Reviewer’s report
Title: 
Impact of childbirth history on dense breast in mammographic screening:
a cross-sectional study
Version: 
0 
Date: 
27 Mar 2022
Reviewer's report:
This is a cross-sectional study to investigate the relationship between parity and
dense breast among 16,864 healthy Japanese women. The findings suggest the
same inverse trend of more parous associated with less dense breast among both
premenopausal and postmenopausal women, which is important to understand
the etiology of dense breast as well as the design of screening program. This is a
straightforward study in which the research question was answered by the study
design. The strength of this study is large sample size in Asian women with less
evidence, while the limitations are, as authors mentioned, temporal issue and
unknown confounders. Here are some comments:
1) The authors showed the same trend among both premenopausal and
postmenopausal women. However, these two groups seemed to have very
different distribution in terms of breast density (Table 1). Can authors provide
some explanation?
2) It appears nulliparous women had worse lifestyle, such as more drinking and
smoking, which are also the risk factors associated with dense breast. Authors
are suggested to consider mediation effects rather than confounders.
Alternatively, authors should at least address this relationship in Discussion,
which might lead to different findings from the other studies.
3) Do authors have any plan to follow up these women for breast cancer? 
4) The abstract in the first page is different from the abstract in the main body, in
which single parity and nulliparity were compared to more than one parity while
the main results were null, single and two vs. more than two.
5) Authors are suggested to remove the repeated results from the first paragraph
in Discussion.
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Is the study design appropriate to answer the research question (including the use
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