🎗️Childhood Exposure to Pesticides Linked to Changes in Age at First Menstruation

Can pesticides affect the timing of puberty? Researchers followed girls from childhood and found associations between substances present in the body following exposure to certain pesticides and the timing of the first period. The results are intriguing but do not yet prove a cause-and-effect relationship.

The first period is one of the most important signs that a girl's body is entering puberty. The age at which a girl has her first period indicates more than just when menstruation begins; it can also signal when the body started undergoing the major hormonal changes associated with puberty. Throughout the 19th and 20th centuries, the average age of the first period dropped from approximately 16 or 17 years to about 13 years.

Nowadays, it typically occurs between the ages of 12 and 13. While improved nutrition and health conditions are likely reasons for this shift, researchers believe other factors may also be involved. Excess weight during childhood, for instance, is linked to earlier puberty.

Furthermore, certain chemicals in the environment can interfere with the hormones responsible for growth and reproduction. These substances are known as endocrine disruptors because they can mimic, block, or alter the action of the body's natural hormones.

Pesticides are used to protect crops against insects, fungi, and other threats. Although they are important for food production, small amounts of these substances, or their breakdown products, can enter the human body.

For the general population, a primary source of exposure is diet, particularly conventionally grown fruits and vegetables. Some pesticides can interfere with the hormonal system; consequently, researchers sought to determine whether exposure during childhood might influence the timing of puberty.

To investigate this possibility, they specifically analyzed five substances that appear in the body after the metabolism of various pesticides. These included a substance related to the insecticide chlorpyrifos and another known as ethylenethiourea, which is produced through the transformation of certain fungicides.

Researchers followed 506 Spanish girls who were part of a large research project on child health. Urine samples were collected when the girls were between 7 and 10 years old. This step was important because scientists did not have to simply ask families whether the girls had come into contact with pesticides; instead, they looked in the urine for small molecules produced when the body breaks down certain pesticides.

It is like looking for a "fingerprint" of exposure. Five indicators were analyzed: one related to chlorpyrifos, one to diazinon, a group of products resulting from the breakdown of organophosphate insecticides, an indicator of exposure to pyrethroid insecticides, and ethylenethiourea, which is related to certain fungicides.

After the initial collection, the girls continued to be monitored for several years. Between the ages of 7 and 16, the girls, and in some cases, their parents, answered questionnaires indicating whether menstruation had begun and at what age.

The researchers did not look only at pesticides. They also recorded the girls' height and weight at the time the urine samples were collected. This allowed them to calculate body mass index (BMI) and determine whether weight might modify the relationship between pesticide exposure and the age of first menstruation.

This is an important issue because adipose tissue plays a role in hormonal function, and girls who are overweight or obese may experience changes in the timing of puberty onset. The researchers then used statistical models to compare the age at first menstruation among girls with varying levels of exposure.

They also took into account other factors that could influence the results, such as age, growth, maternal education, urine concentration, and simultaneous exposure to other pesticides. In doing so, they sought to isolate the potential effect of the pesticides from other characteristics that might also be linked to puberty.

The results revealed two noteworthy associations. Girls showing signs of chlorpyrifos exposure in their urine experienced their first period later than those in whom this indicator was not detected. This difference was more pronounced among girls who were overweight or obese, although the researchers did not find a statistically significant interaction between body weight and exposure.

The most intriguing result involved ethylene thiourea. The higher the concentration of this substance in the urine, the greater the tendency for the first period to occur earlier. Girls with higher levels were about 39% more likely to have reached menarche at some point during the follow-up period compared to those in whom the substance was not detected.

The association appeared even stronger among girls who were underweight or of normal weight, though this difference was not statistically significant.

Does the study prove that pesticides cause changes in the timing of puberty?

No. This is a crucial point. The study found associations but cannot prove that pesticides were responsible for causing the first period to occur earlier or later. Exposure was measured at a specific stage of childhood, and there are many other factors that can influence pubertal development.

Furthermore, the interaction between body weight and pesticides was not statistically significant, so researchers consider this part of the findings inconclusive. Even so, the discovery is important because it adds to the evidence that certain environmental exposures during a particularly sensitive developmental stage may be linked to the timing of puberty.

The researchers themselves emphasize that further studies, involving different populations and more detailed monitoring of exposure throughout childhood, will be needed to confirm the results and understand exactly how these substances might interfere with the hormones that control puberty.

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Age at menarche and exposure to non-persistent pesticides in Spanish girls from the INMA (environment and childhood) project

Carmen Freire, Alicia Olivas-Martinez, Francesca Castiello, Alba Jimeno-Romero, Sara Lopez de Calle, Beatriz Suárez, Adonina Tardón, Marta M. Rodriguez-Suarez, Mònica Guxens, Martine Vrijheid, Andrea Beneito, and Maria-Jose Lopez-Espinosa

Environmental Research. Volume 300, 15 June 2026, 124450

DOI: 10.1016/j.envres.2026.124450

Abstract: 

Our objective was to assess the longitudinal association between exposure to non-persistent pesticides and the age of menarche in Spanish girls. The study was conducted in a sample of 506 girls from the INMA Project. Pesticide exposure was assessed by quantifying four insecticide metabolites (TCPy, chlorpyrifos metabolite; IMPy, diazinon metabolite; DETP, non-specific organophosphate metabolite; 3-PBA, pyrethroid metabolite) and the dithiocarbamate fungicides metabolite ethylene thiourea (ETU) in spot urine samples collected at the age of 7-10 years (2013-2016). Information on the age at menarche was collected from questionnaires completed by the girls and/or parents at successive visits from the age of 7 to 16 years. Associations between categorical exposure biomarkers and age at menarche were examined using Cox regression models adjusted for sub-cohort; maternal schooling; age, urinary creatinine, body mass index (BMI) z-score, and height of girls at the age of 7-10 years; and other pesticide biomarkers. Effect modification by BMI was also examined. Menarche was later in girls with detected versus undetected concentrations of urinary TCPy (Hazard ratio [HR] = 0.71; 95%CI = 0.54–0.92); this association was stronger among those with overweight or obesity (HR = 0.69; 95%CI = 0.47–0.99), although the interaction was not statistically significant. By contrast, menarche was earlier in girls with increasing ETU (HR = 1.32; 95%CI = 1.00–1.73 for concentrations between limit of detection and 75th percentile [P75]; HR = 1.39, 95%CI = 1.03–1.89 for concentrations > P75 versus undetected ETU); this association was stronger but not significant in girls with underweight/normal weight. These results are in line with previous findings from the INMA Project on the association of ETU exposure with earlier puberty onset in girls and provide previously unreported evidence of its association with TCPy. Further studies are needed to verify these findings and to elucidate the role of BMI in these associations.