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Calculating Risk


So, let’s take an example. Let’s say that you’re 33 and I’m going to show you how the alpha-fetoprotein test is used to calculate a different number. Okay. So if we have a 33-year-old mom and we were talking about her risk of Down syndrome in the second trimester. Let’s say one in 500, okay, as a nice round number. So if we took her one in 500 risk that’s related to her age, so we start with her age related risk, and now we’re going to see where she lands if we draw her blood. If she lands at one, back to that average, we would take her one in 500 risk and multiply it by one over one and her risk is still one in 500. Okay? But if she landed up there at two, we take her one in 500 risk that she starts with, and we multiply that by one over two and her risk goes down. Now it’s one in a 1000 and that makes sense because she’s going on that side of the curve, right? So if she landed down there at a half, we’d take her one in 500 risk, multiply that by one over a half, and now it’s one in 250 and her risk went up. So that’s all we’re doing, is adjusting the risk by taking age, plus the blood test and where you landed on the blood test, and coming up with a number one out of something. Okay? That was the first test … the alpha-fetoprotein.

Now, if we were to do a quad screen, it becomes a longer calculation, but it works the same. What we do is we just measure the alpha-fetoprotein plus the estriol and HCG and another hormone, called inhibin, and that’s how we come up with a calculation that gives you your age-related risk. Okay?

Now one thing that I want to make sure that is clear. Age plays into this. Right? Because the first number is your age. So if we had a mom who didn’t start with a one in 500 risk but starts with a one in 200 risk, even if she landed in the same places, her risk comes out higher because her one in 200 is multiplied by one or by one half, right? Things change based on risk.

All right. One more effect of age — not only does mother’s age affect these numbers but so does the baby’s. Okay, so let’s say we’re assuming that the baby’s 17 weeks along. There’s the normal curve for a 17-week baby and the Down syndrome curve for a 17-week baby. But here’s the normal curve for a 16-week baby. And here’s the normal curve for a 15-week baby. So what happens if you’re really 15, but I think you’re 17, and we’re doing calculations based on 17? The risk becomes greater. Yeah. It can make you have a falsely elevated risk. So one of the causes of an abnormal test is when we don’t have good dating. That’s why we take care in our system to try to make sure that we know your dates early on in your pregnancy so we know exactly how far long you are.

So here’s a question, we’re coming out with numbers: one out of something. My question for you is what’s a lot? Right? When do you draw a line? When do you crossover the line and say, oh, that sounds like a lot? If I said the risk for Down syndrome was one of out of two, 50 percent. How many of you would say that’s a high risk? Extremely high. It’s a lot. Isn’t it? Yes. All of us would agree 50 percent risk is kind of scary. So how about one out of 10? Ninety percent chance the baby is normal; 10 percent chance the baby has Down syndrome. Is that high risk? Feels like it. Some say yes, some say no. It’s a little bit more shaky, isn’t it? How about one percent? Ninety-nine percent chance the baby is normal; one percent chance the baby has Down syndrome? Is that a lot? Most people say somewhere between one and 10. Somewhere in that 10 percent range to that one percent range, I get more comfortable with it and things are okay. And most humans respond by saying less than one percent is low risk. It would be nice if we did it that way. But it’s not quite that simple in medicine because where do we draw the line? We’re trying to do this in some scientific way not a subjective way cause you notice some of you had a little different of opinion?

So let me ask you this. Back to that question I asked you earlier. So what happens if I make you nervous? I say, “You’re high risk.” Might you want to know? Right, you might want to know for sure. If I say, “Oh, you’re risk is high.” That’s a lot. If I said to you, “Oh that one percent, ooh that’s high risk.” That’s a lot of risk, you know. You might believe me right and you might say that’s a lot. Or that 10 percent, that’s a high risk. Some people would say it’s a very high risk, okay. So if I tell you you’re high risk, how are we going to know for sure? How are we going to find out if the baby really has Down syndrome? Testing. What kind of test? Do you remember? Amnio. Yeah, like an amnio. Okay, so if you might be willing to have an amnio because I made you nervous. Right? Because that’s the only way you can become un-nervous or find out for sure. Right, is to ask yourself that question. So what is the risk of the amnio? What is the risk of that yes or no testing? Do you remember? What were the chances of causing a miscarriage? One in 270. Yeah, exactly … one in 270. It goes back to this picture here. Remember that. So that’s how medicine does it right now.

So if we drew your blood and you had a quad screen for example and it came out one out of 274. We would say that’s low risk. The same way we’d say one of out 7000 was low risk cause it’s less than one out of 270. But if it was one out of 269. That’s the phone call, right? That’s the cutoff. Why? Because if we make you nervous and you have a diagnostic test, we don’t want to cause more problems than we could find. Now whether or not you think that risk is equal, is up to you. It shouldn’t be a scientific discussion or decision, ultimately. Because you have to think, am I really nervous about this number or this risk? What would I do with the information? Am I willing to do an amniocentesis? What if we found out the baby had a problem? Would we continue or not continue? The answer to that, is what helps you decide should I have the tests in the beginning? Okay, from the get go.

Now remember besides screening for Down syndrome, we can use the alpha-fetoprotein to screen for spina bifida. Remember how that was high in babies who have spina bifida. That’s one of what we call a neural tube defect. There’s another one called anencephaly — that’s another serious one. That’s where the top of the baby’s head isn’t formed. Fortunately, it’s very uncommon, but the alpha-fetoprotein test can help screen for those.

Now, if you’re going to do an ultrasound, then you probably would see that anyway, right? The other thing that the alpha-fetoprotein can suggest is multiple gestations. More babies, more alpha-fetoprotein, or if the baby had a little hole in the abdominal wall which is another rare finding. So the alpha-fetoprotein can help look for those things. Now again, if you’re going to have an ultrasound, that’s how we screen for those things also. So you don’t have to have it, if that’s the case.

Now here’s what I really hope that you’ve gotten out of this so far. The blood tests, they’re not just a blood test. Yeah we can draw your blood that’s easy. Easy for me, I guess. If we draw your blood and we get a result that comes back, then what? Because some people say well Dr. Fausett, I just want to know. Does drawing your blood tell you? It doesn’t really tell you. It just changes your risk. So I say, okay, I understand if you want to know. Are you willing to do diagnostic testing? Because if you say, no. Then maybe you shouldn’t do the screening test. If you say, I’m not sure. Then maybe you should. You think about it, you talk about it, and you decide what to do. But it really depends on what you would do with the information whether or not the test is useful for you.

So, back to the beginning. What I was telling you is some of these tests we want you all to do because they clearly help us. These tests they’re not really for us. They’re for you, so you have to decide what would I do with the information. Okay? So you’re not a bad mom if you choose not to have tests because it doesn’t really change things for the babies. It just depends on you, and what you would do with it. Okay?