Mid-pregnancy pollution exposure linked to postpartum depression, new study suggests

Around 10% of people in Europe suffer from depression after giving birth, experiencing symptoms that include sadness, agitation, and difficulty concentrating. For such a common condition — and one that has been linked to poorer outcomes for both the baby and the parent — postpartum depression has been relatively under-studied, write the authors of new work in Science of the Total Environment. In a bid to help address this, they ran a detailed study of one proposed risk factor — exposure to air pollution during pregnancy.

Yuhong Hu at the University of California and colleagues studied 361 women from the start of their pregnancies. These women were predominantly low-income and mostly Hispanic/Latina women living in Los Angeles. The team gathered weekly data on levels of air pollution near their homes, as well as the women's levels of depressive symptoms over time.

One year on from giving birth, overall, measures showed that 17.8% of these women were suffering from postpartum depression. After two years, the figure was 17.5%, and after three years, it was 13.4%. (These percentages are higher than recent figures from the US CDC, the team adds, suggesting that postpartum depression may be more prevalent than thought, especially among more at-risk groups.)

When comparing these women's rates of depression against the local levels of various air pollutants during their pregnancies, they found some clear associations. Women who fell into the top quarter of levels of exposure to NO2 (which mostly comes from fossil-fuel-burning vehicles) or PM10s (tiny particles that can be inhaled into the lungs, including dust as well as pollutants from factories and wildfires) during their second trimester of pregnancy were almost four times more likely than those in the bottom quarter to develop postpartum depression — and that risk remained higher for the full three years of the study.

Two other types of air pollution were also considered by the team: PM2.5s (even smaller than PM10s) and ozone. Interestingly, they did not find a link between greater exposure to these particular pollutants and postpartum depression risk.

These new findings come against a background of other findings linking air pollution and depression. Epidemiological studies have associated long-term exposure to air pollution to an increased risk of depressive and anxiety symptoms in adults, for example. Also, studies on mice have found that air pollutants can induce depressive- and anxiety-type symptoms, probably by increasing inflammation in the brain and triggering oxidative stress.

In work published earlier this year, some members of the team involved in the new research also reported a link, for a larger group of women enrolled in the same long-term study, between increased exposure to NO2 from traffic on nearby major roads and postpartum depression one year after giving birth — and this risk was especially high for women who suffered from high blood pressure during pregnancy. "Pregnancy may be a particularly vulnerable window of air pollution exposure for maternal health effects," that team wrote.

Exactly why the second trimester — specifically the period between week 13 and week 29 of pregnancy — should have emerged in this new study as being the sensitive period isn't clear. One suggestion is that the air pollutants hiked already raised levels of inflammation, from other causes, in the women's bodies at this stage of pregnancy. But further work is now needed to replicate these findings, and explore exactly how these pollutants affect pregnant women — as well as to investigate whether the results from this particular group of participants (mostly low socioeconomic status, mostly Latina/Hispanic residents of Los Angeles) will be the same in other populations.

It is worth noting that other studies have linked traffic noise to a higher risk of depression, including postpartum depression. Though the team controlled for a number of factors that might potentially have affected their results, levels of traffic noise wasn't among them. However, given that more traffic noise generally goes hand in hand with higher levels of NO2 pollution — and both have been linked to increased stress and inflammation — it may be difficult to separate their unique real-world effects (especially as electric vehicles are also quieter than their fossil-fuel-burning cousins).

When it comes to what the new results should mean for pregnant people, the team recommends limiting exposure to air pollution during the second trimester. In practice, this could mean avoiding exercising outdoors during morning and evening rush hours, for example, and staying inside, away from roads, during the hottest parts of the day. But the results do also suggest that a move to increasing the proportion of electric vehicles could boost the wellbeing of new parents, and their babies.

Read the paper in full:
Hu, Y., Niu, Z., Eckel, S. P., Toledo-Corral, C., Yang, T., Chen, X., Vigil, M., Pavlovic, N., Lurmann, F., Garcia, E., Lerner, D., Lurvey, N., Grubbs, B., Al-Marayati, L., Johnston, J., Dunton, G. F., Farzan, S. F., Habre, R., Breton, C., & Bastain, T. M. (2024). Prenatal exposure to ambient air pollution and persistent postpartum depression. The Science of the Total Environment, 953, 176089–176089. https://doi.org/10.1016/j.scitotenv.2024.176089

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