History of breast feeding and risk of incident end

History of breast feeding and risk of incident endometriosis: prospective cohort study

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History of breast feeding and risk of incident endometriosis: prospective cohort study

Farland et al. 2017. BMJ; 358:378



A/ Are the results of the study valid?

Screening Questions


1. Did the study address a clearly focused issue?


HINT: A question can be focused in terms of:

- the population studied

- the risk factors studied

- the outcomes considered

- is it clear whether the study tried to detect a beneficial or harmful effect?


The study aimed to determine the association between exclusive breast feeding, duration of total breast feeding and amenorrhea and incidence of endometriosis among parous women.


Looking for a protective effect of exposures.


2. Did the authors use an appropriate method to answer their question?

HINT: Consider

- Is a cohort study a good way of answering the question under the circumstances?

- Did it address the study question


1. This cohort study is a prospective longitudinal study which involves repeated observations of various exposure variables over time to establish a temporal sequence to events (allowing risk factors to precede outcome).  


Recall Bradford Hill Criteria which states the minimal conditions necessary to provide adequate evidence of a causal relationship between an incidence and a consequence. One criterion is ‘temporality’ ie the effect has to occur after the cause (and if there is an expected delay between the cause and expected effect, then the effect must occur after that delay)


2. The cohort study was an appropriate method for answering the questions. The cohort population of nurses aged 25-42 years were likely to be exposed to the risk factors. The large number of endometriosis cases in this prospective study allowed the researchers to more adequately estimate the association between breast feeding, amenorrhea and incidence of endometriosis in these women.  Access to follow-up data was possible. Duration of follow-up unclear?


3. Was the cohort recruited in an acceptable way?


HINT: We are looking for selection bias which might compromise the generalisability of

the findings:

- Was the cohort representative of a defined population?

- Was there something special about the cohort?

- Was everybody included who should have been included?


The cohort comprised registered nurses in the USA aged 25-42 years.


Healthy volunteer bias? Probably lacks generalisability to women who are not registered nurses and women outside USA. Because education and income will be more similar in this cohort than in women of the same age from the general population it limits the ability to look at social determinants of health.


4. Was the exposure accurately measured to minimize bias?


HINT: We are looking for measurement or classification bias:

- Did they use subjective or objective measurements?

- Do the measures truly reflect what you want them to (have they been validated)?

- Were all the subjects classified into exposure groups using the same procedure?


How was breast feeding and amenorrhea measured?

Breast feeding was self-reported and exposure categories were converted to continuous outcomes therefore misclassification of exposure was possible. Social desirability bias and recall bias would be a problem as not easy to remember the duration (weeks) of breast feeding.


5. Was the outcome accurately measured to minimize bias?


HINT: We are looking for measurement or classification bias:

- Did they use subjective or objective measurements?

- Do the measures truly reflect what you want them to (have they been validated)?

- Has a reliable system been established for detecting all the cases (for measuring disease occurrence)?

- Were the measurement methods similar in the different groups?

- Were the subjects and/or the outcome assessor blinded to exposure (does this matter)?


Outcome was a laparoscopically confirmed endometriosis reported by the woman and confirmed by medical record review. The validity of this approach was verified by a previous study.


Unconfirmed cases were censored which means that they were taken out of the analysis at that point as it is unclear whether or not to assign them to becoming a case or non-case.


All participants had the same procedures for measurement of exposures and outcomes.


Blinding was unclear, but it is unlikely that endometriosis results were influenced by knowledge of exposure status.


6. A. Have the authors identified all important confounding factors?

List the ones you think might be important, that the authors missed.


B. Have they taken account of the confounding factors in the design and/or analysis?



- Look for restriction in design, and techniques eg modelling, stratified, regression, or sensitivity analysis to correct, control or adjust for confounding factors


Understanding confounding is critical in determining what inferencescan be drawn from study findings.


When establishing relationships of cause and effect, internalvalidity represents the truthfulness of conclusions about causalrelationships.  Internal validity means that a true cause-and-effectrelationship exists between an exposure (the cause) and outcome(the effect) variable.

Confounding is one of several threatsto the internal validity of a research study. Confounding can be defined as a possible source of bias in studies in whichan unmeasured third variable (the confounder) is related tothe exposure of interest (although not causally) and the outcome of interest.


Researchers in this study adjusted hazard ratios for the effect of total breastfeeding and exclusive breastfeeding on the incidence of endometriosis adjusted for age, current BMI, BMI at age 18, smoking, history of oral contraceptive use, pregnancies lasting at least 6 months, age at menarche, infertility history, and time since last birth (Table 2).


7. A. Was the follow up of subjects complete enough?

B. Was the follow up of subjects long enough?



- The good or bad effects should have had long enough to reveal themselves

-The persons that are lost to follow-up may have different outcomes than those available for assessment

- In an open or dynamic cohort, was there anything special about the outcome of the people entering the cohort?


Duration of the study was 1989-2011. Unclear what the total person-years of observation was and how long each participant was followed up for.


B/ What are the results?

8. What are the results of this study?


- What are the bottom line results?

- Have they reported the rate or the proportion between the exposed/unexposed, the ratio/the rate difference?

- How strong is the association between exposure and outcome (RR,)?

- What is the absolute risk reduction (ARR)?

Table 3: The results indicate that for each additional 3 month increase in duration of total breast feeding per pregnancy women experienced an 8% lower risk of endometriosis (HR 0.93 95% CI 0.90 to 0.94), and 14% lower risk with every additional 3 months of exclusive breast feeding per pregnancy (HR 0.86, 0.81 to 0.90).


9. How precise are the results?

How precise is the estimate of the risk?


- Size of the confidence intervals

(95% CI: 0.90 to 0.94) fairly narrow CI 95% certain true population effect between 10% and 6% reduction in risk.


10. Do you believe the results?



- Big effect is hard to ignore!

- Can it be due to bias, chance or confounding?

- Are the design and methods of this study sufficiently flawed to make the results unreliable?

- Consider Bradford Hills criteria (eg time sequence, dose-response gradient, biological plausibility, consistency).


Results in Table 3 statistically significant at 1% level so minimised chance effect.

Bias and confounding were minimised but still possible.

Temporality: Cause preceded effect as all women were negative for endometriosis at study inception and incident cases measured.

Strength: Not a big effect

Consistency:  In agreement with some other studies – but overall the evidence inconclusive.


Biological plausibility:   See first bottom column 2 page 1 for plausibility.


Dose-response relationship was demonstrated (Table 2 and 3)


11. Can the results be applied to the local population?


HINT: Consider whether

- The subjects covered in the study could be sufficiently different from your population to cause concern.

- Your local setting is likely to differ much from that of the study

- Can you quantify the local benefits and harms?


Results may not be applicable to other general population samples.


12. Do the results of this study fit with other available evidence?


Yes, other recent studies include the following


References provided in paper of additional studies.

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