Humid Heat Reshapes Pregnancy Timing and Raises Termination Risk in Nigeria

In Nigeria, the most populous country in Africa and home to one of the world's highest fertility rates, the growing combination of heat and humidity is quietly rewriting the timing of pregnancies and raising the risk that they end early. A new study led by researchers at the University of Michigan, published in the Proceedings of the National Academy of Sciences, is among the first to examine how intensifying humid heat affects pregnancies in a lower- and middle-income country, where the health consequences of climate change remain the least studied. The work, supported by the NASA Land-Cover and Land Use Change Program, offers public health officials and communities across Nigeria evidence they can use as they navigate pregnancies through increasingly oppressive conditions, and it arrives at a moment when the country's combination of high fertility and acute climate vulnerability makes such evidence especially urgent.

The research team chose to focus not on temperature alone but on humid heat, a distinction with real physiological consequences. High humidity limits the body's ability to cool itself through the evaporation of sweat, which increases the risk of heat stroke and places additional strain on anyone exposed to it. Despite this, humidity has received limited attention from studies investigating the effects of climate change on fertility and human health, particularly in lower- and middle-income countries including Nigeria.

"There's a fair amount of literature in epidemiology that looks at temperature and pregnancy outcomes, but most of it is in the U.S. and other high-income countries," said Nina Brooks, assistant professor at the U-M School for Environment and Sustainability. "Humidity plays a really important role in pregnancy heat and we wanted to focus on the role of not just heat, but humid heat." To capture that combined influence of air temperature and humidity, the researchers calculated high-resolution measurements of wet-bulb globe temperature, a metric that integrates moisture in the air with thermal conditions to estimate the heat stress experienced by the human body.

The team assembled daily maximum wet-bulb globe temperature values across Nigeria between 1983 and 2016 by combining data derived from land-based weather stations with satellite observations. This fusion of ground and space-based measurement matters enormously in a country like Nigeria, where few weather stations hold long historical records. "When combined with stations, satellites can improve heat stress measurement in countries like Nigeria where we have few weather stations with long historical records," said Cascade Tuholske, a study co-author who is now a professor at Aix-Marseille University in France.

The approach demonstrates how satellite and remotely sensed data can be integrated with large-scale health surveys to close persistent gaps in climate and health research. For information on pregnancies themselves, the team turned to two waves of the Nigeria Demographic and Health Surveys, which included data spanning from 2003 to 2016. The survey is supported by agencies within Nigeria and abroad, including, until recently, the U.S. Agency for International Development.

By linking the survey records of pregnancies across the country to the daily heat stress estimates, the researchers could ask how exposure to extreme humid heat in a given month altered the probability that a conception occurred or that a pregnancy ended early. The findings were published in the Proceedings of the National Academy of Sciences under the title "Humid-heat shifts conceptions and increases pregnancy termination risks in Nigeria." The headline result is stark: increasing humid heat is strongly associated with an increased likelihood of early pregnancy terminations.

The researchers are careful, however, to note an important caveat in the underlying data. The surveys do not ask whether terminations are induced or spontaneous, so the study cannot distinguish between pregnancies ended deliberately and those lost naturally. "There are certainly reasons to think that either of those channels could be affected," Brooks said.

"But we tend to think that physiological, spontaneous termination is more plausibly linked to heat exposure." That caution matters for how the finding should be read: the study establishes an association between humid-heat exposure and early pregnancy terminations as recorded in the surveys, while the mechanism linking heat to those terminations remains an open question that the data cannot resolve on their own. The effects on conceptions reveal a subtler and in some ways more surprising pattern. During a month of exposure to high wet-bulb globe temperatures, conceptions fell by about 1.4 percent compared with the survey-wide average of just above 15 conceptions per 1,000 woman-months.

But the story did not end there. Conception rates grew relative to that average by 2.3 percent the following month and by 5 percent after two months. The net result is that the overall probability of conception increased, but the timing of pregnancies shifted, with couples apparently postponing conception through the hottest periods and resuming afterward.

In other words, humid heat in Nigeria is not simply suppressing conceptions; it is redistributing them across the calendar, even as the overall number of births trends upward in the country. That redistribution carries consequences of its own. "While shifts in timing alone might indicate effective adaptation by heat-exposed populations, this net increase in conceptions may bear social costs for households if they conflict with women's desires around childbearing," said Katie McMahon, a study co-author and doctoral candidate at the University of California Santa Barbara.

"If these changes primarily affect socially vulnerable households, we may also worry that they will exacerbate existing inequalities." A shift in conception timing that looks like successful adaptation at the population level may, at the household level, mean pregnancies arriving at moments when families are least prepared for them, or when they conflict with women's own preferences about if and when to have children. The researchers' framing makes clear that a population-level statistical rebound in conceptions cannot simply be celebrated as resilience without asking who bears the costs of the shift. The burden of these shifts was not distributed evenly.

There were also marked geographic differences that correspond to Nigeria's religious and economic divides. In northern Nigeria, where the majority of communities are Muslim, shifts in pregnancy timing were more pronounced, while in the south, where Christianity is the major religion and which is the wealthier part of the country, timing was less impacted or even trended in the other direction. "The regional differences in outcomes observed in this study reflect longstanding regional disparities in Nigeria's development which have yet to be addressed by social policy," said Kwaw Andam, a study co-author who was the country program leader for the International Food Policy Research Institute in Nigeria at the time of the study.

The geographic pattern suggests that the same climatic exposure can produce different reproductive outcomes depending on the social and economic conditions of the communities experiencing it. For the researchers, the practical value of the work lies in what it can offer the people managing these risks on the ground. Understanding the impacts of humid heat, and how those impacts vary between communities, will be helpful to public health workers around Nigeria as they advise women throughout their pregnancies.

Brooks emphasized that a goal moving forward is linking high-level studies like this one to lived experiences in the communities, to ensure that women receive the best possible public health advice. "I think a really important step forward is doing work on the ground to really understand how people perceive these effects and how they perceive risks to their health," Brooks said. The study also carries a methodological lesson that extends well beyond Nigeria.

By demonstrating that satellite-derived heat stress measurements can be married to large-scale demographic surveys, the research provides a template for studying climate and reproductive health in countries where ground-based weather records are sparse. For now, the Nigerian evidence makes clear that it is not just the heat, and not just the humidity, but the two together that are reshaping some of the most personal decisions and outcomes in human life, and that understanding those effects requires measurement tools capable of seeing them where they occur.

Subject of Research: The effects of intensifying humid heat on pregnancy outcomes and conception timing in Nigeria

Article Title: It's not (just) the heat, it's the humidity: Examining the impact of intensifying humid heat on pregnancies

Article References: It's not (just) the heat, it's the humidity: Examining the impact of intensifying humid heat on pregnancies. Original publication (Bibliographic details unavailable.)

Image Credits: AI Generated

DOI: Not provided

Keywords: humid heat, wet-bulb globe temperature, Nigeria, pregnancy, conception timing, pregnancy termination, climate change, satellite data, Demographic and Health Surveys, public health, regional inequality, PNAS