Mailbag 35: “What does the Fox say” – With Dr. Nathan Fox

All right. Hey, everyone. Let’s get started with “Mailbag 35, What Does the Fox Say?” Our first question today is from Beth. All right. “Thanks for the great podcast. I’ve learned so much. I had my twins at 34-1 after they identified that both babies had elevated flow on their Doppler, and twin B had intermittent no flow at my 32-week appointment. They are thriving one-year-olds now. But I’ve always wondered, with elevated flow and intermittent no flow, how do you know things won’t go bad in between appointments? Meaning, I had Dopplers twice a week and non-stress tests three times a week between 32 and 34 weeks, but over the weekends especially, worried that if something changed, no one would know and twin B would be hurt. Can you explain Dopplers and how you know how long you can wait?”

All right, Beth. Great question. So, as background, Doppler is basically the technology we use in ultrasound in order to detect blood flow. 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 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 are we doing 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’s not functioning well. Sometimes we call that placental insufficiency. 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, if that’s going on, how bad is it? How concerned should we be about it? And you would think, logically, you’re like, all right, if I’m going to 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, 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, 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. We’re 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 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 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 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. Why is that? Well, there’s technical reasons why we do it in terms of the angle and this and this.

But basically, when the baby’s heart is pumping, there’s all this force pushing the blood out of the baby into the umbilical cord towards the placenta. So, we sort of see a spike in how fast the blood is flowing. And then when the heart relaxes and there isn’t that pressure pushing the blood out, we sort of see a nice flow. And we measure the ratio of the first one, which is higher to the bottom one. So, when you say, Beth, that your baby had elevated Dopplers, what we mean is not that there’s elevated flow. It means that there’s an elevated ratio of the systole over diastole. We call it an SD ratio. And that’s the first stage of abnormal Dopplers. So, they’re abnormal, but it’s sort of the first stage of it.

If the placenta starts getting worse and more resistant, what we actually see is, during diastole when the heart relaxes, there’s a point when the blood actually stops flowing forwards. Meaning, the heart pumps during systole, there’s a big sort of burst of fluid of blood from the baby into the umbilical cord towards the placenta, then as the heart relaxes, that speed slows down. And if there’s a lot of resistance in the placenta, the flow actually stops. And that’s what we call absent flow. That’s stage number two. It’s getting a little bit worse.

Stage number three, which is when it’s even worse than that, is we actually see, at a certain point, reverse flow. We see the flow moving backwards, in the artery instead of from the baby towards the placenta. There’s so much resistance in the placenta that it actually starts moving the wrong direction, so to speak. We call that reverse flow.

And so, when we’re doing these Dopplers in a situation where a baby is small, the first thing we’re doing is it helps us maybe determine the etiology, the reason why the baby is small. And if we see abnormal flow in the umbilical cord, these abnormal Dopplers, it tells us that most likely the problem is the placenta as opposed to something else. That’s number one.

Number two, based on how abnormal the Dopplers are, it helps us determine how significant of a situation is this, and things like how often do we need to test? Do we need to do other testing, like non-stress tests or something else like you had? Do we need to deliver early because we’re worried that the placenta is going to get even worse? And so, we use sort of the Doppler studies to help do that.

Now, exactly how you do it depends on so many other factors. How small is the baby? Are there any medical issues? How far pregnant are you? Is the baby moving well? What’s the fluid amount? So, there’s all these things that sort of go into the hopper with us deciding when to deliver. And in your situation with twins, that adds another level of complexity. Are both babies’ Dopplers abnormal, just one, how bad, how far pregnant are you, and all these things. So, ultimately, they made a decision, sounds pretty reasonable, to follow you very, very closely and then deliver around 34 weeks. Okay. That’s all the background.

So, in terms of your specific question in terms of, well, what about in between those days? So, you are correct that it is possible in between testing that something could happen, so to speak. It’s possible. It’s possible with anybody in any situation that between doctor visits, between ultrasound visits, between whatever, something could happen. But the expectation is that if you interpret the Dopplers and the other tests correctly, you can have a pretty good sense of whether something really bad is going to happen in the next week or the next three days or four days. And assuming that chance is very, very low, that’s when we say to come back.

So, if I think everything is really, really fine, very, very low chance of anything happening in the next week or so, I’ll say, okay, come back in a week. If I’m worried that maybe there’s a slightly higher chance that something could happen, I’ll say, okay, come back twice a week. So, there’s less time in between visits. Or if I’m worried that there’s even higher chance of something happening, like obviously with you, I’d say come back three times a week.

And then ultimately, if we get to a point where we’re like, hey, I don’t think that’s even frequent enough. I think we need to monitor you more frequently than three times a week. Well, what do you do in that situation? Either you say, you know what, maybe it’s time to deliver, or maybe we should admit you to the hospital so we can test you once or twice a day. I mean, obviously, it depends so much on the circumstances. But that is something that is a calculus that we’re doing in our heads all the time with this kind of testing. What is the chance of something happening in between visits? And based on that, how frequently do we need to see you to prevent that from happening?

Now, unfortunately, there’s no strategy on the planet that universally removes all risk and all chance of something bad happening. But we’re just trying to minimize it and cater it and individualize it to each person, each specific situation based on all of the data that comes in. I hope that answered your question. It’s a very long answer to a really good, but it’s a complex question. And we decide this literally person by person based on the situation. It’s hard to just follow one specific formula because there’s so many other features that come into it.

Okay. Next, I have two questions from Matt, also on twins. “Hi, Dr. Fox. Thank you for the podcast and amazing information, as well as being our doctor.” He actually said amazing doctor, “As well as being our amazing doctor,” smiley face emoji. “I have two questions. One, can you talk about twin growth divergence for twins versus singleton and when to start expecting this? What percentile is generally considered healthy after we start to see divergence? Two, can you explain how you look at cervical length in twins along with FFN swabs? Are they both leading indicators? Do they help inform each other? Does one override the other? IE, short cervix and negative FFN isn’t as concerning. A long cervix, positive FFN also isn’t concerning, and so forth.”

All right. So, I’m going to take these questions one at a time on twins. So, the first question from Matt on twins was about twin growth and divergence for twins versus singleton and when to start expecting this. So, that’s really a fascinating topic about twin versus singleton growth. And the short answer is we don’t really have this all set and figured out yet, and I’ll explain why.

It is very, very well known that twins grow at a different rate inside than singletons. Meaning, twins on average are going to be smaller than singletons when you measure them on ultrasound and then when they’re born. And this is not just because they’re born earlier, but they’re actually smaller. A 34-week twin is typically smaller than a 34-week singleton.

When does this start happening? Typically, to answer your question directly, Matt, in the third trimester. So, usually, they’ll measure pretty similar, very similar in the first trimester, pretty similar in the second trimester, and in the third trimester, they start to diverge. And the question is, okay, we know that’s true. Option A is this is a bad thing. Meaning, option A is the twins, since there’s two placentas, they’re getting less nutrients per twin than a singleton would. And this is a pathologic process. Even though it’s common, it’s not good, and they’re getting sort of underfed compared to a singleton’s, and we should have heightened awareness for this and potentially deliver early because of this and do all this monitoring and watching. Fine.

Option B is no, no, no, this isn’t a problem. It’s an adaptive process that twins do. They don’t grow as big because if they both got really big, the uterus would start to contract and you would deliver prematurely. And so, it’s sort of an adaptive process that twins have “learned” over thousands, millions of years, whatever it is, in order to stay inside longer. And so, option A is sort of like common and abnormal, pathologic. Option B is it’s common, but sort of physiologic and normal.

And the reason we have these two options and we’re not really certain is because different people manage it differently. I would say, around the world, most people, when we’re measuring twins on ultrasound, we will use the norms for singletons. Meaning, if, let’s say, our growth curves where we establish what is the 50th percentile, what is the 90th percentile, what’s the 10th percentile, meaning what’s big, what’s small, what’s average, all these things, for singletons, when we take twins and their weights, we’ll plug them into the same charts.

And so, you’ll frequently see the twin percentile start to drop because that’s compared to a singleton. And then if it drops below a certain percentile, we start doing things like Dopplers, like we mentioned in the last question, or non-stress tests, or biophysical profiles, or deliver earlier and whatnot. And the thought is, hey, these twins are not growing as well. It’s a form of fetal growth restriction. I’m going to treat them like I do singletons and deliver them at the same way I would do it. Fine.

But some people say, you know what, maybe we should develop specific growth curves that are for twins and say, okay, no, this is maybe small for a singleton, but it’s normal for a twin. And maybe we should sort of change the percentiles. And it’s very hard to know which of those two strategies is better without doing a very large randomized trial where you take 1,000 women with twins and 500 of them, you manage them with one kind of growth curves and the other 500 with another kind of growth curves and see who does better.

And so, there’s data out there which will suggest, do this one. Data says do the other one. But it’s not perfect. And so, we still don’t exactly know what is the best management. I would say, in our practice and most people in the U.S., again, we’ll go according to the singleton growth curves, which ends up…if you do that option, you might be diagnosing more people with twins with fetal growth restriction, but maybe missing less problems, potentially. And if you go the other option, you’re going to diagnose fewer people with fetal growth restriction, but maybe miss more problems. Again, these are all maybes. We don’t know for certain. So, we do it the way most people do it in the U.S., which is to go according to the singleton growth curves and make our decisions about testing, about delivery based on that.

Okay. Your second question about twins was about measuring the cervical length and doing fetal fibronectin and how do we use those tests. So, this is also an area of controversy with managing twin pregnancies. And because not everyone does it, not everyone does it the same way, there isn’t one universally recommended or not recommended way to do it. What I would say is, in our practice, we do check twins with cervical lengths. That’s with a vaginal ultrasound measuring the length of the cervix. And we also, not always, but often utilize fetal fibronectin testing, which is another screen for preterm birth. So, basically, both of those are intended to screen for the risk of preterm birth.

So, someone walks in, they’re carrying twins, they’re whatever age. Let’s say they’re 24 weeks. And all twins have an increased risk of preterm birth. But if you do these tests, you measure the cervical length, you measure the fetal fibronectin, and you use that data. It can help give you a sense, is this person at a very low risk of preterm birth, a medium risk of preterm birth, a high risk of preterm birth, a very high risk? It can sort of help stratify people into what their risk is. And the cervical length is the shorter the cervix, the higher the risk. And with the fetal fibronectin, it comes back either positive or negative. Positive is bad, negative is good. We do that to confuse people, I think.

But essentially, the highest risk patient is someone with a short cervix and a positive fetal fibronectin. The lowest risk patient is someone with a long cervix or a normal cervix and a negative fetal fibronectin. And the people in the middle are those with, let’s say, a short cervix and a negative fetal fibronectin. One test is sort of higher risk, one test is lower risk, or a normal cervix and a positive fetal fibronectin, which is what you indicated in your question. Exactly how those risks play out depends on the circumstances, how far pregnant, how short is the cervix.

We actually developed a calculator that’s available on our website. I think the link is mfmnyc.com/twin, if I’m correct. If not, there’s definitely a link on our website where you could plug in a cervical length and a fetal fibronectin result at their various gestational age and it’ll tell you your risk of preterm birth and before a certain gestational age as it’s based on data we published.

And so, we sort of…again, we take all of this into account, the cervical length, the fetal fibronectin, how far pregnant she is, what’s her history like, sometimes if she’s contracting or not or feeling contractions or not, and make a decision about what to do. The reason not everybody does this is because it’s unclear what you’re going to do about this. Let’s say you find out someone’s high risk versus low risk. What practically are you going to do? And that’s a valid argument to say it’s not clear that there’s something you can do.

Our argument is that better to know than not know. There are things you could potentially plan for. Sometimes the analogy that I’ll give is, when I wake up in the morning and ask Alexa what the weather is going to be today and she tells me whether it’s going to rain or not rain, I can’t impact whether it rains or doesn’t rain, but I could bring an umbrella. That helps, keeps me dry.

And so, the same thing. Let’s say someone finds out they’re at a higher risk for preterm birth of twins compared to somebody else. You may not be able to stop at the preterm birth. I mean, we try, but you may not be able to stop it. But what if they were otherwise planning to go to Mexico that week? Or let’s say they live very far from the hospital. Or let’s say, for example, we made a decision, hey, you’re such high risk, maybe we’re going to give you a course of steroids, which help improve outcomes for newborns if they’re going to be born prematurely.

And so, that’s sort of the reasoning behind doing the screening. And again, it’s done differently around the country. Different people do it different ways. In our practice, we do these screens typically, but not everyone does it. In terms of your question about figuring out exactly how one plays with the other, cervical length, FFN, which has more weight, I would say the easiest way to do that is go to the calculator and play around with it. Play around with the numbers and see how it looks, and you can get a sense of how one is weighted versus another.

Okay. Our next question is from an anonymous listener. “Hi. I actually just joined your practice after listening to your podcast and thoroughly enjoyed it.” Hey. Awesome. “I found it incredibly informative and reassuring, especially as this is my first pregnancy.” All right. Welcome aboard. Great to have you. Question. “Since routine prenatal visits are typically spaced out and blood pressure is usually only checked during those appointments, how would a patient know if they are developing preeclampsia in between visits? Are there early signs or symptoms patients should be especially aware of? And do you recommend that patients monitor their blood pressure at home as a precaution? Thank you again for the work you do, both in the practice and through the podcast. It really makes a difference.”

All right. Thank you so much to this anonymous listener. You know who you are. I really thank you for the kind words. Okay. You are correct. We check blood pressure… Not just we. Pretty much everyone checks blood pressure in every prenatal visit, or almost every prenatal visit. And the main reason we’re doing it is to screen for preeclampsia, which usually if it’s going to come, it’s going to come later in pregnancy. But early in pregnancy, it’s helpful to know what someone’s baseline blood pressure is, or if they have hypertension, high blood pressure, and didn’t know about it.

And you’re also correct that for most people, if their blood pressure starts going up, they don’t have other symptoms. Now, you can have symptoms if your blood pressure goes up. Some people get headaches. Some people don’t feel right. Some people, if they’re getting preeclampsia specifically, they’ll have things like increased swelling or weight gain. The problem is that’s very common in a lot of pregnant women without preeclampsia. So, it’s hard to know, hey, is my increased swelling specifically preeclampsia? Again, sometimes people with preeclampsia can also have things like headaches. They can have abdominal pain. They cannot feel well. There are other symptoms potentially preeclampsia. But you are correct that for many people, if their blood pressure is going up or they’re developing preeclampsia, they may not have any symptoms.

So, while we give people warning signs about preeclampsia, again, things like headaches, things like increased swelling or whatever, it’s hard because a lot of people have that who don’t have preeclampsia. So, the question is, what do you do about that?

And so, traditionally, what we do about that is, as the risk of preeclampsia starts going up, the frequency of prenatal visits where we check your blood pressure also starts going up. And so, that’s one of the reasons why prenatal visits in the beginning of pregnancy and the middle pregnancy tend to be closer to once a month. Again, this is in standard, low-risk patients. There’s other reasons to have more frequent visits, but sort of as a standard. And as you get closer to your due date, the visits become every two weeks and every week and whatnot. And part of that is because we need to start checking the blood pressure more frequently because you don’t want to go a month, per se, for example, towards the end of your pregnancy of not having your blood pressure checked.

But your question is, well, if we’re going to check it more frequently, why not check it every day? Maybe people should just get a blood pressure cuff and check their blood pressure at home every day. It’s a really interesting thought and concept. And there are definitely people, for lack of a better term, toying around with this idea and trying to figure out what to do. And there are some, let’s say, hospitals or system or OBs or places where they will tell their patients, hey, pick up a blood pressure cuff and check your blood pressure every day. The upside is people will know and they’ll check it. The downside is you can have a lot of people, maybe with false elevation, a lot of scares or this or that. Okay.

Then there are places that will do that, but just for high-risk patients or certain high-risk patients. Like in my practice, I don’t typically tell everybody to get a blood pressure cuff and check it every day. But certain high-risk women, I will. People, for example, maybe who had a history of preeclampsia in a prior pregnancy or maybe women who have hypertension and take medicine for it. So, I potentially might tell them, hey, I think you should check your blood pressure once a day or twice a day for the exact reason that you mentioned, that they wouldn’t otherwise know if their blood pressure is going up.

This is something that’s also done after delivery. And a lot of women who, let’s say, had preeclampsia during this delivery or their blood pressure was higher, they’re at high risk for developing preeclampsia, maybe after they deliver, we’ll send them home with a blood pressure cuff or tell them to pick one up. And same questions. Who should get that? How often should they do it? What’s the best paradigm? What should you do? And I think that’s still being worked out. And I don’t think everyone’s going to do it exactly the same way.

But conceptually, there is probably a group of women who should be having their blood pressure checked more frequently than “standard.” But figuring out exactly who that group is, when should it start, how often should they do it, for how long should they do it after they deliver, is probably going to differ from place to place and from doctor to doctor. But that is certainly concept. Whether we’re going to get to the point where someone says, hey, I think every pregnant woman should check her blood pressure every day, everywhere, I don’t know. Maybe yes, maybe no. I think that might be a lot. But I’m not sure how it’s going to land.

But it is a really good point that you bring up. And certainly, if you feel like you’re at higher risk for preeclampsia than average, or for whatever reason, you’re concerned about it, it’s something to bring up with your own doctor. Now, you are one of our patients. You can bring it up with us. But for someone else listening, bring it up with your doctor, your midwife, and say, hey, for whatever reason I’m concerned I might get preeclampsia. I’m thinking about checking my blood pressure more frequently. What do you think about that? Should I check it at home? Should I come to the office more? Am I just being paranoid? Whatever. You have a conversation about it and land on something that makes sense for you. But you are correct. The tenet is that most people, when their blood pressure starts going up, they will not know it’s happening.

All right, guys. Great questions this week. Thank you very much. We’ll see you all next week.