So now we're going to move from the passenger to the next P or passageway. We're talking
about the pregnant patient's anatomy this time only. So, no fetus involved here. The
passageway actually refers to the bony pelvis, the soft structures of the cervix, the vagina,
and the introitus. The introitus, just to jag your memory, is the opening of the vagina. We
want to think about some landmarks because as we move into the types of pelvic shapes,
it's important that we remember our orientations. So looking from the top view, you can see
the pelvic inlet. So what the fetus is going to enter in first as it engages in to the pelvis?
We also have a bottom view and so this again is looking at the patient's pelvis from the
lithotomy view and we can see what we would see as the baby comes out. So we have an
inlet and we have an outlet. So there are 4 basic shapes. Now I'm going to say they are
4 basic shapes knowing that we're humans and as humans there are all kinds of variations,
but there are 4 very common shapes that we're going to talk about today. And it's important
that we talk about these shapes because they have, again, an implication on the way the
fetus navigates the pelvis. So the way we determine the types of pelvis has to do with the
inlet and some other elements we'll talk about in just a second, but I want you to look at
the shapes of the inside of the pelvic structures so you can start to commit to memory what
each one represents. So if you look at the first one, we have a gynecoid pelvis, and this
gynecoid pelvis is sort of round and it's a round shape. Remember the fetus churns, and as
the fetus churns we want to use the circle structure to make space for that to happen.
In the android pelvis, you'll notice the shape is now more of a heart shape. Can you see
that? So, this is more likely to be associated with the male-shaped pelvis. This is going to
create some problems for delivery we'll talk about in just a minute. We have an anthropoid
pelvis and you begin to notice that that shape has sort of stretched out so it's more
elongated. And we have a platypelloid pelvis which is elongated this way. So these are our 4
structures. So let's break this down in a little bit more detail so we have a really good
understanding of what each one looks like. Let's start with the gynecoid pelvis. The gynecoid
pelvis is the female pelvis. More than 50% of female patients are going to have a
gynecoid-shaped pelvic structure. Again, the internal pelvis is round. Now we want to look at
the sidewalls. So the sidewalls in this graphic are straight. We don't want convergent
sidewalls because that's going to make it harder for the fetus to come through. These are
going to be straight. We can look at the ischial spines and they're blunt, they're not sharp.
And we're going to look at the pubic arch and what we notice is that the pubic arch is wide.
This is associated with the birth in the OA position and now you know what that means. So
let's take a look at that pelvis. Okay, so we have a round pelvic structure here at the inlet.
The sidewalls are going to be represented by the inside of the pelvis here. These sidewalls
are going to be straight. The ischial spines are here and these are blunt, and the pubic arch
is going to be wide. So remember, this is associated with the fetus in the occiput anterior
position. This is normally the shape of the female pelvis, 50% prevalence and it's the one that
we want to have most of the time. Okay, let's talk about the android pelvis. As we move
through this series, think about what's different. It's always helpful to compare one to the
other to help you remember how they're different. So, in the android pelvis, this is the
opposite. This is the pelvis that's most likely associated with the male type pelvis. The
frequency is a lot less, it's less than 25%, which makes sense. The shape of the inlet now is
no longer round, it's more of a heart shape. And the sidewalls now are not straight, but
they're convergent. So that means they kind of move in towards each other. And if you
take a look at the pubic arch instead of being wide, it's now narrowed and this is associated
with an increase risk of failure to progress and what that means is because the shape is now
more constricted it's going to be less likely the patient will be able to achieve a vaginal
vaginal delivery. So the baby might get stuck and not be able to rotate through the pelvis
necessitating a Caesarean birth. So, lucky for us, the pelvis is flexible so we can show you
the same thing on the pelvis again. So instead of that nice round shape of the gynecoid
pelvis, we now have more of a heart shape. So let's go over the landmarks again. So this
time the sidewalls, you can see them here. Instead of being straight, they're now convergent.
So can you imagine how much more difficult it would be for the fetus to come through. And
the ischial spine is here instead of being blunt are now going to be longer and prominent.
So kind of think about vampire fangs. Now they're not that sharp, but you'll get that idea
and then you'll never forget. And this is the pelvic arch. So instead of being nice and flat
and open here, it's now very narrow, making the points of the end of the heart. So you can
see how the space in the pelvis has significantly decreased. Moving on to the next pelvic
shape. We want to think about less about a heart and think about an oval. The next 2 pelvic
shapes really use that kind of imagery. So this is an anthropoid pelvis. So now the heart is
kind of stretched out like this. It has about a 25% prevalency and it's an oval. Okay? The
sidewalls for this particular pelvic shape are now straight. The ischial spines are still
prominent and the pubic arch is going to be narrow. So, it's going to be narrow this way, but
the anterior to posterior diameter is going to be a little bit wider. So let me show you on my
pelvis. Okay, let's look at the anthropoid pelvis in real time. So, thinking about our pelvic
shape, remember this is more of an oval so it's going to look like this and so what you'll find
is that the anterior posterior is now wider, but the lateral diameter left to right is going to
be much more narrow. Okay? The sidewalls are still straight, which is good, but our ischial
spines are going to be prominent, which is not as good. Because we don't have as much
space on the lateral side, the fetal head is not going to rotate as well. Unfortunately, it's
not likely to get stuck in the anterior position because the pubic arch is going to be a little
bit more narrow. It's more likely to be stuck in the occiput posterior position. So remember
what we talked about with the OP position that the maternal feeling about that maybe one
of pain because we have bone-on-bone. So ask yourself if a patient presents with back pain
in labor that doesn't go away, take a guess at what type of pelvic shape they may have.
Anthropoid. Perfect. Now we're on to our last pelvic shape and the most difficult to say.
This is called a platypelloid pelvis. This is also the least frequent type of pelvis so maybe
that's a good thing. So let's look at the shape and again compare it to the other 3 types of
pelvic shapes we've talked about. So, in this particular shape, the sidewalls are flat which is
good. Okay? And they're straight. Also good, but let's look at the ischial spines. What we'll
notice is that they're blunt. That part's good and they're widely spaced. And the pubic arch
instead of just being a little bit wide is really wide. So now we have an oval that's shaped
in the other direction so the AP diameter is short, the lateral diameter is wide, and this
creates whole another set of problems. Let's take a look at the pelvis and see if we can
point it out. So, on a platypelloid pelvis, remember the AP diameter is now shortened and our
lateral diameter is wide so it's really wonderful that our ischial spines are blunt. That's great.
It's great that our sidewalls are straight and flat. That's fine, the problem is there is not any
room here in the anterior posterior diameter for the fetus to rotate so typically we're
going to have an arrest of labor, meaning that we're not likely to achieve a vaginal delivery
unless the patient has a really large pelvis or a really teeny tiny baby because the fetus is
not going to be able to rotate into an OA position or an OP position. They will be stuck in a
transverse position, not a transverse lie necessarily but a transverse presentation. So think
about LOT and ROT and any of those OT positions then this is what that's going to look like.
Alright? So platypelloid, not very frequent and not likely to end in a vaginal delivery. Alright,
we looked at the 4 types of bony pelvis shapes. The takeaway message here is that gynecoid
is the most prevalent and the most desirable for a vaginal delivery.