Strength and limitation
To our knowledge, this is the first large-scale study conducted in Japan with meticulous attention to data collection. Therefore, the results of this study are considered representative of the general pregnant population in Japan.22,23 Randomised controlled intervention trials are more likely the best study design; however, it is impossible to conduct long-term controlled trial examining intake of overall diet as exposure.
Although the present results were not obtained from a randomised controlled study, a prospective birth cohort study can prove the correlation between outcome and exposure assessed before the onset of outcome. Nevertheless, this study also has potential limitations. First, we do not have information on the diagnostic methods of gynaecological complication such as endometriosis, uterine myoma, and adenomyosis. Moreover, of the 3,249 women, the number of women who had active endometriosis during their pregnancies and whether these complications were treated before pregnancy were unclear, because information was based on self-reported questionnaire. Information on these complications in patient’s medical records was insufficient, and we did not utilise past medical records of the participating women. In Japan, gynaecologists usually use transvaginal ultrasonography to examine the conditions of the uterus and ovaries at their first visit. A previous study reported that the diagnostic accuracy of endometriosis by vaginal ultrasonography is almost 90% regardless of its phenotype.24 With regard to obstetric outcomes, data were collected prospectively from the medical records by trained research coordinators and medical doctor at the subject institution for all puerperal patients. Therefore, we expect that the maternal gynaecologic condition before pregnancy and outcome of delivery to be accurate. Second, although we accounted for some confounding factors in large portions of the questionnaire, unknown factors that might have affected the occurrence of PTB, LBW, or HDP might have existed.
Third, the DII score of each study participant was calculated using only the FFQ as the basis of the Japanese lifestyle, and JECS participants were Japanese women. Therefore, the DII of this study is not validated yet, and our results may not be applicable to other ethnicities. Fourth, because the FFQ used for calculating the DII score in this study was based on the self-reported information of women during their first trimester, recall bias may be possible, as participants might have had morning sickness and were asked to recall their diet content before pregnancy.