In this questionnaire study with a relatively large sample size, we found that BMI is not correlated with the length of menstrual cycles and menses, but BMI is positively associated with menstrual blood loss in adult women.
To date, most studies investigating the association between BMI and abnormalities of the menstrual cycle focused on a younger women population, aged between 18 to 25 years and there was no information on history of childbirth [2,3,4,5,6,7,8]. Age is associated with the length of menstrual cycle and women over 35 years had shorter menstrual cycles . In addition, the association between BMI and menstrual blood loss has not been fully investigated, although a high BMI increases the risk of ovulatory dysfunction and subsequently may cause heavy menstrual blood loss . In our current study, we investigated the association between BMI and menstrual cycle characteristics in women with a wide range of ages (aged 17 to 53 years), using ethnicity-specific WHO classification of BMI. This could represent such associations in women of a reproductive age.
Although a variety of sex hormones play an important role in affecting menstrual cycles, studies have suggested that having a high BMI may cause an absence of menstruation, irregular menstruation, heavy or long menstruation and painful menstruation [2,3,4,5,6, 8]. In our current study, we found that 25% of Chinese women had heavy menstrual blood loss which is consistent with a previous study that reported 28% of adolescents had heavy menstrual blood loss. While another study with self-reported data only showed 3% of women with heavy menstrual blood loss , the difference in age between our study and that study could be one of the contributing reasons. In our current study, we investigated women aged 17 to 53 years, while in that study, they only investigated women aged 30 to 49 years. A previous study has reported that women over 35 years had shorter menstrual cycle .
To date, studies about the association between BMI and menstrual blood loss are very limited since most studies investigated the association between BMI and irregular menstrual cycle lengths [2,3,4,5,6,7,8]. A previous study reported a higher incidence of heavy menstrual blood loss and longer menstrual cycle and irregular menstrual cycle in overweight women . However, in that study they did not individually investigate such associations. In our current study, we found that the odds ratio of having heavy menstrual blood loss in obese women was 2.28 (95% CL: 1.244, 4.193), compared to women with normal weight. The odds ratio of having heavy menstrual blood loss in overweight women was 1.26 (95% CL: 0.903, 1.749), compared to women with normal weight, but this odds ratio did not reach to statistical significance (p = 0.08). In addition, we also found that the number of pads used during menses was significantly more in obese women, compared to women with normal weight. Because there was no difference in the length of menstrual cycle and menses in subgroups according to BMI categories in our current study, taken together our data may suggest that obese women are more likely to experience heavy menstrual blood loss, which supports a previous hypothesis .
The underlying mechanism of our findings is unclear, but it may be explained by obesity being associated with excessive levels of estrogen [20, 21], which is the main sex hormone affecting menstrual cycles including menstrual blood loss. Women with very low body fat had significantly lower levels of estrogen . It has been suggested that weight loss can restores regular menstrual function by decreasing the aromatisation of androgens to estrogen in adipose tissue. As obese women commonly have a degree of insulin resistance, a study has found that metformin, a medicine to treat insulin resistance, can restore normal menses in women with heavy menstrual blood loss . In our current study, we also found that the number of women with heavy menstrual blood loss was significantly lower in underweight women than in women with normal weight (The odds ratio of having heavy menstrual blood loss in underweight women was 0.4034 (95% CL: 0.224, 0.725), compared to women with normal weight). Taken together, our data further suggests that adipose tissue may be associated the menstrual blood loss through estrogen production. Our current finding also could support our recent published study which showed a negative association between BMI and the stage of endometrial cancer . Abnormal vaginal bleeding is one of the main clinical symptoms of endometrial cancer in both premenopausal and postmenopausal women, and increased unopposed estrogen (which can be associated with obesity) is associated with abnormal vaginal bleeding.
In our present study we did not find any difference in both the length of menstrual cycles and the length of menses among groups according to BMI categories, although previous studies found such association [2,3,4,5,6,7]. Again, the difference could be explained by the age of women who participated in these studies. In our present study we included a wider range of ages in women who participated.
We acknowledge that there are some limitations in this study. Firstly, all data including weight and height were self-reported which may induce bias, in particular in recalling the number of pads used during menses. There is also a potential bias on the different type of pad with different absorbent capacity used. Second, we do not have data on smoking, physical activity and alcohol consumption, as smoking and alcohol consumption can cause hypoestrogenism  and shorten the menstrual cycle . But the prevalence of women who are smoking or who drink is relatively low in China based on the Chinese traditional culture. In addition, data were obtained from two hospitals from different regions, and BMI and economic levels may vary in China. Therefore, our data may not be representative of China as a whole.