Was my doctor right in diagnosing a cord accident?

by Carolyn Salafia, MD, Ph.D.

A "cord accident" is a very non-specific diagnosis that just means "something" happened to the cord. That "something" could be anything from a cord being wrapped around the baby's neck, to a cord prolapse, to umbilical blood vessel rupture. Cord problems are real problems, especially at term but commonly pathologists believe that a normal cord has enough built-in protection to save itself UNLESS there is some other kind of problem making it harder for the cord to function properly. That's why we look closely at the placenta to determine what might have caused the cord to fail. Although true cord accidents do occur in the third trimester, I have no recollection of ever coming to the conclusion of a pure cord accident in the second trimester when the amniotic fluid volume was normal, and the placenta and cord were healthy. Even in the third trimester I commonly find that a cord accident was a final event and not the cause of the problem that led to the baby's death. I generally conclude that the cord was put at risk because of another circumstance. In future pregnancies many of those circumstances wouldn't necessarily recur as a cord accident. Those maternal problems could manifest themselves in any one of a number of ways such as growth restriction, fetal death, preeclampsia, or preterm birth. That's why it's important for a pathologist to examine your placenta to determine exactly what caused your cord to fail.

To understand why a true cord accident--whereby the cord gets trapped or fails without some other process causing it to fail--is rare it helps to understand how the umbilical cord functions. The cord has two arteries wrapped around a vein. That means it has two outflow tracts and one inflow tract, centimeters at a remove from the baby's itsy-bitsy heart. The force of that little baby's hearbeat has to get blood all the way down the cord through a capillary bed where flow is necessarily very slow and then all the way back up the vein. The idea people have is that the only way Mother Nature allowed us to evolve such a stupid lifeline is because the cord is built to be able rescue itself most of the time. 

To help protect itself, the cord has a substance called Wharton's Jelly around the blood vessels. It takes some force to actually move Wharton's Jelly but after the force is applied, the jelly will move and assume a new position. That means that if the cord is compressed it will remain where it is until the pressure becomes great. Then the Wharton's Jelly will cause it to move into a safer position.

In addition, the umbilical cord is covered by a skin that allows the cord to slip and slide at term even when there's not a whole lot of amniotic fluid, just like your intestines slip and slide in your belly. In fact, the cord is so slippery that when they're trying to encourage the placenta to be born, obstetricians may have to wrap the cord around their hands several times in order to get a good grip on it. If you have adequate fluid volume and a healthy cord I find it hard to understand how a cord could get wrapped several times around a baby's neck and stick there during the second trimester. I find it a lot easier to imagine how a floppy lifeless cord could drape itself around the neck of a child who has died and may be rotating in utero due to maternal positioning and gravity. Then, as the baby is delivered the cord would be pulled and tightened in its position as the baby descends and the placenta is retained attached to the uterine wall. But if there's enough amniotic fluid in the second trimester I can't see how the cord can get trapped. If a cord does get trapped in the second trimester with even a respectable amount of amniotic fluid then I have to worry about abnormal things in the amniotic fluid space that would cause epithelial erosion, like blood.

Since the cord is built to rescue itself under most circumstances, when a "cord accident" is diagnosed by an obstetrician, a reproductive pathologist will look for problems that might have increased the cord's vulnerability. Some of the questions I ask as I examine the placenta are: Was the placenta damaged so that the baby's heart would have had to work harder to pump blood through the placenta? Is there abnormal inflammation? Are there infarcts? Is the cord inserted on the wrong place, i.e. on the membranes instead of on the chorionic plate? Are the blood vessels abnormally straight? (In a normal cord the two arteries wrap around the vein and the pulsations of the artery help milk blood back up the vein. Some ultrasound studies have suggested that when there is not a helical coiling of the blood vessels, the straightness of the vessels is associated with a greater risk of problems. Presumably the idea is that if the arteries aren't lending their support to the vein, there might be an abnormality in how well the cord is functioning and it may be closer to a threshold where it doesn't rescue itself.)

At times (it's quite uncommon, and far less often than my clinician friends think it might be a cord accident!) I do diagnose a true cord accident in the third trimester. These would be:
1. A knot in the cord.
2. Spontaneous cord vessel rupture.
3. Nuccal cord. This is the medical term for a cord being wrapped around the baby's neck.
4. Cord prolapse. This is when the cord precedes the baby out of the uterus.

Also sometimes there is a problem with water flow and the umbilical cord Wharton's Jelly possibly isn't well hydrated. Or there may be something "bad" in the amniotic fluid space--like meconium or blood--so that the umbilical cord epithelium is eroded. Or there may be something abnormal in the organization of the arteries so that maybe the arteries don't help milk blood back along the vein the way they ought to. Then I would diagnose a cord accident.

However more commonly I find other issues that probably caused the cord to fail. There are more common reasons for the cord to get caught and have an accident; the frequency of each really varies with different hospitals and different groups of people. The most frequent causes of cord failure are:
1. Abnormal amniotic fluid volume. In the second and third trimesters amniotic fluid is mainly made up of the baby's urine. Normal kidneys can change their level of urine production depending on how much blood flow comes to them. Blood flow will be shunted away from the kidneys to more vital organs like the heart and the brain if there is a problem with oxygen delivery. If amniotic fluid volume drops off steeply I have to start wondering if there's some reason why fetal urine production is being reduced. If the placenta is insufficient or if the baby perceives low oxygen levels in its tissues, it will start shunting blood away from its kidneys to spare its vital organs. That shunting away will decrease the amount of blood that's filtered in the kidneys and reduce the amount of urine that's produced.
2. An abnormally functioning placenta. There are four main reasons for a placenta with normal genetics to function abnormally: a.) A structural problem in the uterus such as a septum or some abnormality of shape and form. b.) "Immune" issues. The first thing the placenta does when it's forming is invade into the mother's tissues and establish vascular connections. While it remains controvertial exactly how to diagnose immune issues under the microscope it appears clear to me that in some people there may be problems in how the mother's tissues accommodate the foreign placenta. c.) Problems in how the blood vessels are remodeled by the placenta and how that remodeling process is tolerated by the mother. d.) Congenital viral infection. It's not uncommon for me to see intramniotic bacterial infection or congenital viral infection in babies with cord problems. 
3. Abnormal pulse pressure in the cord. This can be related to fetal heart dysfunction (abnormal rhythmn, or can happen if the baby's heart is malformed and more stressed by the normal demands made on it during fetal life. 
4. Velamentous, or membraneous, cord insertion. A normal cord is inserted on the placental side of the chorionic plate. The chorionic plate is the outermost of the two shells in which the baby grows and develops. On the placental side the chorion sprouts the villai that are the structures through which oxygen, nutrients and waste are exchanged between the baby's bloodstream and the mother's bloodstream. The cord's two arteries and one vein enter the chorion at a right angle. The placental side of the chorion is almost like a trampoline surface. If you were going to bend or tug at the cord at the point of insertion (thereby lessening the 90 degree angle at which the cord enters the chorionic plate), the chorionic plate will move in such a way that the blood vessels will stay open. 

However, if the umbilical cord is inserted on the membraneous, or maternal, side of the chorionic plate, that's when troubles can arise. The membraneous side of the chorion is a very flat, paperthin sheet of tissue that runs up against the rigid myometrial wall. It has no give and, because of that, the the blood vessels that run from the cord to the plate are subject to compression or traumatic injury. This risk would increase as the baby approached the third trimester and amniotic fluid volume level started its normal rate of decrease as the baby got larger. At the time of membrane rupture a velamentous cord can be destabilized because the pressure of the amniotic fluid is no longer keeping the angle of the membranes and the plate open. It's very common for the fetal heartbeat tracing to acutely change and an emergency c-section to be required.

In addition to being vulnerable at the insertion site where the blood vessels are making that right angle, the blood vessels are also vulnerable further down along the cord. Those blood vessels have to travel in the membranes to the main body of the placenta, the mass of placental villi that are perfused by the mother's blood stream, and are where the nutrients actually are being extracted. The baby's head, elbow or knee can push against those blood vessels and they will have no place to go. They will be compressed, can be injured, and make it hard for the baby's heart and placenta to maintain their normal function, and keep healthy structure.

I wouldn't consider a velamentous or membraneous cord to be a true cord accident because the cord is mechanically set up to be vulnerable by abnormal placental migration. I can't blame the cord, then, if the root cause of the cord's problem is that the placenta grew out from underneath it. I need to try to figure out if there are any reasons why the placenta grew in such a direction. Sometimes, uterine septum can create a problem for the placenta. Sometimes, implantation down too close to the cervix can be the root cause. These each need to be evaluated carefully by clinician and pathologist to understand if there is any treatable underlying problem that caused the placental migration.

Carolyn Salafia, M.D., Ph.D., is board certified in Anatomic and Clinical Pathology and in Pediatric Pathology. 

Note: This communication is for educational purposes only and should not be used as a substitute for a consultation with your physician.