Mailbag 35: "What does the Fox say" - With Dr. Nathan Fox

So as background, Doppler is basically the technology we use in ultrasound in order to detect blood flow, right? So that can mean a lot of different things.

So Doppler is just a technology.

But when we refer to Dopplers and ultrasound, what we basically mean is that there are times in ultrasound when we're going to measure blood flow in various ways.

blood vessels.

So the type of Dopplers that you're describing are the ones that we use frequently when there are babies, whether you're carrying a singleton or twins, who we suspect have fetal growth restriction, that they're small.

And so what are we doing with these particular Dopplers? So in these particular Dopplers, we're measuring the blood flow in the umbilical cord which connects the baby, your case, the babies, and the placenta.

In your case, I don't know if it was one placenta or two, but let's say two placentas.

And so what do we do in there? So in the umbilical cord, there is one vein which brings blood from the placenta to the baby.

And typically there are two arteries which bring blood from the baby to the placenta.

So when we have a baby that's measuring small, one of the concerns or one of the potential reasons why this could be going on is if there's a problem with the placenta, like the placenta is not functioning well.

Sometimes we call that placental insufficiency, right? That could be one cause for the baby being small.

There are other causes and we've had lots of podcasts and mailbags about growth restriction, but one of the causes is that there's an issue with the placenta.

So what we like to do is figure out sort of if that's going on, how bad is it, right? How concerned should we be about it? And you would think sort of logically, you're like, all right, if I'm gonna measure blood flow in the umbilical cord, to measure the placenta, I should measure the amount of blood flow that's going from the placenta to the baby, right? Because that would be the thing to see if the placenta is working, giving nutrients.

But in fact, we do the opposite.

We check the blood flow that's going from the baby to the placenta, umbilical artery.

So why do we do that? The reason is that sort of from a physiologic perspective, the placenta tends to be what we call a very low resistance organ, meaning the blood that's flowing from the baby to the placenta tends to flow nice and easy, sort of like a river, as I use as an example, very nice and easy towards the placenta.

And then If that's the case and we can see that pattern on ultrasound, we measure the blood flow, like, all right, the placenta looks healthy, which means either there's no problem with the placenta whatsoever and the baby's small for another reason, or if there's a problem with the placenta, it's very, very minor problem.

And nothing really is going on now.

And we can watch and wait.

But then we start to see patterns potentially of resistance in the placenta.

So it's like harder for the blood to go from the baby to the placenta.

And there's various stages of that.

And the way we do it from a technical perspective is we measure blood.

a ratio of the blood flow during systole.

That's when the baby's heart is pumping and diastole when the baby's heart is relaxed.