To the best of our knowledge, few studies have evaluated the association between parity and MUHNW. The present study’s findings demonstrated that multi-parity was associated with an increased prevalence of MetS in postmenopausal women with a BMI lower than 24 kg/m2, which was found to be significant. However, this trend was attenuated and became non-dose-dependent after adjusting for confounding factors. In regard to the components of MUHNW, only WC was significantly associated with parity after adjusting for confounding factors.
Several cross-sectional studies have concluded that parity was independently associated with an increased prevalence of MetS in different races and ethnicities[18-22]. Young Lee et al. conducted a cross-sectional study on 4098 Korean postmenopausal women and reported according to 5 groups of parity (0, 1, 2, 3 and ≥4). Accordingly, only higher parity (≥ 3 live births) was significantly associated with MetS when the parity 2 group was taken as reference (parity 3: OR 1.40 and ≥4: OR 1.38])[18]. Ortiz et al. found that women with MetS were also more likely to have had at least three children (P = 0.05) [23], though the associations were marginally significant. In addition, findings from a U.S. Hispanic/Latina study have shown that compared to one birth, those with four births had the highest odds of overall MetS (OR=1.4, 95%CI 1.0, 2.0) after adjusting for confounding factors including education, marital status, income, nativity, smoking, physical activity, menopausal status, oral contraceptive use, and hormone therapy[21]. However, Shamima Akter concluded that only pre-menopausal women with the highest parity (≥ 4) had 1.65 times higher odds of having MetS compared to those in the lowest parity (0-1), but not among postmenopausal women[20]. All of the aforementioned studies were conducted using general MetS, and none of these studies have focused on the relationship between parity and MUHNW. Specifically, only YAO et al. has compared the associations between parity and MetS as well as its components in two groups according to BMI (normal weight vs overweight)[24]. Here, they concluded that there was a significant statistical difference between normal weight BMI and higher weight BMI in terms of associations between parity and MetS (P-interaction<0.001)[24]. However, they did not further evaluate the associations between parity and MetS as well as its components in the normal weight group. In this study, normal-weight women with multiple parturition (more than 3 times) were found to be at high risk of MUHNW. Although the ORs for the parity 4 group was lower than that for the parity 3 group, it was thought that the fewer individuals present in group ≥4 attributed to the results.
By further analyzing the components of MetS, no consensus was reached regarding the association between parity and WC as well as other metabolic disorders. Blaudeau TE conducted a cross-sectional study in order to assess total body fat and intraabdominal adiposity in 170 nonsmoking Caucasian women and found no relationship between parity and waist circumference (P= 0.16) [25]. Moreover, parity was also found to not be associated with central obesity (wc=88cm) both in an unadjusted and multivariable adjusted model (p= 0.66)[20]. Koch et al found that parity was associated with BMI, but was not related to WC[26]. However, A.A. Mansour et al. concluded that the number of births remained significantly and independently associated with increased WC after adjusting for age, BMI, employment, education, and marital status[OR=1.10, 95%CI(1.06,1.12)][27]. In addition, researchers also found that parity was significantly associated with risk of abdominal obesity measurements (WC), exhibiting a greater OR than general obesity measurements (BMI)[28]. The present results demonstrated that high parity was found to be associated with increased WC. Potential reasons pertaining to the disparate results from this study could be due to the heterogeneity in the background of the study populations. Moreover, our study was conducted in women of normal weight, thus, it may be plausible that parity serves a risk factor of MUHNW irrespective of BMI, and WC may play an important role in metabolic disorders.
Potential biological mechanisms may explain the association between parity and WC. Excessive fat accumulation and postpartum weight retention induced by excess calories taken during pregnancy play a role in the pathophysiological mechanism of abdominal obesity. Studies have suggested that intra-abdominal adipose tissue increases with increasing parity[25], which has larger influences on central obesity than on overall obesity[22]. Furthermore, insulin resistance triggered by hormonal changes during pregnancy and relative increments in insulin may promote lipid synthesis[29] and triacylglycerol surplus to deposit as visceral adipose tissue[30], whereas repeated pregnancies may amplify such effect. In addition, the release of placental corticotropin-releasing hormone during pregnancy may result in excess cortisol exposure, causing intra-abdominal adipose tissue accumulation[31].
In the present study, the association between parity and other metabolic components (high fasting blood glucose, high blood pressure, elevated triglycerides and low HDL cholesterol) were not detected. The aforementioned studies have previously reported inconsistent results regarding the relationship between parity and other metabolic components. In this regard, it is believed that the socio-economic status of the study populations as well as different correction factors may partially account for the discrepancy.
Various limitations exist in the present study. First, similar to previous studies, this is a cross-sectional study, hence, a causal link between parity and MUHNW cannot be established. Second, recall bias was inevitable as information about reproductive factors was collected based on electronic medical records. Third, variables of pregnancy related complications, such as gestational diabetes and pregnancy induced hypertension, were not taken into account in the data analysis as most postmenopausal women were not even aware of the related history of pregnancy complications. Furthermore, a history of lactation and oral contraceptives was not adjusted, although previous studies have reported that breast feeding was associated with a decreased risk of obesity[32] while a longer duration of breast feeding could help reduce postpartum weight retention[33]. In terms of the strengths of this study, this is first study that validates the association between parity and MUHNW in Chinese postmenopausal women. In addition, researchers have previously reported that pregnancy loss was associated with MetS[34] and other metabolic disorders such as diabetes or nonalcoholic fatty liver disease (NALFD)[35]. In this study, reproductive variables including duration of reproductive years and pregnancy loss were adjusted as confounding factors when exploring this association, whereas pregnancy loss was ignored in most previous studies.