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«Case study epidemiology Maternal self-reported exposure to pesticides in a Polish rural population and its effect on birthweight – example of ...»

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Case study epidemiology

Maternal self-reported exposure to pesticides

in a Polish rural population and its effect on birthweight – example of

cross-sectional study

Case study – epidemiology

Children's Health and the Environment

CHEST Training Package for the Health Sector

Case study epidemiology

Aim of the exercise:

• To use an epidemiological approach to examine

the effect of exposure of pregnant women to

pesticides on the birthweight of the infants.

• To explain step by step, the elements of crosssectional study (i.e. the selection of study population, methods preparation, results recording, analysis of results and their interpretation).

Case study epidemiology Introduction (1) In which situations pregnant women living and working in rural areas might be exposed to pesticides?

• The women living on farms are rarely involved, especially during pregnancy, in the mixing of chemicals, loading and cleanup of equipment and disposal of empty containers.

• However, they may be present on the field during the application of pesticides, take care of the clothing worn by the applicators and be subject to re-entry exposure while entering the field after spraying.

Case study epidemiology Introduction(2) Have there been reports that farming activities might affect the birthweight? Do we really need an epidemiological study to address this problem?

• Farmers in general, compared to non-farmers, have more favourable birth characteristics including lower rates of smallfor-gestational age infants (SGA), which may obscure the potential risk factors inherent in farmers’ jobs.

• There were few epidemiological studies indicating that maternal exposure to pesticides may contribute to SGA births.

• However different scenarios of exposure in different countries or even country region did not allow for definite statements.

Case study epidemiology Introduction (3) Which other factors affecting birthweight should be taken into account?

• It is well known that in the absence of chronic maternal and foetal disease, birth weight is affected by the duration of pregnancy, sex of the infant, socio-economic variables (education, marital status) and intrauterine exposure to tobacco smoke.

Case study epidemiology Study goal How would you describe “study goal”?

• The goal of the present study was to evaluate the impact of maternal exposure to pesticides on birth weight in a population of farmers in rural Poland.

Case study epidemiology

Methods (1) – definition of study population:

To proceed further you should decide about the study population and study period.

• The study population included women living in Zadzim, Poddębice County, Central Poland, who had delivered at least one child between January 1, 1994 and December 31, 2000.

• Identification of all women (N=123) who met this criteria was provided by the County Maternity Unit.

Case study epidemiology Methods (2)- recruitment of subjects

• Ten women identified were excluded due to chronic health problems during pregnancy (diabetes, hypertension, heart or kidney problems).

• Of the remaining 113 women, 104 (92%) participated, six women refused and three were permanently away from home or unable to be reached/or contacted.

Case study epidemiology Methods (3) - questionnaires Questionnaire was administered by a midwife to all recruited subjects.

A questionnaire included:

• mother’s demographic

• anthropometric characteristics (weight and height),

• job history,

• smoking

• reproductive history (spontaneous and elective terminations, stillbirths and live births, including number, duration, birth weight and sex of each infant)

• type of farming (crops, orchards, around home activities)

• mother’s and other family members’ involvement in field labor,

• other non-farming jobs

• trade names of pesticides used within the last six years (openended questions).

Case study epidemiology

Methods (4) –exposure assessment

Information on the trade names of pesticide (s) used

Each mother was asked to:

• verify the trade name and the timing of the pesticide(s) used, with the person directly involved in the application (most often her husband).

•describe her personal involvement (applicator, observer, not involved) in each pesticide applications session.

•provide history of pesticide use on the farm during the three months’ immediately preceding conception period and in three trimesters of pregnancy.

Based on the trade names of pesticides reported, the names of active ingredients were identified using a database of registered pesticides in Poland and classified into chemical groups.

Case study epidemiology

Methods (5) - statistical analysis

• To test for differences between subgroups the T test was used.

• In order to determine whether the birthweight differed between infants of mothers exposed and unexposed to pesticides multiple linear regression model was built by SPSS (Statistical Package for Social Science) software.

• The model included pregnancy duration, exposure to pesticides as well as other variables known to affect birthweight (infant sex, maternal prepregnancy weight, height, smoking during pregnancy, calendar year of birth, involvement in field work).

Case study epidemiology

–  –  –

• Most frequent among the examined population were mothers engaged in crop farming (50%), followed by those involved mostly in non-farming jobs (32.7%) and lastly orchards owners (17.3%).

• Farms reported were usually family run and were approximately 50 000 –100 000 m2 in size.

Case study epidemiology

–  –  –

• Among mothers working in orchards, 44.4% reported involvement in field work compared to 26.9% engaged in crop farming.

Case study epidemiology

–  –  –

• Phenoxyacetic acid derivatives were the most frequently reported type of pesticide used (58.8%), followed by synthetic pyrethroids (51%), benzensulfonothioate derivatives (33.3%) and organophosphorous compounds (27.5%)

• Except for benzenothiosulfonate derivatives, all of the above mentioned pesticides were used more often in orchards than in crop farming.

• Exposure to other types of compounds was much less prevalent in each group.

Case study epidemiology

–  –  –

Results (7) – socio-economic characteristics of women involved in different types of farming Have the women involved in different types of farming in such characteristics as weight, height and smoking?

–  –  –

Results (8) - birth weight and type of farming

• The mean birth weight of infants delivered by mothers working in crop farming was slightly lower compared to those with non-farming jobs.

• In contrast, mothers working in orchards delivered infants with a slightly higher birth weight than non-farming women did.

• The average pregnancy duration was similar in all the examined groups.

• Mothers in each group were also similar with respect to other factors affecting birth weight, such as maternal weight and height.

• The observed differences in birthweight could not be related to cigarette smoking, as mothers who reported either crop farming or work in orchards were characterized by a very low prevalence of smoking.

Case study epidemiology Results (9) - pregnant women involvement in field work and pregnancy outcome Have there been differences in pregnancy outcome by type of involvement in field work?

If so how would you evaluate the statistical significance of them?

–  –  –

Results (10) - birth weight and field work during pregnancy

• Mothers who reported involvement in fieldwork had a similar pregnancy duration but delivered infants with a significantly higher birth weight than mothers not reporting such activities.

Case study epidemiology Results (11) socio-economic characteristics by women involvement in the work in the field

–  –  –

Results (12) socio-economic characteristics by women involvement in the work in the field

• Both groups had a rather low proportion of smokers and similar height

• Women reporting field work had a slightly higher body weight.

Case study epidemiology Results (13) – use of pesticides and pregnancy outcome

–  –  –

Results (14) - birth weight and pesticide exposure

• Children of women living on farms where no pesticides were used during the first or second trimesters of pregnancy, despite the higher proportion of smokers, had slightly higher birth weight.

• Pregnancy duration and anthropometric characteristics of mothers were similar in both groups.

Case study epidemiology Results (15) – how to control confounders?

• The groups of women compared with regard to pesticide use in the first and second trimesters of pregnancy differed in respect to potential confounding variables.

• What type of statistical analysis you would propose to overcome this problem?

Case study epidemiology Results (16) - Linear regression model

–  –  –

Results (17) How would you describe the results received?

• The exposure to synthetic pyrethroids pesticides was found to be an independent factor negatively associated with birth weight.

• Other factors identified as significant included:

pregnancy duration, maternal prepregnancy weight, year of birth and involvement in field work.

Case study epidemiology Discussion (1)- What is the main limitation of the presented study The major limitation was assessment of pesticide exposure based on questionnaires.


• several steps to obtain valid assessments were undertaken.The source of information about pesticide exposure was the person directly involved in farming and he was encouraged to recall the details of exposure using leaflets from the pesticide manufacturers and labels from used containers.

• The pattern of pesticide use was very much the same every year, so even the information referring to relatively remote events could be treated as highly reliable.

Case study epidemiology

Discussion (2) - how valid is the use of questionnaire forexposure assessment?

• The results of the National Cancer Institute studies indicate that the data obtained from standardized questionnaires may be reasonable indicators of occupational exposure when biomarker data are not available.

• The farmer carefully selects the pesticide most likely to be effective, purchases the pesticide, records the purchase (costs are tax deductible), mixes and applies the pesticide and evaluates the effectiveness of treatment. All these activities tend to reinforce their memory.

• All together, the validity of retrospective exposure assessment to pesticides, in spite of some limitations, is quite reassuring.

Case study epidemiology

Discussion (3) -should the biologic monitoring methods be used?

• Although the methods of biological monitoring seem to be the optimal way of assessing individual exposure to pesticides, such an approach is possible only in the case of cross-sectional (hospital deliveries) and prospective cohort studies.

• These methods have not as yet been introduced to retrospective assessment but have been strongly recommended.

• However, it has been recently stated that ”biological monitoring may not be suitable when the nature of the work makes it difficult to pinpoint a priori which exposures among a broad range of possible contaminants are more likely than others”.

Case study epidemiology

Discussion (4)- consistency with results of other studies

The present study revealed a small but statistically significant effect of self-reported maternal pesticide exposure during the first or second trimester of pregnancy trimester on birthweight.

This was found after adjustment for pregnancy duration and the confounders including smoking and physical work during pregnancy.

Are results received consistent with other studies?

Case study epidemiology Discussion (5)- results of studies in agriculture or farming population Only a few reports addressed the risk of SGA in farmers’ infants. Two of them related the risk to the occupation labeled as “agriculture” or “farmer”.

• In Scotland, an analysis of 252,147 livebirths documented slightly decreased risk of SGA which varied little with mother’s and father’s occupation including agriculture. (San Jose 1991).

• The comparison of almost 200,000 births to farmers with those to nonfarmers (1967-1989) in Norway, revealed fewer SGA births among farmers when adjusted for the year of birth, maternal age and geographical region (Kristensen 1997).

Case study epidemiology Discussion (6) – results of studies of women exposed to pesticides

• Three other studies related the risk of SGA to pesticide exposure.

• A case-control study in 29 hospitals in Shanghai, China, covered 75.756 infants with birthweights of ≥ 1000g.

Women exposed to pesticides during pregnancy had an approximately threefold higher risk of having an SGA infant after adjusting for the sex of infant, fetal number, birth defects, maternal chronic illness and pre-eclampsia (Zhang 1992).

Case study epidemiology

• Based on the National Natality Survey and National Fetal Mortality Survey in US an elevated risk of delivering an infant with very low birth weight was found in the group of women reporting exposure to pesticides at work OR=2.4 95% CI(1.1.-5.0) (Savitz 1989).

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