Rising heat and humidity increase pregnancy termination risks in Nigeria

New research led by the University of Michigan contains valuable insights for public health officials and communities in Nigeria as they navigate pregnancies through increasing heat and humidity.

The research team focused on Nigeria, the most populous country in Africa, because it has one of the highest fertility rates in the world and is particularly vulnerable to the impacts of climate change. But high humidity, which limits people's ability to cool off and increases their risk of heat stroke, has received limited attention from studies investigating the effects of climate change on fertility and human health. That's especially true of lower- and middle-income countries, including Nigeria.

This study, which was supported by the NASA Land-Cover and Land Use Change Program, shows how satellite and remotely sensed data can be integrated with large-scale health surveys to change that.

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. Humidity plays a really important role in pregnancy heat and we wanted to focus on the role of not just heat, but humid heat."

Nina Brooks, assistant professor, U-M School for Environment and Sustainability, or SEAS

For data on pregnancies, the team turned to two waves of the Nigeria Demographic and Health Surveys, which included information spanning from 2003 to 2016. This survey is supported by agencies within Nigeria and abroad, including, until recently, the U.S. Agency for International Development. 

To estimate the combined influence of air temperature and humidity, the team calculated high-resolution measurements of what's called wet-bulb globe temperature. The researchers obtained the daily maximum wet-bulb globe temperature across Nigeria between 1983 and 2016 by combining data derived from land-based stations with satellite data.

"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 findings were published in the journal Proceedings of the National Academy of Sciences. Overall, the team found that the number of births is trending upward in Nigeria, but there are interesting and important wrinkles in the research. 

For example, increasing humid heat is strongly associated with an increased likelihood of early pregnancy terminations. A caveat, though, is that the surveys don't ask whether terminations are induced or spontaneous.

"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."

Another important finding is that, during a month of exposure to high wet-bulb globe temperatures, conceptions fell by about 1.4% compared to the average of just above 15 conceptions per 1,000 woman-months. But those conception rates grew relative to that average by 2.3% the following month and by 5% after two months. The net result is that the overall conception probability increased, but the timing of pregnancies shifted.

"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."

These shifts were particularly common for women with less education and women in rural areas. There were also marked differences geographically, which correspond to religious and economic differences in Nigeria.

In northern Nigeria, the majority of communities are Muslim, and Christianity is the major religion of the south. The south is also the wealthier part of the county. Shifts in pregnancy timing were more pronounced in the north, while timing was less impacted, or trended in the other direction, in the south. 

"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. Andam was the country program leader for the International Food Policy Research Institute, or IFPRI, in Nigeria at the time of the study.

The team said understanding the impacts of humid heat and how those vary between communities will be helpful to public health workers around Nigeria as they advise women throughout their pregnancies. Brooks added that a goal moving forward is linking high-level studies like these to lived experiences in the communities to ensure women are getting the best 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. "You can then use that to design improved public health guidance."