamentous cord insertion, being pregnant with more than one baby, and placenta previa (when the placenta is located low in the uterus, covering some or all of the cervix) all increase the risk for vasa previa. Vasa previa can sometimes cause painless bleeding in the second or third trimester, so be sure to talk to your provider if you experience any bleeding during your pregnancy. If you are diagnosed with vasa previa, it’s likely that a C-section around 35 weeks will be recommended due to the risk of stillbirth.
After birth
After your baby is born, your provider will clamp your baby’s umbilical cord. Studies have shown there may be benefits to delayed cord clamping, which you may want to discuss with your provider ahead of time. Cord blood banking is also something you may want to learn more about. During the third stage of labor, you will deliver your placenta. Providers will often ask if your partner (or you!) would like to cut the umbilical cord, so be sure to let your providers know if that is something you are interested in.
After the umbilical cord is cut, your baby will have a small umbilical stump. In the coming days or weeks, this stump will fall off, leaving behind your baby’s cute little belly button (also called an umbilicus).
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