While there are many sites that explain what a non-stress test (NST) is, it isn’t often we get to see what they look like.
Here, I will describe what we are seeing, playing tour guide around a strapped belly that kindly allows us a vantage point.
There are two colors of the elastic straps; the color is inconsequential, a random choice as to which monitor it goes on. As you can see, the colors, pink and blue, are deliciously sexist.
The pink strap here is holding the fetal heart monitor on the mom’s belly. The elastic has slits in it so the band can be adjusted, tightly, around mom’s stomach, holding the monitor in place. Women need to sit still with the monitor on lest it wiggle out of listening zone on the baby’s upper back (where the heartbeat is heard clearest). Laughing, turning from one side to the other, lying back or sitting up more can all require a re-adjustment of both monitors. You can see why, in the middle of labor, it would be easier to have a mom monitored with an epidural; women without one move all over the place, requiring the nurse to adjust and re-adjust it over and over again.
The monitor on the mom listens and the bedside machine (also called the monitor) clomps out the sound of the baby’s heartbeat as well as blinks the heart rate to one side of the machine. When the monitor is “listening” accurately, it can sound like a horse galloping down the street. Clompity clomp, clompity clomp, clompity clomp… at a clip of 120 to 160 beats per minute. Looking at the bedside monitor strip (see picture below), the baby’s heartbeat is the top squiggly line. With education and experience, a nurse/doctor/midwife is able to look at the line and determine the baby’s well-being. A line that doesn’t change much is considered “flat” and not a positive sign, especially if it stays flat during a contraction, meaning the baby isn’t reacting to the increasing pressure. We’d want to see the baby have some reaction, hopefully the heart rate going up slightly.
While listening can be helpful, it really is easier to look at the strip, comparing the contraction to the baby’s heart rate. There are nuances that can give information about how the baby is doing during labor. When we hear (which usually comes first) a beat that’s half that fast, the first thing to do is make sure the mother’s pulse isn’t being picked up, quickly adjusting the monitor and, at the same time, feeling for the mother’s pulse on her wrist, seeing if the beats match. If they do not and the heart rate does not recover with position changes or stopping contractions, mama’s (most likely) off to have a baby in the operating room – fast.
Here is a fabulous site that discusses the variations of normal and abnormal when looking at fetal heart tracings in labor.
The monitor under the blue strap measures uterine contractions (UCs). The technical name for monitoring a contraction is tocodynameter (I was corrected. Sorry for the initial incorrect name.). You might hear the nurse say, “Toco” and she’s talking about the transducer and its need to to be adjusted or some other reference to the monitor.
This monitor is placed on the fundus, the top of the uterus. A nipple-like button rests between the round monitor and the mom’s skin. As a contraction comes, the button gets squeezed and measured on the bedside monitor just like the fetal heart beat one, but on its own line (the bottom line in the picture).
I turned the picture on its side so you can see the two lines. See the bottom one with the mountain? That mountain is a contraction. This mom was not on pitocin.
Just like the fetal monitor, if mom wiggles or shifts positions, the monitor can slide down and not register any contractions if mom is having any. Or, if mom shifts in a way that the monitor is tighter, it can look, to the lay person, that the mom is having some helacious contractions when, in fact, the baseline has moved way up. Put another way, while the fetal monitor blinks out the actual heart rate of the baby (actually, it’s about 1-2 seconds behind the beats), the UC monitor “rates” the level of contractions, from 0 to about 100. If the monitor is placed correctly, the machine is “zero’d out” and the monitor's numbers dive down to the steady 20 of no contraction. As the contraction builds, the number climbs, usually slowly, over the course of about 45 seconds or so and then starts its descent, ending again at 20. Women having mild contractions might register about 45 or so whereas I have seen pitocin’d women go so high the monitor doesn’t register it and the paper has a flat line across the top until a contraction goes down again and is able to be seen on the paper.
A comment from Cindy says it in a more technical manner:
"The appearance of the contractions on the tracing is based on maternal habititus, placement of the monitor, fetal position, and lastly ctx strength. When the belt is moved, it is completely random where the ctx. tracing now prints. WIth an Intra-Uterine Pressure Catheter (IUPC) the ctx. tracing would show resting tone and strength."
If the monitor is properly placed and calibrated, going that high would, in my experience, prompt the nurse to cut the pitocin off so mom’s uterus doesn’t explode or the baby doesn’t suffocate from the intensity of the squeezing of the uterus.
Look again at the picture above. Look on the left page and see how the bottom line is very close to the bottom of the paper? That’s pretty zero’d out. Then, after the contraction, see how much higher the line is? Mom must have moved and shifted the “zero” on the monitor. So, if she had another contraction with that higher baseline, it can look like she’s having a much bigger contraction when it really was about the same as the one before. If people don’t know about the zero-ing the machine out, they could think, erroneously, that the contraction is “a huge one!” when, in fact, the gauge began at 70 and the UC was quite mild.
Hey, here’s an idea for the family and support people. Instead of watching the monitor for all these numbers, how about we watch and be with the mother! She will tell you, accurately (if she doesn’t have an epidural, of course), what the contraction is doing. She can feel it going up, topping out and then going down. Isn’t that amazing?! I always think contractions are so incredible… that our bodies have the wherewithal to propel a baby out of the host body. It boggles the mind sometimes.
Now, look at the top picture again. See the thingie the mom is holding with a button on the end? This is a marker for mom to press when the baby moves. This puts a hash mark on the bedside monitor’s paper so the person reading the strip can see how the baby does with fetal movement. Little tiny arrows point, next to the upper/fetal heart rate line to make it easier to read. Experienced nurses sometimes forego the button pushing because, with practice, it is possible to see the baby’s movements with the heart rate shiftings.
So, there’s your tour of the image of a Non-Stress Test. This, of course, doesn’t speak to the need of an NST or how to prepare for one, but this is a start. Hope you enjoyed yourself!