Exposure to drought during pregnancy associated with higher risk of preterm birth

Climate change is making droughts more frequent, prolonged, and intense in many regions of the world, increasing the need to understand their effects on health. An international study scientifically coordinated by the Sant Pau Research Institute (IR Sant Pau), together with the Center for Data and Knowledge Integration for Health (CIDACS, Brazil), and based on nearly 30 million births recorded in Brazil over two decades, shows that exposure to drought during pregnancy is associated with a higher risk of preterm birth and, when it persists through much of the pregnancy, with a greater likelihood of the baby being small for gestational age. Concurrent episodes of extreme heat further increase the risk of preterm birth.

The research, published in The Lancet Regional Health-Americas, is part of the project "Influence of Adverse Climate Events on Birth Outcomes and Maternal and Child Nutrition Using Data From the 100 Million Brazilian Cohort," funded by the Gates Foundation through its Grand Challenges program in collaboration with the Brazilian Ministry of Health (DECIT) and the National Council for Scientific and Technological Development (CNPq). The analysis focused on a particularly socioeconomically vulnerable population and also assessed how the effects of drought vary according to the environmental and social conditions of each area.

Dr. Aline Rocha, a researcher with the SalutData Lab-Data and Health Laboratory at IR Sant Pau, is the project's principal investigator and first author of the article. Dr. Otavio Ranzani, head of the same group, is the project's scientific coordinator and a coauthor of the study. Dr. Rocha is currently completing a postdoctoral fellowship at Sant Pau under Dr. Ranzani's supervision, during which they are expanding this line of research to analyze the potential effects of drought on childhood growth through age five.

Our results show that drought is not only an environmental or agricultural phenomenon but also a potential determinant of maternal and child health. It is a clear example of planetary health: how changes in the climate and natural systems can ultimately affect human health."

Dr. Aline Rocha, researcher, SalutData Lab-Data and Health Laboratory, IR Sant Pau

Twenty years of birth and climate exposure data

The study analyzed births recorded in Brazil between 2001 and 2020 by linking data from the 100 Million Brazilian Cohort and the country's Live Birth Information System. The cohort was developed and is maintained by CIDACS. Environmental information was provided by the Wellcome Trust-funded CIDACS-Clima platform.

The cohort consists of people registered to access social programs and therefore represents the country's most socioeconomically disadvantaged half of the population. This feature made it possible to study the impact of climate events on a group particularly exposed to factors such as food insecurity, difficulties accessing safe drinking water, and barriers to receiving health care.

The researchers assessed three adverse outcomes: birth before 37 weeks of pregnancy; low birth weight among full-term babies, meaning those born after at least 37 weeks; and babies born small for gestational age, whose weight was below the expected level for their gestational week and sex.

Exposure to drought was estimated using the Standardized Precipitation-Evapotranspiration Index (SPEI), which accounts for both precipitation and temperature-related water loss. The analysis considered the cumulative water deficit during the three, six, and nine months preceding birth, making it possible to examine both relatively brief exposures and those lasting throughout almost the entire pregnancy.

Although the research focused on Brazil, its findings may be relevant to other regions increasingly exposed to drought and extreme temperatures. In the Mediterranean basin, these events are becoming increasingly frequent and may intensify as a result of climate change, making it important for countries such as Spain to understand their potential effects on pregnancy. However, the researchers caution that the estimates cannot be directly extrapolated from one area to another because the impact depends on climate conditions, infrastructure, social inequalities, and each population's capacity to adapt.

"Brazil offers an exceptional scale of analysis and allows us to study the impact of drought on a particularly vulnerable population, but the problem is not limited to this country. In the Mediterranean, we are increasingly exposed to periods of drought and extreme temperatures. We therefore need to anticipate their effects and understand how they may affect particularly sensitive groups, such as pregnant women and children," explains Dr. Ranzani.

Drought increases the risk of preterm birth and may affect fetal growth

Exposure to drought was consistently associated with an approximately 2% relative increase in the likelihood of preterm birth across the three time scales analyzed, representing between 1.4 and 1.7 additional cases per 1,000 births. The relationship was particularly clear when examining cumulative exposure over six months: the risk increased progressively with drought intensity, from approximately 1% during moderate episodes to 3% during severe episodes and 5% during extreme episodes. The findings also suggest that the middle and late stages of pregnancy may be particularly sensitive periods.

"The relative magnitude may appear modest, but it must be interpreted from a population perspective. Droughts simultaneously impact vast areas and millions of people, so even a small increase can translate into a considerable number of preterm births," notes Dr. Rocha.

Droughts lasting six or nine months were also associated with a 1% to 2% increase in the likelihood of a baby being small for gestational age, particularly when exposure occurred during the early and middle stages of pregnancy.

The study also analyzed low birth weight-less than 2,500 grams-exclusively among babies born at full term. This distinction is important because a preterm baby may have a low weight simply because they were born before completing the pregnancy, whereas being small for gestational age means that the baby weighs less than expected based on the duration of the pregnancy and the baby's sex.

This analysis found no consistent association between drought and low birth weight among full-term babies. The findings therefore suggest that the identified effects are mainly concentrated in the risk of preterm birth and, following prolonged exposure, lower-than-expected fetal growth. They do not support the conclusion that drought generally increases the likelihood of low birth weight among babies born after at least 37 weeks of pregnancy.

Although the study was observational and cannot establish a causal relationship or demonstrate the mechanisms involved, the authors propose several possible pathways, including dehydration, heat stress, inflammation, hormonal changes, and difficulties accessing food, water, or prenatal care. Prolonged exposure could also affect maternal nutrition and placental function, reducing the supply of oxygen and nutrients needed for fetal development.

The risk of preterm birth was even higher when drought coincided with episodes of extreme heat, an interaction observed across all three time windows studied. "People are not exposed to drought in isolation. These episodes may coincide with heat waves, wildfires, air pollution, crop losses, rising food prices, or difficulties traveling to health care facilities. The accumulation of risks can considerably increase vulnerability during pregnancy," explains Dr. Ranzani.

Socioeconomic conditions amplify the impact

The impact of drought was not uniform across the population. In municipalities with a very low Human Development Index, cumulative exposure over six and nine months was associated with approximate increases of 8% and 10% in the likelihood of preterm birth, respectively. These figures are considerably higher than the increase observed across the study population as a whole.

The findings indicate that socioeconomic conditions can increase vulnerability to climate events. A lack of resources limits people's ability to cope with food shortages or rising prices, maintain continuous access to safe drinking water, ensure adequate nutrition, and receive prenatal care. Drought can therefore worsen preexisting inequalities and have a greater impact on those with fewer means to protect themselves from its consequences.

"Climate events do not affect everyone equally. Social conditions largely determine people's ability to protect themselves from food and water shortages, maintain adequate nutrition, or access prenatal care. Adaptation measures must therefore pay particular attention to pregnant women in situations of greater vulnerability," emphasizes Dr. Rocha.

Continuing the research at Sant Pau

During her postdoctoral fellowship at IR Sant Pau, Dr. Rocha is continuing this line of work through new research that extends the follow-up beyond pregnancy. The aim is to study whether exposure to drought may also influence children's weight, height, and other growth indicators during their first five years of life.

The authors believe the findings support incorporating climate information into maternal and child health policies. Potential measures include strengthening prenatal care during droughts and heat waves, ensuring access to safe drinking water, providing nutritional support, developing warning systems that integrate environmental and health data, and introducing specific protections for populations with fewer resources.