Originally Posted by doctorjen
The tocometer (which is the monitor that monitors contractions) works as a simple pressure change monitor. It detects changes in pressure. A contraction shows up usually as a smooth hill - pressure gradually increases, peaks, and goes back down gradually. It is the heart rate monitor that is ultrasound. An external contraction monitor also picks up other pressure changes - you will see spike type things with movement, or sometimes see small bumps with each breath. An external monitor can only tell when a contraction is happening - not how strong they are. How high the pressure change registers has to do more with the shape of the mama, where you put the monitor on her belly, and a lot of external factors like that. Sometimes, particulary with fluffier moms, it's hard to trace contractions at all if you can't get the monitor situated well over the uterus. It is not at all uncommon to see huge-looking contractions that one woman can barely feel or not feel at all, and have another woman in active, active labor with contractions that visually barely measure anything. Sometimes, care providers get caught up thinking someone must be laboring because contractions are tracing, or someone must not be laboring because their contractions don't trace or look small. It's important to remember that the monitor can only tell you when a contraction is happening, not anything about it's strength.
An internal contraction monitor (called an intrauterine pressure catheter, or IUPC) can usually accurately measure the strength of a contraction.
I don't understand how a flat thing on an outside of something else can measure pressure though. I get how the IUPC does--it's mechanical so it's easy for me to understand, I guess.
When I was in labor last time I was too active for the external Toco to work so they had me pressing a button every time I had a contraction. Then they stopped believing that I was hitting the button at the same time every ctx (baby was having normal decels, I
could tell they were normal) so they pressured me into a IUPC. I guess it worked out, but yuck anyway. (oooh and while we're on the subject-- What are the risks with IUPCs? I don't think they told me of any risks when I agreed. . .)
Really though, how does a flat object on the outside of a uterus measure pressure? Sorry if I'm being thick