Fetal Anatomy and the Three-letter-system – Passenger (Nursing)

00:00 So as we move to the next discussion about the presentation of the fetus, it's important that we go back and remember some of the anatomy because this will help. On the fetus, we have the sacrum. This is the lower part of the spine. It's a bony prominence and we will use this as a landmark. The mentum we've already discussed is the chin and the occiput is the back part of the fetal head. Sometimes depending on the view, we will actually be looking at the scalp or the skull of the fetus. So let's look at those landmarks. We have the posterior fontanelle which is shaped like a triangle, we have the sagittal suture which you can actually feel with your fingers, and we have the anterior fontanelle which is shaped like a diamond.

00:47 So remember that, we're going to use that in just a minute. Now we have those parts in mind, let's look at the way that we would communicate fetal presentation to other providers.

00:59 There is a 3-letter system that we used that we're going to go through right now. Now, this tells you the order that those letters are going to appear, but I'm going to show it to you differently to make it easier to help you understand. So you see steps 1, 2, and 3, but we're going to look at step 2 first. Step 2, the letters are O, M, and S. Do you remember those from just a little bit ago? O stands for occiput, M stands for mentum, and S stands for sacrum.

01:34 So, looking at the position, look at this diagram again. Occiput is this position; mentum is this position or face, poor flexion attitude, remember that; and sacrum being a breech. So do you see that, it's a little more clear. So first we identify what is coming first. Now we're going to look at the 3rd part. I know this is out of order, but this is just the way that someone decided it should be so we're going to go with it. So, now we want to know the relationship of that presenting part to the pelvis, to the maternal pelvis. Now, typically when we do this we're looking at the bottom view. So imagine a patient in lithotomy position, they're lying on their back and the provider is facing them. All of these orientations are the patient's orientation, not the provider's so don't get confused. So the first letter A could mean that the presenting part is facing the anterior portion of the pelvis. P means posterior or T means transverse.

02:44 So I want to show you that on the pelvis so it makes a little bit more sense. Now I have a pelvis and I want to show this to you close up. So, anterior is this part of the pelvis, posterior is this part of the pelvis, and then transverse could be right or left. Now that's in a lithotomy orientation. I could also turn the pelvis this way and then the anterior would be here, posterior would be here, and right and left transverse would be here. So now let's go back to step 1, and what we're determining here is which side of the maternal pelvis the presenting part is facing. Now we have 2 options. It could be left or right. Let's look at the graphic. So from the bottom view, which is this way, we have right side and left side and from the front view we have right side and left side. So remember this is always the pregnant patient's orientation and not the provider's. So for you, this looks a little bit backwards, but you want to make sure you have that orientation correct or you'll mess up this whole part and we don't want to do that. You're doing great. So now it's time to put all this together. Remember, if at any point it starts to feel overwhelming, just pause the video and go back. Sometimes this is the trickiest part of this whole lecture so it's understandable if you need a little bit more time. No worries. So let's go on and put it together.