Prevalence of anemia and sociodemographic characteristics among pregnant and non-pregnant women in southwest China: a longitudinal observational study

Anemia in women of reproductive age is a global public health concern [2]. In 2011, WHO reported that the global prevalence of anemia for all women of reproductive age was 29.4% (95%CI: 24.5–35.0%), with the highest prevalence in Africa (37.6%), the lowest in North America (12.4%), and about 31.9% in Asia where China was located [10]. In our study, 640,672 women of reproductive age in southwest China were involved, and the overall prevalence of anemia appeared as 18.9% (95%CI: 18.8–19.0%). Among them, the prevalence of anemia in non-pregnant women was 17.4%, in women during the first trimester was 21.6%, and in women during the third trimester was 10.5%, similar to the results of national studies (Pregnancy: 17.2%-42.1% [11, 15]), Non-pregnancy: 15.4%-35.6% [12, 13]), also similar to the results in Guangdong coastal areas (20.1% [20]), Hebei province (11%-16% [21]) and other regional studies. Our study also found that the prevalence of anemia in women of reproductive age had significant declined from 23.0% in 2014 to 16.4% in 2018. In recent years, the anemia status of women in China had been improved obviously, which might be related to the economic development, the improvement of people’s living standards and the improvement of the nutritional status of women of reproductive age in China.

Anemia is more prevalent in women of reproductive age [19]. A Mexican study showed that the national prevalence of anemia in pregnant women was 17.9% (95%CI: 13.5%-23.3%) higher than that in non-pregnant women (11.6%, 95%CI: 10.9%-12.4%) [22]. Lin et al. [16] conducted a multicenter retrospective study in developed areas of China in 2018, showing that among 43,403 pregnant women, the prevalence of anemia was higher in the 2nd trimester (14.7%) and 3rd trimester (16.6%) than in the 1st trimester (2.7%). In our study, the prevalence of anemia in first trimester was 21.6%, significantly higher than that in non-pregnancy (17.4%, P < 0.05). The results were higher than those of the Mexican study and higher than the anemia prevalence among pregnant women in developed areas of China. Our study also found that the anemia prevalence in the third trimester was lower than that in non-pregnant women. This may be related to the high coverage of prenatal care in China (97.8% [23]). In China, prenatal care has been incorporated into the national basic public health service, and pregnant women can receive at least five free prenatal examinations provided by the government during pregnancy. Once a pregnant woman is found to have anemia during prenatal examinations, she will be treated promptly, and then reduce the prevalence of anemia.

Previous studies noticed that demographic characteristics, such as age was associated with the prevalence of anemia in women [17, 24‐26]. Gupta et al. [17] found that non-pregnant women 35–49 years of age had a significantly higher prevalence of anemia than non-pregnant women 20–34 years of age. Shamah-levy et al. [18] also showed that in the two National Nutrition and Health Surveys in 1999 and 2006, the anemia in women aged 36–49 years was higher than that in women aged 35 years and below. Five National Health and Nutrition Examination Surveys circles between 2003 and 2012 in the United States showed that the anemia prevalence among women of reproductive age peaked at 40–49 age groups (6.5%, 95%CI: 5.5%-7.5%) [25]. A European study showed that, being younger than 20 increased the chances of anemia (aOR = 1.4, 95%CI: 1.1–1.9) [26]. In our study, the prevalence of anemia were significantly higher in women aged 18–20 years and over 35 years than in women aged 25–30 (under 20 years: aOR = 1.28; over 35 years: aOR = 1.07). When women of reproductive age are too young, risk factors include incomplete development of organs and tissues, unmarried status, low level of education, poor financial conditions, and low body weight, which can lead to notable increases in the incidence of anemia [27]. Furthermore, the increased risk of anemia among younger pregnant women could be due to unplanned pregnancy and suboptimal nutritional status prior to conception [28], leading to a higher prevalence of anemia than other age groups. For older women, the etiology of anemia is complex and ranges from bone marrow failure syndromes to chronic kidney disease, and from nutritional deficiencies to inflammatory processes including inflammaging in immunosenescence [29]. After the implementation of the Two-child policy in China, the proportion of older women has increased, which indicates that anemia among older women should be paid more attention by women themselves and relevant institutions.

Previous studies have shown that there were racial differences in hemoglobin distribution [12], and race was associated with the prevalence of anemia [17]. A study in the United States showed that significant differences in the prevalence of iron deficiency anemia existed by race/ethnicity. Non-Hispanic blacks had a higher prevalence of iron deficiency anemia than non-Hispanic whites, Mexican-Americans and other races, which was due to the differences in socio-economic status [17]. China has 56 ethnic groups, with Chinese Han the majority. There are substantial differences in genetic background, culture, socioeconomic levels, climate and geographic features of the residential area, lifestyle, and dietary pattern among certain ethnic groups [30]. Xu et al. [31] found that the anemia prevalence among minority pregnant women in southern rural area of Guizhou province was 23.96%, higher than the overall anemia prevalence of Chinese pregnant women (17.20%).Xu et al. [32] found that among women of reproductive age in six counties of Jiangsu and Zhejiang province in China, the prevalence of anemia in minority women was higher than that in Han nationality (51.3% vs. 42.8%). Our study was conducted in southwest China, which is a multi-ethnic region, and found that the anemia prevalence in minority women was higher than that in Han nationality. Most ethnic minority women live in remote areas with lower education level, unique dietary habits and genetic background [30]. In addition, some women of reproductive age lack the knowledge of prenatal health care and are not willing to improve their nutritional status and seek health care services [33]. For some ethnic minorities, where the prevalence of inherited haemoglobinopathies was high, the prevalence of anemia might be increased. Zhang et al. reported that the Dai (25.94%), Zhuang (17.23%), and Hani (5.16%) had a higher morbidity prevalence of alleles of thalassemia than the Han (2.56%) [34]. Yi people mainly eat a plant-based diet, and their high intake of phytic acid may lead to low iron absorption [35]. Most of the Miao people live close to the mountains, and the transportation is very inconvenient. The diet is mainly corn, and the intake of meat, fruits and vegetables is insufficient, which leads to the iron absorption barrier and causes the increase of the anemia prevalence [36]. In addition, the Miao population has the highest infection prevalence of soil-borne nematodes [37], which may also be the reason for the high anemia prevalence. In our study, the prevalence of anemia in farmers was 1.42 times higher than that in workers. The study of Ma et al. [13] on non–pregnant women of reproductive age in rural areas of China also showed that the prevalence of anemia in farmers was significantly higher than that in workers and other occupations (P < 0.05). This might be related to the inadequate access to health services, and farmers were exposed to higher risk of disease [12]. Meanwhile, farmers are usually of low socioeconomic status, which represented insufficient diet, limited education, and they were exposed to occupational hazards, including extreme weather conditions, dangerous chemicals, schistosomiasis, and hookworm infections [12, 38, 39], which may lead them to suffer from anemia.

The physiological haemodilution in pregnancy results in alterations of hematological parameters particularly in a reduction of hemoglobin concentration [4]. Previous studies showed that parity negatively associated with hemoglobin concentration [40, 41]. Compared to the non-pregnant state, every pregnancy carries an increased risk of hemorrhage before, during, and after delivery, especially during short pregnancy intervals, because of limited recovery time. Therefore, higher parity exposes women more frequently to periods of hemorrhage risk [40]. A study in Mexico showed that women with a history of childbirth had a higher prevalence of anemia than women without a history of childbirth (OR: 1.5–1.8) [42]. An Ethiopian study also showed that, mothers with five and above history of parity were found with increased odds of anemia compared to those who had parity of less than two (OR = 4.20, 95%CI: 1.14–15.52) [43]. Our study also found that women with a history of childbirth were more likely to suffer anemia than women without a history of childbirth, which was consistent with the above findings.

The strength of our research was that, to our knowledge, this was the largest population-based longitudinal observational study of anemia in China which covered both pregnant and non-pregnant women. Prior to our study, most of the published literatures in China focused on anemia during pregnancy, and there was no study about anemia in both pregnant and non-pregnant women of reproductive age. We used the data from NFPHEP and electronic medical records of local hospitals from 129 counties in southwest China to obtain a better representation of the anemia prevalence in women of reproductive age in southwest China. At the same time, we adjusted the thresholds for altitude recommended by WHO, enabling our results to compare with other study of the same kind.

Limitations in our study included the following: the sample size of women in the third trimester involved in this study was relatively small, which accounted for a small proportion of the study participants. Therefore, the anemia prevalence in the third trimester in our study might be underestimated. In our study, there were few variables related to reproductive status of women, and only the pregnancy history and childbirth history were included, and for possible factors that affected anemia, such as menstrual quantity, the use of intrauterine device and diet nutrition were not included in the analysis, to a certain extent, may affect the interpretation of the results.