There are two causes for virtually all brachial plexus injuries at birth: (1) a shoulder dystocia combined with (2) excessive force.
A shoulder dystocia occurs when the baby’s shoulders come down the birth canal in an unusual position, and the “top” shoulder gets stuck on the pubic symphysis (the cartilage connecting the right and left pelvic bones), thus preventing the baby from being delivered immediately after the head pops out. In attempting to get the baby out, the doctor or midwife applies traction to (pulls on) the baby, usually in a down and out direction. Excessive force when this is done pulls the baby’s head away from the shoulders, stretching the brachial plexus. Imagine sitting while someone presses down on your shoulders so you can’t move, and someone else grabs your head and pulls up. That kind of stretching is what frequently occurs after a shoulder dystocia, and if the stretch (the applied force) is too severe, the nerves are torn and a brachial plexus injury occurs.
Since virtually all permanent brachial plexus injuries are caused by excessive force when a shoulder dystocia occurs, prevention is based upon reducing the occurrence of shoulder dystocias. The risk factors for shoulder dystocia are widely known and accepted in medical literature, and include:
1. Macrosomia (a large baby)
2. A small or misshaped pelvis
3. Excessive weight gain by the mother
4. Gestational diabetes
5. Short maternal stature
6. A shoulder dystocia at a prior birth
7. A brachial plexus injury at a prior birth
8. Slow dilation/descent
9. Use of forceps or vacuum extractor
Macrosomia simply means a large baby, and the larger the baby, the greater the risk that the baby will get stuck in the birth canal. The definition of a large baby can vary. Most medical literature on which shoulder dystocia research is based defines a large baby as 4000 grams+ (8 pounds 13 ounces or more) or more. However, some of the more recent obstetrical literature defines shoulder dystocia as 4000 grams+ if the pregnancy is complicated by gestational diabetes, or 4500 grams+ (9 pounds 14 ounces ore more) for a normal pregnancy.
Pelvic size or shape (2)
The birthing process can be complicated by either a small pelvis or an unusual shape to the pelvis. Either one increases the likelihood of the baby getting stuck. The size and shape of the pelvis should be determined in the first prenatal examination. Unfortunately many doctors and midwives fail to perform this basic procedure. It may be up to the mother to ensure that this part of the examination is done, simply by requesting it.
Excessive Weight Gain (3)
Excessive weight gain (30 pounds or more) by the mother often correlates with a larger baby and an increased risk of shoulder dystocia. Most mothers are unaware of the risks of too much weight gain. Consult your doctor about how much weight you can expect to gain, and the degree to which you need to monitor your weight during pregnancy.
Gestational diabetes increases the risk of shoulder dystocia because babies born to mothers with diabetes are often very large. If the overall size is combined with excessive deposits of fat and muscle on the arms, shoulders and chest, this increases the likelihood the baby will not easily fit through the pelvis.
Other Pre-Birth Factors (5-7)
A shorter or smaller mother has an increased risk of shoulder dystocia from pelvic size if for no other reason. Prior births involving a shoulder dystocia or brachial plexus also signal a potential for subsequent occurrences of either or both. Even if the obstetrician providing current prenatal and delivery care is the same person as for a prior birth, never assume that the prior history will be remembered—after all, you are not the doctor’s only patient. Whether the doctor is new or “old,” tell him or her of that prior history.
Evaluate Pre-Birth Risk Factors Before Labor Starts
The presence or absence of the seven pre-birth risk factors is something that not only can but should be determined during the course of the pregnancy.
If you have a prior history of shoulder dystocia or brachial plexus injury (Factors 6 and 7), you should insist on a scheduled caesarian section If your doctor refuses, find a new doctor. The literature proves that if it happened once it is likely to happen again. There is no reason to attempt a vaginal delivery in this setting.
If one or more of the five other risk factors are present before you start labor, you should discuss the risk of shoulder dystocia and brachial plexus injury with your physician. The greater the number of risk factors the greater the risk of a permanent brachial plexus injury. Remember that a shoulder dystocia is an emergency. When it occurs there is the risk that the doctor will use excessive traction, thereby injuring your baby. It is better to avoid this risk if at all possible.
Slow Dilation/Descent (8)
Starting in the 1950’s investigators have studied the rates at which women dilate during labor and the rate at which the babies descend in the birth canal. Abnormalities in either dilation or descent should alert the delivery team to the fact that this particular baby might be a tight fit through this particular birth canal. The risk of shoulder dystocia in this setting is well known.
When there is abnormal dilation or descent, before making a decision on what to do, your doctor must first determine why the slowness is happening. The “four P’s” represent the possible causes for your physician to investigate:
Sometimes the mere fact of slow dilation or slow descent will cause a doctor to immediately decide to use forceps or a vacuum extractor to speed the birth. Before doing so, however, it is crucial that the doctor must first have ruled out a “tight fit” as the cause. (That’s also known as cephalopelvic disproportion or fetopelvic disproportion). If the doctor wants to use forceps or a vacuum extractor without having first evaluated the fit of this baby through this pelvis—ask for a c-section instead.
Using forceps or a vacuum extractor is a “procedure” and a caesarean section is an alternative to using those devices. Doctors are required by the ethical guidelines of their profession, as well as by state law, to obtain your informed consent before performing any procedure. In a general sense, that means a doctor must give you enough information about the risks and benefits of a proposed procedure, as well as alternatives, to enable you to make a rational decision. If the risk/benefit/alternatives information isn’t volunteered, you should ask.
Forceps, Vacuum Extractors and C-Sections
The ninth factor for shoulder dystocia is the use of forceps or a vacuum extractor, most often when there is slow dilation and/or slow descent. This is something which can also be discussed ahead of time. In other words, talk about these issues before the emergency occurs.
A shoulder dystocia is an emergency because the baby’s supply of oxygen is reduced when the head is out but the body is still inside. The amount of time that can pass before the onset of brain damage is not precisely known and likely varies depending upon how well oxygenated the baby was before the head delivered. At this point, the key to injury avoidance and sound management by the delivery team is to have a delivery plan that is executed calmly without excessive force or traction.
The primary problem in a shoulder dystocia is that the shoulders are in the straight up/down position rather than at an angle. Doctors are therefore taught specific maneuvers to rotate the baby from that position to the oblique (angled) position. This can be done without injuring the baby. Most protocols call for the nursing staff to put the mother in the McRoberts position, with her legs flexed back toward her chest. The nurses will then apply suprapubic pressure, pushing down at an angle just below the belly button to help the shoulders move into a slanted position.
Other safe maneuvers are available if this approach fails, including pulling the lower arm out then delivering the impacted shoulder, cutting an episiotomy or even using a sling to pull the baby out. Nurses should never apply fundal pressure, where force is applied just below the breast. Fundal pressure simply forces the baby’s shoulder more tightly against the pelvis.
The use of forceps or vacuum extractors significantly increases the risk of shoulder dystocia. The pelvis is oval shaped, so the widest part of the pelvis is at an oblique angle rather than from top to bottom. These instruments do not shift the baby’s position in the birth canal (as the above-described maneuvers do) so that the shoulders are at the right angle for passage through the pelvis. Instead, they artificially interfere with the delivery process by pulling the baby straight down, preventing any rotation of the shoulders into the angle position, and pulling the shoulders into the top of the pelvis, thereby causing the baby to get stuck.
Thus, where there is a size/size mismatch, a c-section is the alternative which eliminates both shoulder dystocia and brachial plexus injury.
There is no absolutely certain way to prevent brachial plexus injuries. There are, however, four things that can be done: