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It has been well established that age, ethnicity, weight, and lifestyle behaviors can affect blood pressure (BP). Co-morbid conditions such as HELLP syndrome (hemolysis, elevated liver enzymes, and low platelets), pre-eclampsia, and previous hypertension diagnosis might also be risks for chronic hypertension among women who have had children. Although parity has been linked to changes in blood pressure in White women, these findings have not been replicated among African-American women. The purpose of this study was to determine if the number of pregnancies urban African-American women have effects BMI and blood pressure readings later in life. Results indicated that women with a previous diagnosis of hypertension had higher SBP and DBP, and a slightly higher BMI than women who had never been diagnosed. Additionally, women with a prior history of hypertension had more children than those without a diagnosis of hypertension. As parity increased, SBP increased. However, DBP decreased after 3 to 4 children, even with increases in BMI. This study shows that parity may increase African-American women’s risk for hypertension in terms of increased SBP and BMI with increased parity. However, increased parity and BMI may also serve as protective factors in lowering DBP. Further studies, with larger samples followed throughout their pregnancies, is needed before more definitive statements may be drawn about the effects of parity on BMI and blood pressure readings among African-American women can be made.
Numerous factors influence one’s risk for developing hypertension. Age, body mass index, diet, physical activity, race, genetic factors, menopause, and use of oral contraceptives can contribute to a woman’s risk for hypertension (Chobanian et al., 2003; Rosenthal & Oparil, 2000). Little research has been conducted regarding the relationship among childbearing history, parity (number of pregnancies), and blood pressure (BP). One study has reported significant effects of parity and age on decreases in both systolic (SBP) and diastolic (DBP) blood pressure (Blackburn, 2002). According to Blackburn, the differences in blood pressure readings were greatest when nulliparous women (women who have never having given birth to a child) were compared to primiparous women (a woman who has given birth only once). Although these findings are a step in increasing the knowledge base in the area of parity and blood pressure, the Blackburn (2002) study was not specific to African-American women. The present study adds to the current knowledge base by examining the effects of parity on blood pressure among urban African-American women.
The number of pregnancies a woman has may affect blood pressure due to physiological changes in blood perfusion during gestation. Blood volume increases throughout pregnancy, peaking at term near 40% to 45% above non-pregnant levels (Olds, London, Ladewig, & Davidson, 2004). Increased heart rate, myocardial contractility and pre-load and reduced after-load can influence cardiac performance in pregnancy by expanding cardiac output (Girling, 2004). No increase has been noted in pulmonary capillary wedge pressure or central venous pressure due to decreases in systemic vascular resistance and pulmonary vascular resistance. These decreases in systemic and pulmonary vascular resistance allow circulation to adapt to higher blood volume while maintaining normal vessel pressures and increasing cardiac output. According to Girling, the femoral venous pressure rises as the uterus enlarges and exerts pressure on return blood flow. In addition to an increased tendency toward stagnant blood flow in the lower extremities, a reduction in plasma colloid osmotic pressure also occurs. Postural hypertension, caused by increased blood volume in the lower extremities, is a risk to pregnant women. The blood pressure of pregnant women decreases slightly during the first trimester, and is lowest during the second trimester. Blood pressure decreases between 0–9 mmHg SBP (Girling, 2004) and DBPhas an average decline of 10–15 mmHg in the first trimester (Caulin-Glaser & Setano, 1999; Lockitch, 1997). During the third trimester, blood pressure readings gradually return to baseline levels.
It is well documented that African-American women tend to be more overweight and obese and to have a higher incidence of developing hypertension during pregnancy compared to White and Hispanic women (Hilmert et al., 2008; American Heart Association, 2008). Increases in obesity may be a factor with increased parity and it has been shown to be a major risk factor for hypertension. However, obesity related to parity among African-American women has not been well defined. Therefore, the effect of parity on increasing the risk of developing hypertension during the lifetime has been understudied, and the present study will contribute to this body of knowledge.
The purpose of this study was to determine if parity has an influence on blood pressure readings and BMI among urban African-American women. This study will answer the following research questions.
Findings from some research studies have noted that blood pressure is affected by a number of factors including race, age, genetic factors, and body mass index (BMI) (Olds, London, Ladewig, & Davidson, 2004). Pregnancy can also affect blood pressure, with numerous hemodynamic shifts occurring. During pregnancy, blood perfusion and flow to the kidneys and uterus increases, whereas cerebral blood-flow and flow to the liver remains the same (Olds, London, Ladewig, & Davidson, 2004). The additional cardiac output flow increases blood perfusion to the uterus by 400 ml/min extra, to the kidneys 300 ml/min additional, as well as to the skin, breasts, gastrointestinal tract, and cardiac and respiratory muscles (Girling, 2004). In addition, the number of pregnancies a woman has, her history of preeclampsia or hypertension before pregnancy, or preeclampsia during pregnancy may also influence a woman’s risks for developing hypertension during her lifetime.
Race can be a factor for an increased risk of hypertension. In the general population, African-Americans have a greater incidence of hypertension (defined as SBP greater or equal to 140 mmHg and DBP greater or equal to 90 mmHg) than Whites (American Heart Association, 2005; Hilmert et al., 2008). African-Americans experience earlier onset and higher prevalence and severity of hypertension than other ethnic populations in the United States (Cooper, Liao, & Rotimi, 1996). Approximately twice as many African-American women enter pregnancy with preexisting hypertension than White women (Hilmert et al., 2008; American Heart Association, 2008). Although chronic hypertension is more prevalent among African-American women than in other ethnic groups, the true relationship between hypertension and the number of pregnancies is unknown (American Heart Association, 2005; Chobanian et al., 2003). Hypertension is a major health concern for the African-American population, but effects of parity related to high blood pressure have not been studied.
Blood pressure has been found to increase with age in developed countries, beginning in childhood and continuing into adulthood (Franklin et al., 1997; Taylor, in press). A greater increase in SBP than DBP is seen during the middle adult years (Franklin et al., 1997) when women are of childbearing age. A woman’s fertility peaks between 20 and 24 years of age, during which time, blood pressure increases slightly. However, fertility rates remain relatively constant through the early 30s, after which they begin to decline. For example, at 30 to 35 years of age, fertility is 15% to 20% below maximum and from 35 to 39 years of age, the decrease is 25% to 50%.
Obesity and overweight have been associated with increases in blood pressure (Masuo, Mikami, Ogihara, & Tuck, 2000; World Health Organization, 2000). Women who experience increases in BMI had significant increases in SBP and DBP compared to those with a stable BMI (Drøyvold, Midthjell, Nilsen, & Holmen, 2005; Hilmert, et al. 2008). Rooney, Schauberger, and Mathiason (2005) concluded that excessive gestational weight gain and failure to lose pregnancy-related weight by six months postpartum constituted important predictors of obesity in midlife. Data on body weight after more than 12 months postpartum are scarce because other social and behavioral changes may add to the retained weight (Schmitt, Nicholson, & Schmitt, 2007).
The present study used a correlational research design. The study was nested within a larger study that was designed to examine genetic polymorphisms associated with hypertension among three generations of African-American women. The investigator and research assistants recruited African-American women who lived in a large Midwestern urban area into the three-generation genetic study. Details of the recruitment strategies can be found in Taylor (2009) and Wu, Prosser and Taylor (under review). Multiple procedures were used to recruit African-American women for the multigenerational genetic study. Specifically, women were recruited using the following methods: (a) the use of flyers posted in neighborhood areas including local stores, markets, and community centers, (b) advertisement and announcement at local churches, (c) advertisement and announcement at historically Black sororities, and (d) and use of participant resource pools from University of Michigan and Wayne State University (Taylor, 2009). After agreeing to participate in the study, informed consent was obtained during home visits that served as the site for data collection. Research assistants were trained by the principal investigator regarding all data collection methods, home visitation, and coordinating visits. Details regarding recruitment strategies can be found in Taylor (2009). Participants in the present study were a subgroup of African-American women from the parent study who had at least one biological child. Data associated with all study variables were collected at baseline.
The sample included 126 (N = 126) African-American women from the parent study. All of the women in the study had given birth to at least one child. The women in the study ranged in age from 20 to 93, with a mean of 52.15 (SD = 16.87) years of age. The largest group of women (n = 40, 31.7%) reported that they had completed some college, with 24 women (19.0%) indicating that they had obtained bachelor’s degrees. The household income levels ranged from less than $10,000 to more than $80,000. Most of the women had incomes of greater than $30,000 (N = 77, 56.4%). The majority of the participants (N = 115, 91.3%) reported they had health insurance (see Table 1).
The inclusion criteria were as follows: 1) participants must self-identify as African-American and female; 2) must have the ability to read and write in English; 3) must have had at least one viable pregnancy and delivery in their lifetime.
Participants were excluded from participation if the following existed: included: 1) having a co-morbidity of substance abuse, mental illness, end-stage cancer, end-stage renal disease or other terminal illness.
Recruitment strategies commenced after approval was sought and obtained from the Institutional Review Boards (IRB) at the University of Michigan and Wayne State University.
The demographic questionnaire collected information from the participant regarding age, educational level, household income, number of children, and hypertension diagnosis. This instrument was investigator developed in the parent study in order to provide a demographic profile of the participants and it used forced-choice options for answering the questions.
The investigator developed the natal history questionnaire based on the GTPAL system (Varney, Kriebs, & Gegor, 2004) to obtain information on the number of pregnancies, full-term deliveries, pre-term deliveries, miscarriages, abortions, live births, vaginal deliveries, cesarean section deliveries, and total number of living children. Participants were also asked if they had been diagnosed with hypertension or if they had any other health-related complications with any of their pregnancies. Additionally, participants reported in a forced-choice “yes/no” format if they had received prenatal care during their pregnancies. A Cronbach’s alpha of 0.85 was obtained for the present study.
Blood pressure was measured using a digital blood pressure monitor with size appropriate upper arm cuff (model # A&D UA767PC). Reported blood pressure measurements represent an average of three seated blood pressure readings. Procedures for participant preparation for blood pressure measurement are in accordance with JNC-7 (Chobanian et al., 2003) recommendations. Each of the three blood pressure readings was taken 5 minutes apart. Participants were wearing unrestrictive clothing, had their feet on the floor, their backs supported, and their arm at heart level.
BMI is the relationship between weight and height that is associated with body fat and health risk. The formula for calculating BMI is weight in pounds divided by the height in inches squared (World Health Organization, 2000). The result is then multiplied by 703 to obtain the BMI. BMI over 25 is considered overweight, with BMI outcomes greater than or equal to 30 being indicative of obesity. An electronic scale (BWB/807 Tanita Tokyo, Japan) was used to measure weight, with height measured using a portable stadiometer (Model 214 Road Rod, Seca Corporation, Hanover, MD).
SPSS version 16.0 was used to analyze the collected data. Cross-tabulations were used to provide a profile of the participants who had been previously diagnosed with hypertension and of those who had not been diagnosed. Descriptive statistics were obtained for blood pressure and body mass index (BMI). The average blood pressure readings and BMI were compared across the number of pregnancies (1–2, 3–4, and 5 or more) using one-way analysis of variance (ANOVA). Data on the pregnancies were compared between the two groups (those who had been diagnosed with hypertension and those without a diagnosis of hypertension) using t-tests for two independent samples. All decisions on the statistical significance of the findings were made using a criterion alpha level of .05.
The mean SBP for women who had been previously diagnosed with hypertension was 145.83 (SD = 23.18), while women who had not been diagnosed with hypertension had a lower mean SBP (M = 128.32, SD = 15.46). When mean DBP was compared between the two groups, women who had been previously diagnosed with hypertension had higher levels (M = 83.29, SD = 12.21) than those who had not been diagnosed with hypertension (M = 81.61, SD = 12.04). The women who had been previously diagnosed with hypertension had slightly higher BMI (M = 32.79, SD = 7.41) than those without a hypertension diagnosis (M = 32.14, SD = 7.87) (see Table 2).
The blood pressure readings and BMI for the women were compared by the number of pregnancies (1–2, 3–4, and 5 or more). As shown in Table 3, average SBP increased with the number of pregnancies, while average DBP increased between 1–2 and 3–4 pregnancies and then decreased with 5 or more pregnancies. BMI increased as the number of pregnancies increased. The results of the one-way ANOVA were not statistically significant.
The women’s natal histories (number of pregnancies, number of full-term deliveries, number of premature deliveries, number miscarriages/abortions, and number of living children) were compared between the women who had been previously diagnosed with hypertension and those who had not been diagnosed. Women who had been previously diagnosed with hypertension had a greater number of pregnancies (M = 4.26, SD = 2.54) than those who had not been diagnosed with hypertension (M = 3.25, SD = 1.80), t (124) = 2.55, p = .012. The women who had been previously diagnosed with hypertension had a higher number of full-term deliveries (M = 3.29, SD = 2.35) than those without this diagnosis (M = 2.23, SD = 1.43), t (124) = 3.01, p = .003. The number of live births reported by the women previously diagnosed with hypertension (M = 3.58, SD = 2.32) was higher than those reported by women without a diagnosis of hypertension (M = 2.42, SD = 1.31), t (124) = 3.36, p = .001 (see Table 4).
As expected, women who had been previously diagnosed with hypertension had higher SBP and DBP, as well as a slightly higher BMI. However, the present study’s findings suggest both deleterious and protective aspects parity may have on blood pressure readings among African-American women. As the number of the participants’ pregnancies increased, so did SBP. Respondents who had 1 to 2 pregnancies recorded a mean SBP of 135.32 with a SBP average of 140.84 for those who had had 5 or more pregnancies. This data illuminates a positive correlation between parity and blood pressure, revealing a heightened risk for hypertension with each pregnancy.
The increase in SBP may be accounted for by the age of the participants at the time of the study. It is widely known that SBP increases as age increases thereby increasing risk for hypertension and other related cardiovascular disorders (Taylor, Washington, Artinian, & Lichtenberg, 2007). In addition, the increase in SBP with increased parity may be affected by the index used for number of children. The index increased incrementally until 4 children, then 5 or more children were grouped together with the greatest number of children being 12. However, the increase in SBP and BMI among African-American women was related to the number of pregnancies, particularly with women having 5 or more pregnancies. These findings support previous research linking obesity and high blood pressure (Masuo, Mikami, Ogihara, & Tuck, 2000; World Health Organization, 2000).
The data also indicate that the respondents’ DBP begins to decrease after 3 to 4 births, even though their weight increases. It has been shown in a genetic study of hypertension that increased adiposity may act as a protective mechanism in decreasing DBP in African-American women (Taylor, Sun, Chu, Mosley, & Kardia, 2008). The genetic study showed that the metallopeptidase 3 (MMP3) polymorphism rs676920, which is involved in embryonic development and reproduction, can be detrimental to arterial stiffening and atherosclerotic syndromes among African-American women when their BMI is low to normal. When the BMI is high, indicating obesity, the MMP3 polymorphism is protective in lowering DBP. Therefore, African-American women may not be at risk for developing heightened DBP readings with increased parity and BMI.
Overall, participants who had been previously diagnosed with hypertension had more children than those who had not been diagnosed with hypertension. Because information regarding blood pressure readings with each pregnancy was not available in this study, no conclusions can be made regarding the relationship between hypertension diagnoses due to pregnancy. However, the data in this study clearly support that as the number of pregnancies increase, so does the SBP and BMI. It has been well established that increases in SBP and BMI are major risk factors in the development of hypertension. This study shows that although parity may increase African-American women’s risk for development of hypertension in terms of increasing SBP and BMI, parity and BMI may also be protective by lowering the risk of increased DBP.
One of the limitations of this study was that the age of hypertension diagnosis was not known. It is unknown whether the diagnosis of hypertension was made before, during, or after any of the pregnancies. The timing of the hypertension diagnosis may have an influence on the results of this study. However, none of the women reported having had a history of preeclampsia, eclampsia, preeclampsia superimposed on hypertension, or HELLP syndrome during any of their pregnancies. Additionally, lifestyle factors (dietary sodium intake, physical activity, and alcohol intake), known to increase the risk for hypertension development, were not assessed in this study. Inclusion of these factors could also alter the results. Another limitation of the study is the use of self-report demographic and natal history surveys. The participants may have over-reported or under-reported their responses in an attempt to be socially acceptable. Although this type of bias is inherent in self-report measures, it is typically accepted in research as relatively accurate.
The study was also limited to only urban African-American women. The results cannot be generalized to women of other ethnicities or geographic backgrounds. Further longitudinal genetic studies of African-American women followed from before their first pregnancy, throughout their pregnancies, and to the end of their childbearing years is needed before more definitive statements about the influence of parity on hypertension can be made. Larger samples of women, including multiple ethnicities from various geographic locations is also recommended in order to generalize the findings.
This data illuminates a positive correlation between increased parity, BMI, and SBP, revealing a heightened risk for hypertension with each pregnancy and greater weight gain. Based on the findings of this study, hypertension should be a concern for African-American women, particularly if they plan to have more than one child. Existing programs for detection and treatment of hypertension are neither identifying African-American women with high blood pressure nor effectively helping those who have been diagnosed to maintain or improve their lifestyle behaviors. Nurses should caution women of these findings at pre-natal visits and educate them about healthy lifestyle behaviors that can help to reduce their risk for hypertension development. Interventions for healthy lifestyle behaviors for women of child-bearing age (particularly those who have had children previously) must be encouraged and need to include the following: weight reduction programs prior to conception (eg., walking, aerobics, pilates, etc.), safe weight maintenance programs during pregnancy (eg., pre-natal yoga, walking, swimming), and healthy eating menus and advice (eg., increased fresh fruits and vegetables, baking and grilling foods instead of frying, increases in potassium intake, reduction of sodium, reduction of sodas and juices with high fructose corn syrup, and increases in water intake).
Prior to making recommendations for changes in lifestyle behaviors, nurses should examine birth history when developing plans of care for African-American women at risk for hypertension. Nurses need to refer families to appropriate networks (i.e., dieticians, nutritionists, exercise therapy, and support groups, etc.) for further education and follow-up care as needed. Because nurses specialize in providing education on health promotion and risk reduction activities, they need to act as educators for the entire family, especially in terms of nutrition and exercise, to create a family system that works to protect the health of all members. Nurses are very well positioned to provide dietary and exercise interventions that include the integration of a multidisciplinary team of nurses, nutritionists, and exercise therapists who can guide families in selecting healthy nutritious food, how to read food labels when preparing heart-smart meals, and participating in safe exercise programs pre and post pregnancy.
The present study was an initial attempt to develop a knowledge base about the effects of parity on blood pressure readings among African-American women. Based on results of the present study, additional research needs to be completed to develop possible nursing interventions that can help African-American women reduce their risks for developing hypertension with each subsequent pregnancy. It is recommended that this study be replicated with a larger sample, drawn from a cross-section of African-American women that represent various socioeconomic levels, living arrangements, and lifestyle behaviors to determine if the findings of the present study are valid indicators of parity and blood pressure outcomes. Experimental studies with culturally appropriate interventions for dietary and exercise recommendations (i.e., provide low fat, low salt cookbooks written by African-Americans, such as: Oprah Winfrey or Patti Labelle) would also be beneficial in determining ways to reduce blood pressure among African-American women at risk for hypertension. Interventions should also include an education component that would teach African-American women how to plan, shop, and cook nutritious meals and how to integrate an exercise program into their daily lives for a healthier pregnancy and beyond.
It is crucial for women to manage their blood pressure and BMI through healthy eating habits and regular physical activity before and after deliveries of their babies to reduce their risks for hypertension. Health professionals should encourage African-American women to seek pre-natal care and hypertension screening, even if their first or second pregnancy was normal, due to their heightened risk for development of hypertension with subsequent pregnancies. It is imperative that African-American women are screened before conception and early in their pregnancies for hypertension, so interventions may be implemented early to reduce complications for mother and baby. Future longitudinal studies of blood pressure readings among African-American women followed pre-conception through the end of the childbearing years is warranted to validate the findings of this study and to determine the long-term effects of risks of parity on blood pressure.
Funding for this research was provided in part by National Institutes of Health Grants 5-P30-AG015281-07 and 1 KL2 RR024987-01 to Jacquelyn Taylor.
Jacquelyn Y. Taylor, Yale University, School of Nursing in New Haven, CT.
Angelina N. Chambers, Yale University, School of Nursing in New Haven, CT.
Beth Funnell, School of Nursing at the University of Michigan in Ann Arbor, MI.
Chun Yi Wu, University of Michigan, School of Public Health, in Ann Arbor, MI.