3.1. Descriptive statistics
In the present work, the age of pregnant women was in the range of 15-45 years with an average of 27.11± 6 years (Mean ± SD). Descriptive statistics and the median values (Q1-Q3) for Cd, As, Pb, and Se in the blood of placenta and umbilical cord samples are shown in Table 1.
Table 1
Descriptive statistics for values of toxic metals (µg/L) in the placental blood and umbilical cord blood samples according to related variables
Independent variables | As | Se | Cd | Pb |
Placenta blood | Umbilical cord blood | Placenta blood | Umbilical cord blood | Placenta blood | Umbilical cord blood | Placenta blood | Umbilical cord blood |
Median (Q1-Q3) | Median (Q1-Q3) | Median (Q1-Q3) | Median (Q1-Q3) | Median (Q1-Q3) | Median (Q1-Q3) | Median (Q1-Q3) | Median (Q1-Q3) |
Living area | Urban | 0.95 (0.66-1.42) | 0.80 (0.47-1.61) | 19.10 (10.64-34.41) | 14.92 (8.46-30.32) | 0.095 (0.0-0.57) | 0.095 (0.0-0.19) | 23.66 (4.23-58.79) | 23.05 (2.94-57.50) |
Rural | 5.27 (0.58-10.45) | 3.18 (0.47-5.41) | 95.53 (19.58-113.11) | 68.15 (14.92-142) | 0.19 (0.095-0.28) | 0.095 (0.0-0.19) | 27.75 (19.011-35.26) | 27.56 (12.26-33.46) |
Sex | Male | 1.37 (0.66-8.46) | 1.04 (0.28-4.18) | 26.61 (11.31-98.85) | 19.15 (9.55-121.38) | 0.19 (0.09-0.33) | 0.095 (0.0-0.19) | 28.42 (16.53-41.44) | 26.42 (13.02-43.29) |
Female | 1.04 (0.66-2.94) | 1.66 (0.66-3.61) | 29.56 (17.20-44.77) | 21.95 (11.69-46.19) | 0.095 (0.0-0.32) | 0.095 (0.0-0.19) | 19.29 (4.75-39.44) | 25 (3.56-43.29) |
Filled teeth | 0 | 1.42 (0.66-8.46) | 1.52 (0.47-4.65) | 32.41 (17.01-100.66) | 30.89 (10.64-110.55) | 0.19 (0.095-0.47) | 0.095 (0.0-0.19) | 27.94 (10.26-41.44) | 25.38 (12.26-36.69) |
1≤ | 0.95 (0.66-1.99) | 1.09 (0.28-2.89) | 19.58 (10.64-42.20) | 15.96 (10.59-3.12) | 0.095 (0.0-0.28) | 0.095 (0.0-0.19) | 19.86 (4.08-43.91) | 27.89 (2.66-53.89) |
Mother's age (Years) | <25 | 1.23 (0.66-2.94) | 1.04 (0.47-3.89) | 24.80 (17.01-43.48) | 18.67 (11.69-34.31) | 0.095 (0.0-0.28) | 0.095 (0.0-0.19) | 20.81 (7.31-41.44) | 19.58 (1.18-43.72) |
25≤ | 1.28 (0.66-8.84) | 1.71 (0.38-3.99) | 38.30 (10.97-103.56) | 30.89 (8.46-135.07) | 0.19 (0.095-0.47) | 0.095 (0.0-0.19) | 30.51 (7.17-50.33) | 27.56 (14.54-45.81) |
Abortion | 0 | 1.33 (0.76-6.17) | 1.14 (0.47-4.18) | 29.08 (11.31-98.85) | 23 (13.16-87.30) | 0.19 (0.095-0.47) | 0.095 (0.0-0.19) | 27.66 (9.6-41.44) | 27.56 (11.21-47.62) |
1≤ | 0.85 (0.47-2.37) | 1.61 (0.47-3.32) | 25.47 (17.96-44.48) | 14.16 (8.46-34.69) | 0.095 (0.0-0.28) | 0.095 (0.0-0.19) | 22.14 (4.37-42.77) | 21.57 (0.0-36.69) |
Number of pregnancy | 0 | 1.33 (0.76-6.65) | 1.52 (0.76-4.94) | 24.14 (11.31-106.46) | 22.33 (14.92-106.46) | 0.14 (0.047-0.42) | 0 (0.0-0.19) | 25.11 (7.31-41.44) | 23.28 (3.80-46.15) |
1≤ | 1.14 (0.66-2.5) | 1.14 (0.38-3.18) | 29.08 (17.01-54.27) | 16.49 (8.46-63.35) | 0.14 (0.047-0.28) | 0.095 (0.0-0.19) | 27.99 (7.50-42.77) | 27.56 (11.78-46.15) |
Median total | 1.28 (0.66-5.03) | 1.52 (0.47-3.99) | 28.42 (14.92-94.34) | 21.29 (10.64-65.49) | 0.14 (0.0-0.38) | 0.095 (0.0-0.19) | 27.56 (7.31-42.77) | 26.71 (9.88-45.81) |
The total median values in all samples, except As, were observed to be higher in the placental blood rather than the umbilical cord blood. Numerous studies have reported that the amount of Pb, Cd, and As in the umbilical cord blood is lower than the maternal blood (Korpela et al. 1986; Soong et al. 1991; Baranowska 1995; Osman et al. 2000; Sakamoto et al. 2010). Among the studied toxic metals, the ratio of the Cd present in the umbilical cord blood to the placental blood was lower, indicating that the placenta acts as a barrier against Cd transmission and limits the transfer of Cd from the mother to the fetus, which may be due to the metallothionein presents in the placenta (Goyer et al. 1992; Breen et al. 1994). In a study, Tekin et al. (2012) proposed that the difference in placental Pb levels could be influenced by polymorphisms in genes that coded for metallothionein, apparently similar to that of Cd (Tekin et al. 2012).
The US centers for disease control (CDC) have recommended a reference exposure levels of 50, 0.315, and 1 µg/L for Pb, Cd, and As, respectively (Woo et al. 2018). The results of our study showed that 78.48%, 73.81% and 39.29% of pregnant women and 83.53%, 43.53%, and 80% of the umbilical cord samples were below the recommendations of CDC levels for Pb, Cd, and As, respectively. While 21.52%, 26.19%, and 60.71% of maternal blood samples and 16.47%, 56.47%, and 20% of umbilical blood samples were higher than the CDC recommended levels for Pb, Cd and As, respectively.
The presence of toxic metals even at low concentrations in the blood of pregnant women can also damage the fetus (Ha et al. 2009; Lin et al. 2018). Job-related contacts and foods are effective factors in exposure to toxic metals (Martí-Cid et al. 2008). Studies have revealed that approximately 88% of rice consumed in Iran (including Iranian, Pakistani, and Indian brands) does not meet national standards and WHO/FAO guidelines, and the health risks are unacceptable (Sharafi et al. 2019). The risks of As and Pb in the Indian rice and the Cd in Iranian rice are significantly higher than other types. Contamination of the consumed rice can increase the concentration of toxic metals in the maternal blood (Sharafi et al. 2019).
A study in Sabzevar disclosed that the concentration of toxic metals in agricultural products irrigated by effluent wastewater was higher than the maximum permitted level (Mazhari et al. 2019). These toxic metals can enter the food chain through bio-concentration and bio-magnification phenomena (Martí-Cid et al. 2008).
Selenium (Se) is an essential element for the human body. Some authors have reported the range of 60-140 µg/L to be safe for humans (Fairweather-Tait et al. 2011; Llorente Ballesteros et al. 2020). The statistical results of this study showed that the Se concentration levels in 72.5% of the pregnant women were below 60 µg/L, while 27.5% were above this value. Besides, 6% of pregnant women had Se levels higher than 140 µg/L. Table 2 demonstrates the comparison of toxic and essential metals levels in Iran and other countries (Vigeh et al. 2006; Rudge et al. 2009; Sakamoto et al. 2010; Al-Saleh et al. 2011; Ghaemi et al. 2013; Jin et al. 2014; Ettinger et al. 2017; Polanska et al. 2017; Lin et al. 2018; Kobayashi et al. 2019; Ballesteros et al. 2020). According to this table, the level of Se in pregnant women in this study was very low compared to other countries.
Table 2
The levels of estimated toxic metals in some cities and countries
Country | Cadmium levels µg/L | Lead levels µg/ dL | Arsenic levels µg/L | Selenium levels µg/L | References |
Cord | Maternal | Cord | Maternal | Cord | Maternal | Cord | Maternal |
Iran, Sabzevar | 0.095 | 0.14 | 26.71 | 27.56 (µg/L) | 1.33 | 1.23 | 21.29 | 28.42 | This study |
Iran, Tehran* | 0.34 | 0.54 | 4.30 | 5.09 | - | - | - | - | (Vigeh et al. 2006) |
Iran, Shiraz | - | - | - | - | - | - | - | 71.22 | (Ghaemi et al. 2013) |
Poland* | - | - | - | - | - | - | - | 48.35 | (Polanska et al. 2017) |
Saudi Arabia* | 0.780 | 0.986 | 2.551 | 2.897 | - | - | - | - | (Al-Saleh et al. 2011) |
South Africa** | 0.02 | 0.15 | 1.54 | 2.3 | - | - | - | - | (Rudge et al. 2009) |
Japan** | - | - | - | - | - | - | - | 170a | (Kobayashi et al. 2019) |
Spain, Madrid* | - | - | - | - | - | - | - | 73.06 | (Ballesteros et al. 2020) |
China, Shenqiu* | 0.11 | - | 3.37 | - | 0.92 | - | - | - | (Lin et al. 2018) |
Japan, Munakata* | 0.22a | 1.97 a | 13.2 a | 26.4 a | 3.76 a | 6.16 a | 227 a | 192 a | (Sakamoto et al. 2010) |
Canada* | - | - | - | - | - | 0.6 | - | - | (Ettinger et al. 2017) |
China, Shanxi** | - | -0.47 | - | 24.48(µg/L) | - | 0.52 | - | - | (Jin et al. 2014) |
*Geometric mean **Median a ng/g |
Investigations have revealed that the amount of Se in the body varies significantly between residents of different countries. This difference is due to the amount of Se in the soil and the intake of Se from the diet (Zachara et al. 2001). Several studies have shown that the concentration of Se in the blood decreases significantly during pregnancy (Lyons et al. 2005; Pieczyńska and Grajeta 2015). The reasons for Se deficiency are included: the transfer of Se to the fetus (Imai et al. 2001), underlying active disease (Navarro-Alarcon and López-Martınez 2000), and a diet containing toxic metals (Navarro-Alarcon and Cabrera-Vique 2008). Se deficiency in pregnant women may lead to dysfunction of the developing fetal nervous system (Cengiz et al. 2004).
3.2. Correlation between the toxic metals
Based on Table 3, the results of the test showed a positive and meaningful correlation between Pb, Hg, Cd, As and Se levels in the blood samples of the placenta and umbilical cord.
Table 3
Correlations of toxic metals in placental blood and umbilical cord blood
| As P | As U | Se P | Se U | Cd P | Cd U | Hg P | Hg U | Pb P | Pb U |
As P | 1.000 | | | | | | | | | |
As U | 0.615 (0.001*) | 1.000 | | | | | | | | |
Se P | 0.816 (0.001*) | 0.561 (0.001*) | 1.000 | | | | | | | |
Se U | 0.661 (0.001*) | 0.724 (0.001*) | 0.661 (0.001*) | 1.000 | | | | | | |
Cd P | 0.176 (0.112) | 0.111 (0.319) | 0.161 (0.152) | 0.069 (0.531) | 1.000 | | | | | |
Cd U | 0.080 (0.477) | 0.269 (0.013*) | 0.104 (0.361) | 0.047 (0.665) | 0.414 (0.001*) | 1.000 | | | | |
Hg P | 0.639 (0.001*) | 0.558 (0.001*) | 0.828 (0.001*) | 0.409 (0.002*) | 0.435 (0.001*) | 0.194 (0.087) | 1.000 | | | |
Hg U | 0.539 (0.001*) | 0.602 (0.001*) | 0.388 (0.004*) | 0.931 (0.001*) | 0.285 (0.008*) | 0.318 (0.003*) | 0.627 (0.000*) | 1.000 | | |
Pb P | 0.000 (0.999) | -0.138 (0.229) | -0.220 (0.051) | -0.184 (0.105) | 0.461 (0.001*) | 0.180 (0.113) | 0.491 (0.001*) | 0.370 (0.008*) | 1.000 | |
Pb U | -0.217 (0.050) | -0.030 (0.780) | -0.256 (0.022*) | -0.211 (0.051) | 0.398 (0.002*) | 0.410 (0.001*) | 0.318 (0.004*) | 0.407 (0.001*) | 0.654 (0.001*) | 1.000 |
* Statistically Significant Note: P: Placental blood, U: Umbilical cord blood The figures in parentheses refer to p values. |
Many similar studies have shown that Pb, Cd, As, and Hg can pass through the placental barrier (Marques et al. 2007; Ha et al. 2009; Kim et al. 2011; Johnston et al. 2014; Lin et al. 2018; Hadavifar et al. 2019). Moreover, a positive and meaningful correlation was found between the levels of Hg and Cd, Hg and Pb, and Hg and As in the maternal blood and the umbilical cord blood. Another positive and significant correlation was observed between Pb and Cd levels in the maternal blood and the umbilical cord blood. These results are consistent with the studies of Lin et al. 2018 (Lin et al. 2018). The presence of Se in maternal blood prevents many toxins from entering to the fetus (Magos et al. 1980).
Se is a known antagonist of Hg, As, Pb, and Cd (Käkelä et al. 1999; Musik et al. 1999; Lazarus et al. 2011). It has antioxidant properties that reduce the toxicity of toxic metals by forming neutral metal selenide complexes such as CdSe and HgSe (Yang et al. 2002).
According to Table 3 and Figure 2, the results revealed that there was a positive and significant association between Se and Hg (r=0.82, p <0.001) and As (r=0.81, p <0.001) in the maternal blood (Figure 2-A) and meaningful correlation between Se and Hg (r=0.93, p<0.001) and As (r=0.72, p<0.001) in the umbilical cord blood (Figure 2-B). Earlier studies confirmed the positive and significant correlation between Se and Hg and As levels (1). This association implies that with the increase of toxic metals the umbilical cord absorbs more Se from the maternal blood to decline the toxic metals entering the fetus (Kantola et al. 2000).
The reaction of selenite (Se (IV)) with arsenite (As (III)) is a glutathione-mediated redox reaction that forms a non/less-toxic compound in the body (Sun et al. 2014).
The present study demonstrated a meaningful negative correlation between maternal blood Se level and Pb in the umbilical cord (r=-0.256, p=0.022), which is similar to that of Bo Xu et al. (1994) (Xu et al. 1994).
3.3. As levels and its association with the independent studied variables
The statistical data of this study in Table 1 showed that the levels of As in analyzed samples of rural areas were higher than the urban areas. In such a way that the As level in the umbilical cord and maternal blood samples in the rural areas was about 3-5 times higher than those in urban areas. Food and water are considered the main sources for As exposure. A study in Bangladesh reported that pregnant women were chronically exposed to a range of natural As compounds in their drinking water (Kwok et al. 2006).
One of the major factors is the absorption of As in the mineral form in rice (Awata et al. 2017). Probably one of the reasons for the high As levels in rural areas could be attributed to the consumption of Indian rice, which is lower-cost than other Iranian rice types due to their weaker economic conditions.
Investigations have shown that the risk of As existing in this type of rice is higher than the other types (Sharafi et al. 2019). The agricultural pesticides can also be effective in exposing rural women to As, which needs further investigation.
The results of univariate linear regression analysis displayed that As level has a significant association with the living area of the pregnant women (Table 4). According to Table 5, the results of multiple linear regression analysis exhibited that the As levels had a meaningful association with the living area of the pregnant women (p<0.001), while no significant association was found with other studied variables.
Table 4
Association between toxic metals and independent variables according to univariate linear regression analyses
Independent variables | As | Se | Cd | Pb |
βa (95% CI ) | P value | βa (95% CI ) | P value | βa (95% CI ) | P value | βa (95% CI ) | P value |
Living area (rural) | 35.78 (-23.24, 94.82) | 0.017 | 52.38 (33.7, 71.07) | <0.001 | -0.16 (-0.45, 0.11) | 0.241 | -10.84 (-25.11, 3.42) | 0.130 |
Sex (Female) | -0.88 (-2.71, 0.95) | 0.342 | -10.76 (-32.84, 11.32) | 0.335 | -0.036 (-0.32, 0.25) | 0.804 | -8.05 (-22.48, 6.38) | 0.271 |
Filled teeth (1≤) | -22.02 (-82.72, 38.66) | 0.470 | -23.41 (-45.60, -1.22) | 0.038 | 0.04 (-0.24, 0.34) | 0.757 | -7.11 (-21.95, 7.72) | 0.342 |
Mother age (25≤) | -26.47 (-85.33, 32.39) | 0.373 | 16.93 (-4.66, 38.52) | 0.122 | 0.14 (-0.13, 0.43) | 0.308 | 5.80 (-8.52, 20.13) | 0.422 |
Abortion (1≤) | -15.72 (-83.92, 52.46) | 0.647 | -14.4 (-39.11, 10.30) | 0.249 | -0.14 (-0.47, 0.18) | 0.397 | -4.98 (-21.23, 11.26) | 0.543 |
Number of pregnancy (1≤) | -32.9 (-92.21, 26.39) | 0.272 | -8.15 (-30.18, 13.86) | 0.463 | -0.028 (-0.31, 0.26) | 0.845 | 5 (-9.43, 19.43) | 0.492 |
a Regression coefficient from univariate linear regression. |
Table 5
Association between toxic metals and independent variables according to multiple linear regression analyses a
Independent variables | As | Se | Cd | Pb |
βb (95% CI ) | P value | βb (95% CI ) | P value | βb (95% CI ) | P value | βb (95% CI ) | P value |
Living area (rural) | 4.56(3.06, 6.13) | <0.001 | 47.77(3.83, 45) | <0.001 | -0.18(-0.49, 0.12) | 0.232 | -14.86(-29.98, 0.25) | 0.054 |
Sex (Female) | 0.14(-1.49, 1.77) | 0.862 | 7.47(-12.9, 27.85) | 0.467 | 0.02(-0.29, 0.34) | 0.893 | -4.92(-20.9, 11.07) | 0.541 |
Filled teeth (1≤) | -0.99(-2.7, 0.71) | 0.251 | -20.49(-42, 1.01) | 0.061 | -0.05(-0.39, 0.28) | 0.742 | -11.67(-28.48, 5.13) | 0.170 |
Mother age (25≤) | 1.39(-0.27, 3.06) | 0.100 | 24.41(3.83, 45) | 0.021 | 0.2(-0.12, 0.53) | 0.224 | 7.93(-8.31, 24.17) | 0.334 |
Abortion (1≤) | -1.03(-3.04, 0.97) | 0.308 | -14.5(-39, 10.05) | 0.243 | -0.15(-0.54, 0.24) | 0.447 | -6.9(-26.17, 12.37) | 0.478 |
Number of pregnancy (1≤) | -1.38(-3.13, 0.37) | 0.121 | -13.14(-34.84, 8.56) | 0.230 | -0.01(-0.35, 0.34) | 0.967 | 7.45(-9.76, 24.67) | 0.391 |
a Calculated regression coefficients were adjusted for all variables in the table. b Regression coefficients from multiple linear regression model. |
Studies in Canada have also shown a significant association between the levels of As in the blood of Canadian and non-Canadian pregnant women, which shows that the place of residence of pregnant women affects the level of As, which is comparable to the results of the present study. Furthermore, it is reported that there is a meaningful association between the age of the individuals and the As blood level (Ettinger et al. 2017), but in this study, no significant association was discovered.
3.4. Pb concentration and the association with the independent studied variables
According to Table 1, the results presented that the median of Pb concentration values in rural pregnant women was higher than that in the urban areas. It is proposed that mothers living in rural areas do more outdoor activities. So the higher Pb levels in rural living mothers may be due to the more inhalation dust and particulates. Sabzevar city has been located in an arid area. From other hand, the study of Hosseini et al. (Hosseini et al. 2016) in Iran has demonstrated that dusty and windy days enhances the level of heavy metals in the air and their findings have been significant for Pb levels. Pb blood concentration of pregnant women increased with age. Some studies confirm this claim (Wells et al. 2011). Besides, Pb concentrations were lower in women who had experienced more abortions. The reason for this decrease could be related to the elimination of Pb through abortion. In the pregnancy the mobilization of Pb from maternal bones (as the main source of body Pb) to the fetus enhanced that exposes the fetus to the toxicity and abortion. However this phenomena caused to decrease the maternal Pb levels (Gulson et al. 1997). The results of univariate linear regression showed that none of the studied independent variables had a significant association with the amount of Pb in the pregnant women's blood (Table 4).
The multiple linear regression results, which are brought in Table 5 demonstrated that Pb concentration had a meaningful association with the living area of pregnant women (p <0.054). Al-Saleh et al. (2014) reported that Pb concentration had a significant association with the place of residence of pregnant women (Al-Saleh et al. 2014), which is similar to the results of this study.
3.5. Cd concentration and its association with the studied independent variables
Statistical results revealed no meaningful variation in Cd concentration according to independent variables (Table 1). Additionally, univariate and multiple linear regressions showed no important relation between Cd concentration and the independent variables (Tables 4 and 5).
In a comprehensive study, Johnston et al. (2014) examined 1,027 pregnant women and reported that age, race, and gender of fetus did not significantly affect Cd levels (Johnston et al. 2014). A more recent investigation by Motawei et al. (2015) indicated that the age and the number of filled teeth had a significant association with Cd levels, while the place of residence showed no important relation. Some studies have shown that geographic location plays an important role in the Cd blood level of pregnant women. The difference in Cd levels is mostly due to the existence of steel factories (Lagerkvist et al. 1996) and mines (Reichrtova et al. 1998) in the vicinity of the residences. Furthermore, some studies have noted that urban pollution increases the amount of Cd in the placenta (Falcon et al. 2003).
3.6. Se concentration and its association with the studied independent variables
Se is one of the few elements that are necessary for the normal growth and development of the organs. According to Table 1, the amount of Se in rural pregnant women is about 5 times more than urban pregnant women.
Based on the univariate linear regression results in Table 4, Se levels had a significant association with the living area of pregnant women and the number of filled teeth. The results of multiple linear regression showed that Se concentration had a significant association with the living area (p <0.001) (Table 5). In different areas, the amount of Se in the blood samples of the population changes according to its absorption, which depends on not only the Se content in the soil but also the grains and plants that have grown on the soil (Oldereid et al. 1998).
Studies demonstrated that the serum selenium concentrations in the United States and Canada vary depending on the amount of Se in the soil and the local food consumption. For example, the concentration in the residents of the Midwestern and Western regions of the United States is higher than the Southern and Northeastern regions (Niskar et al.; Kafai and Ganji 2003). A study by Pettigrew et al. (2019) displayed that the amount of Se in the meat and egg yolk of the chickens bred in the rural areas was higher than those of which were sold in urban food stores. They suggested that rural chickens may collect Se while they are free to scratch the ground and eat insects and other foods (Pettigrew et al. 2019). Similarly, it seems that in this study, rural pregnant women have higher access to Se-rich resources than urban women do.