Dr. Lyndsey Darrow: Pregnancy and extreme heat

[00:00:19]

LYNDSEY DARROW: My name is Lyndsey Darrow. I’m a professor of epidemiology in the School of Public Health at the University of Nevada, Reno, and I’m an environmental epidemiologist. So, I study the health effects of environmental exposures like air pollution, water pollution, and climate-related exposures like heat waves. I have a particular interest in trying to understand the maternal and child effects of these exposures because these are physiologically unique life stages that might have unique vulnerabilities.

[0:01:02]

LYNDSEY DARROW: People who are pregnant are more vulnerable to heat-related illnesses than people who are not pregnant, and that is just because of the normal physiological processes that happen during pregnancy. So, there is a large expansion of the blood supply. There is a change in circulation patterns, and so thermal regulation—the ability for the body to keep a steady temperature—becomes more difficult. Hydration needs go up, and it’s hard to stay, you know, avoid dehydration.

[0:01:44]

LYNDSEY DARROW: So, one of the pregnancy complications that has been associated with extreme heat is the hypertensive disorders of pregnancy. So that’s outcomes like pregnancy-induced hypertension, preeclampsia, eclampsia, and these conditions pose serious risks to both the mom and the baby. They’re predictors of maternal mortality, and they also have implications for a long-term cardiovascular health in the women. The women who develop these conditions, there are a number of other pregnancy complications that have been linked to extreme heat, including gestational diabetes. There are quite a few studies at this point indicating that extreme heat is associated with preterm birth and fetal death, so miscarriage and stillbirth. There’s still a lot of uncertainty about these relationships and effects during specifically during pregnancy, but we do know more about the relationship between more extreme heat and mortality, and we know that extreme heat increases the risks of cardiovascular and renal-related deaths. And so, it makes sense that these would be vulnerabilities during pregnancy, when these organ systems, the cardiovascular system and the renal system, are already under enormous strain.

[0:03:08]

LYNDSEY DARROW: One way extreme heat can have an effect on infant health is through the increased risk of being born too early. So, a couple of outcomes that are associated with extreme heat are preterm birth and early term birth. Preterm birth is birth less than 37 weeks gestation, and early term birth is 37 to 38 weeks, so a little bit early than the full period of 40 weeks. But we care about these outcomes because they’re strong predictors of both infant mortality and health trajectories over the life course. So, they’re associated with neurological development respiratory diseases like asthma, and so when you have babies born too early, that has an effect long-term on health. There’s also, you know, once infants are born, they are more vulnerable to extreme heat. They have immature thermal regulation. They have more surface area relative to their size, and so as a result, they’re more susceptible to extreme heat. And we see evidence that there are increased hospitalizations during heat waves due to a range of issues in infants, including diarrhea and digestive issues, breathing problems. And so, this is a very serious issue as well. Another serious outcome that has been associated in multiple studies in the U.S. and in Canada at this point is the increased risk of SIDS—sudden infant death syndrome—during extreme heat events.

[0:04:47]

LYNDSEY DARROW: Our research group has been looking at the acute, or the immediate, effects of heat waves on several birth outcomes, so preterm birth, early term birth, and stillbirth. And we conducted a couple of large studies in the U.S. one of them is looking at 50 cities—50 of the largest U.S. cities. Another looks in more detail at a sample of eight states that includes the whole state. And what we’re looking at is whether the rates of these pregnancy outcomes increase immediately following a heat wave. And what I mean by heat wave is a multi-day streak of unusually hot temperature for the location, so outside the norms for temperature. And what we’ve seen in those studies is that heat waves are indeed followed by an uptick in the rates of those adverse pregnancy outcomes, and there’s more of an increase when we see longer and hotter heat waves. The effect sizes we’re seeing are very small, but I’ll note that this is the increase we see among the whole population, and that’s after any behaviors pregnant women in the population are actually making to avoid the heat. So, the fact that we can actually see a small uptick in the rates at the population level suggests that there are subpopulations of women that are experiencing that heat and having much larger increases in risk of these outcomes. And even within our study, there’s a limited number of things we can look at when we’re dealing with birth records. These studies have been based on millions of birth records and fetal death records, but even with that limited data, we can actually see that some subgroups are particularly vulnerable, younger women, women with less educational attainment, and women who are living in areas of more poverty, for example.

[0:06:49]

LYNDSEY DARROW: So, the first is better understanding the specific meteorological conditions that lead to poor health outcomes—especially among pregnant women. We don’t know what specific temperatures and duration of heat, where we start to see these health risks increase. Is it the high nighttime temperatures because people can’t cool off? How does wind and humidity play into these risks? And when we start to learn more about this, it’s likely that determining the exact conditions is going to differ by location. Acclimatization does play a role in how, when, what conditions under which people feel hot, and so if we’re going to have effective public health warning systems like heat advisories, then we have to better define the weather conditions that confer harm. The second thing I’ll mention is a better understanding of biological mechanisms. So how specifically is extreme heat leading to problems like preterm birth? Is it dehydration that’s leading to decreased blood flow around the uterus and triggering contractions? Is there an inflammatory pathway involved that is premature rupture of the membranes, which is what a woman’s water breaking. Is that involved? There is some evidence for that. And so, the better that we understand the mechanisms that are biological relationships between these exposures and outcomes, the better we can come up with ways to prevent or potentially even intervene on those specific mechanisms.

And then the last thing I’ll mention is additional work to understand both the social factors and the biological factors that might increase a person’s vulnerability to extreme heat. We have a lot of growing evidence that social and demographic factors, like economic disadvantage, like maternal age, like occupation, all those things contribute to exposures and vulnerability to extreme heat. But there’s more to learn, and identifying vulnerable populations is part of determining where to put our efforts to maximize our impact in improving public health. And then biologically, how do certain maternal chronic diseases affect vulnerability? Are there concerns about specific prescription drugs people are taking that are known to actually affect a person’s ability to thermoregulate? And so, there’s a world of research that we’ve barely touched on about biological risk factors as well. So, I hope research in this area can continue to be funded so we can address these areas with the goal of ultimately mitigating these emerging risks.

[0:09:43]

LYNDSEY DARROW: On an individual level, the practical ways to avoid heat stress are what you might expect. So, avoiding being outside and avoiding physical exertion in the hottest parts of the day. Trying to stay hydrated. Staying on top of hydration. Trying to have access to adequate air conditioning or seeking out cooling centers if they’re available. Having a backup plan in case your air conditioning goes out in the worst heat wave of the year. In other words, modifying behavior to the extent possible to avoid overheating. The problem, of course, is that many times the reason someone who is pregnant is being exposed to prolonged, extreme heat is out of their individual control, and so that could because the person is working in an outdoor occupation, for example, a farm worker or a postal worker. It could be because they have to stand outside waiting for a bus in the sun. It could be someone who can’t afford the costs of air conditioning during those extreme events, or that their air conditioning has broken during that event. So, a lot of times it’s out of an individual’s power. So that’s where the public health policies and infrastructure has to come in to provide potentially low-income energy assistance, cooling centers, setting occupational heat standards to protect people, increasing green space in urban heat islands. All of that is public policies that we can pursue to mitigate those individual-level risks.

[0:11:28]

LYNDSEY DARROW: Climate change is what’s driving the increase in extreme heat events that we see happening globally. We are seeing—when we see heat waves, we’re seeing them last longer. We’re seeing them reach record temperatures, and we’re seeing an expansion of the warm season. So, in the past, when we weren’t seeing extreme heat waves in May or October, now we’re seeing potentially severe heat waves happening in these border months, right that weren’t traditionally the warm season.