PREVENTIVE MEDICINE 15, 685-691 (1986)

SHORT REPORT

Reproductive History and Body Mass Index in Black and White Women’

JEAN L. FORSTER, PH.D.,M.P.H.,~ ELLENBLOOM, M.D.,M.PH.,3 GLORIAN SORENSEN, M.P.H. ,PH.D.,~RoBERT W. JEFFERY, PH.D.,

ANDRONALD J. PRINEAS, M.B. ,PH.D.

Division of Epidemiology, School qf Public Health, University qf‘Minnesoto, Stadium Gate 27, 611 Beacon St. S.E.. Minneapolis. Minnesota 55455

The relationships of timing, spacing, number of births, and demographic variables to body mass index were examined in 844 white and 289 black women. Subjects were inter- viewed in 1978-1979 as mothers or female guardians of a stratified random sample of all Minneapolis children in grades 1-3. Results indicate that among black and white women, number of births, age at last birth, and years between first and last births were positively associated with body mass index. However, when age, education, and income were in- cluded in the regression equation for black women. none of the reproductive variables pre- dicted body mass index. When number of births, age at last birth, and years between first and last birth were included in the same regression for white women, only number of births was independently associated with body mass index. ‘G 19X6 Academic Pre\\. Inc.

INTRODUCTION

It is commonly believed that weight gained by women during pregnancy often is not completely lost afterward. Available data seem to support this belief (1, 2, 6, 10, 11, 14). However, the few studies that have examined this question empiri- cally have been limited in scope and methods. All reports are from European populations, the analyses are limited to number of births, and most reports give data for women at the end of or beyond reproductive years.

This study seeks to investigate for the first time the relationship between obe- sity and childbearing in a U.S. population. We explored the hypotheses that timing, spacing, and number of births are related to degree of overweight in adult women. Because the population of women used for this study was enriched for minorities, the hypotheses were examined for black women as well as white women.

Data Collection METHODS

In 1977-1978, 99% of the children in Minneapolis public schools, grades 1-3,

’ Supported by NIH Grants 5-ROI-HL19877 to Dr. Prineas, 5-ROl-AM26542 to Dr. Jeffery, 2-T32- HL07328 to Dr. Sorensen, and 5-T32-HL07036 to Dr. Bloom.

* To whom reprint requests should be addressed. 3 Current address: Honolulu Heart Study, 347 Kuakini St., Honolulu, Hawaii 96817. 4 Current address: Division of Preventive and Behavioral Medicine, Department of Medicine, Uni-

versity of Massachusetts Medical School, 55 Lake Avenue. Worcester. Mass. 01605.

685

0091-7435186 $3.00 Copyright G 1986 by Academx Press, Inc. All rights of reproduction in any form reserved.

686 FORSTER ET AL.

were screened by the Prevention of High Blood Pressure in Children Study. Of these, 2,640 children (representing the top and bottom 5% of the systolic blood pressure distribution, a 50% random sample of the remaining black children, a random sample of one of nine of the remaining white children, and all other mi- norities) were chosen for follow-up (12).

Home interviews with the mother, female guardian, or other adult living with the child were completed for 1,509 of these children. Interviews were consented to by 67% of white parents and 48% of black parents. No other information is available about nonrespondents. During these interviews sociodemographic in- formation, extensive medical histories, and anthropometric measurements were collected for the index child and for the male and female guardians. Weight was measured without shoes or jackets in indoor clothing, to the nearest 0.25 kg, with balance scales that were calibrated daily. Height was measured without shoes to the nearest centimeter.

The female guardians of the index child constitute the sample for this study. Women were eliminated from the sample if they were pregnant, postmenopausal (either surgically or naturally), of ethnic origin other than black or white, living outside the home of the index child, or nulliparous, resulting in a sample of 844 white and 289 black women.

Analysis

The dependent variable for the study was body mass index (BMI, kg/m2). Be- cause wt/htr5 has been shown in women to correlate best with skinfold thickness and to be most independent of height (9), the analyses were repeated using kg/m1.5 as the dependent variable. The results were, in all analyses, unchanged. The reproductive history variables used in this study were number of births (live births plus still births), age at first birth, age at last birth, years since last birth, and years between first and last births (age at last birth - age at first birth + 1). These variables were collected via self-report; no attempt was made to establish their validity. Age, education, and family income were used as covariates. Other sociodemographic variables (smoking status, employment status, marital status, occupation) were not correlated with BMI, and so were not included in this study. To fulfill the assumption of normal distribution for regression analyses, variables with a significantly skewed distribution were transformed as indicated in Table 1.

One-way analysis of variance was used first to compare means between black and white women. Simple correlations were then computed for BMI and repro- ductive history and demographic variables, separately by ethnic group. Multiple regression was used next to analyze the relationship of each reproductive variable individually to BMI for each ethnic group, adjusted for age, income, and educa- tion.

Because the reproductive variables and age were highly intercorrelated, results of a simultaneous regression including these variables might not provide an accu- rate estimate of the relative contribution of each of the reproductive variables to BMI (13). To overcome this problem, a regression was run for each reproductive variable, with age, education, income, and the remaining reproductive variables

REPRODUCTIVE HISTORY AND BODY MASS INDEX 687

TABLE I REPRODUCTIVEVARIABLES,DEMOGRAPHICVARIABLES,ANDBODY MASS~NDEXBY

ETHNIC GROUP”

Transformation

Independent variables Birthsb Age at first birthb Age at last birth Years since last birth Years between first and last birth@

Covariates Age’ Education:b <High school (%,)

High school (%) SHigh school (%)

Family income:b <$lO,OOO (‘S) ~$10,000 < $20,000 (%) ~$20,000 (%I

Dependent variable Body mass indexb

In In In

square In

In

In (In)

Black (N = 289)

3.3 2 0.1 20.6 2 0.2 26.5 ? 0.3

6.2 ? 0.2 6.9 i 0.2

32.8 i: 0.3 33.9 L 0.2 24 I2 48 4s 28 43 48 22 32 38 20 40

26.8 2 0.4

White (N = 844)

2.6 -t 0.1 22.7 t 0.1 27.2 k 0.2 6.6 -’ 0.1 5.5 k 0.1

24.5 k 0.2

a Values given are untransformed means 2 SE, or % in each category b P < 0.0001. c P < 0.003.

forced to enter the regression first. This method provides a conservative estimate of the independent contribution of each reproductive variable to BMI.

RESULTS

Table 1 gives the means and standard errors of the reproductive and demo- graphic variables used in this study for black and white women, and the results of one-way analyses of variance for differences of the means. The black women in this sample had significantly higher mean BMIs, were less educated, were younger, and had lower incomes than the white women. With respect to the re- productive variables, black women had more births, and were younger at first birth, and their average span between first and last births was longer than that for white women.

Simple correlations between the reproductive and demographic variables and BMI are shown by ethnic group in Table 2. For both black and white women, number of births, age at last birth, and years between first and last births were significantly correlated with BMI in a positive direction, but age at first birth and years since last birth were not associated with BMI among either black or white women. Income and education were negatively correlated and age positively cor- related with BMI among white women, but only age was significantly correlated with BMI among black women.

Table 3 shows mean BMI by number of births, age at last birth, and years between first and last births separately for black and white women. These rela-

688 FORSTER ET AL.

TABLE 2 CORRELATION OF BODY MASS INDEX WITH REPRODUCTIVE AND DEMOGRAPHIC VARIABLES FOR

BLACK AND WHITE WOMEN

Black (N = 289) White (N = 844)

Y P r P

Age at last birth 0.15 0.01 Years between first and last births 0.17 0.004 Number of births 0.19 0.001 Age at first birth -0.02 - 0.76 Years since last birth 0.002 0.97 Age 0.17 0.005 Family income 0.01 0.99 Education -0.09 - 0.15

0.11 0.003 0.15 CO.001 0.17 <O.OOl

-0.06 0.09 - 0.008 0.82

0.10 0.005 -0.17 <O.OOl -0.20 ‘Co.001

tionships are adjusted for age, education, and family income using analysis of covariance. P values were obtained using multiple regression analyses. After ad- justing for demographic variables, number of births, age at last birth, and years between first and last births, all were significantly associated with BMI among white women, but none of the reproductive variables was associated with BMI among black women.

TABLE 3 MEAN BODY MASS INDEX~ BY NUMBER OF BIRTHS, AGE AT LAST BIRTH, AND YEARS BETWEEN

FIRST AND LAST BIRTHS, FOR BLACK AND WHITE WOMEN

Black means & SE (N)

Births 1 25.5 2 0.9 (50) 2 26.4 5 0.7 (76) 3-4 27.3 IT 0.7 (89) >4 27.8 2 0.9 (62)

NS Age at last birthb

Bottom tertile 28.1 ” 0.6 (76) Middle tertile 26.2 k 0.7 (102) Top tertile 26.4 5 0.6 (96)

NS Years between first and last birthsc

Bottom tertile 26.2 k 0.6 (89) Middle tertile 27.1 ? 0.6 (110) Top tertile 27.0 f 0.6 (75)

NS

White means 2 SE (N)

24.0 2 0.5 (139) 24.1 + 0.3 (329) 25.0 ? 0.3 (291) 25.7 k 0.7 (62)

P< 0.001

23.9 iz 0.3 (284) 24.6 ‘- 0.3 (233) 24.8 -+ 0.3 (289)

P< 0.02

24.2 ” 0.3 (233) 24.1 ” 0.3 (332) 25.1 ” 0.2 (241)

P-c 0.02

n Adjusted for age, education, and family income using analysis of covariance. b For blacks, cutpoints were <23, 223 ~28, 328. For whites, cutpoints were <24, 324 ~29, 229.

Ns are not equal because of missing data. c For blacks, cutpoints were <4, ~4 <9, 29. For whites, cutpoints were <3, ?=3 <7, 37. Ns are not

equal because of missing data.

REPRODUCTIVE HISTORY AND BODY MASS INDEX 689

To test for associations between the reproductive variables and BMI indepen- dent of each other among white women in the sample, we performed three mul- tiple regressions. (This analysis was not performed for black women since none of the reproductive variables was associated with BMI in that subsample.) Age, in- come, education, and two of the three reproductive variables (number of births, age at last birth, years between first and last births) were forced to enter first, and the significance of the third reproductive variable was assessed. Only number of births significantly predicted BMI independent of the sociodemographic and re- maining reproductive variables [F(1,692) = 6.29, P < 0.011. The model including age, income, education, number of births, age at last birth, and years between first and last births accounted for 9% of the BMI variance among white women.

DISCUSSION

These results confirm previous reports of a positive association between number of births and BMI among white women, independent of sociodemo- graphic characteristics (1, 2, 6, IO, I I, 14). These findings extend the association to premenopausal women, consistent with Heliovaara and Aromaa’s finding that the relationship between number of births and BMI was strongest in the younger age groups (6). This strong association between number of births and BMI inde- pendent of age suggests that number of births may account in part for weight gain with age seen in white women.

Table 3 suggests that the lack of statistical association found between the re- productive variables and BMI adjusted for age, education, and income in black women may be due to small sample size. However, the average adjusted BMI at all levels of parity was higher for black women than for white women, suggesting that a mechanism other than number of births accounts for the differences in BMI between black and white women reported here and in other studies (3, 8).

The lack of association between BMI and age at last birth or years between first and last births when the other reproductive variab!es were forced into the equation is a function of the strong correlations among these reproductive vari- ables. It is impossible to separate the effect of timing or spacing of births on BMI from parity in this sample. One would need to analyze large samples of women where the effects of timing and spacing could be studied within subsamples of women with the same parity.

The sampling frame for this study introduces two potential sources of bias. Though mothers were the subjects of interest, sampling was based on character- istics of their children. One consequence was that mothers of high fertility (3 two children in grades 1-3) were inadvertently oversampled. Our results should be interpreted with some caution since it is possible that highly fertile women are not representative of the general population of women.

One might also speculate that choosing the sample on the basis of children’s blood pressure might impair the generalizability of the results. However, results from the Prevention of High Blood Pressure in Children Study and others indicate that family income, parental education, and occupation are all unrelated to chil- dren’s blood pressure (5, 7).

The low response rate (67% of whites, 48% of blacks) among those selected for

690 FORSTER ET AL.

the home interview also indicates the need for caution in interpreting these re- sults. A study of NHANES II nonresponders indicated that nonresponders had smaller families than responders. However, regarding the demographic charac- teristics of interest here (income, age, education, race), there were no differences between responders and nonresponders (4). No information is available with which to compare BMI among responders and nonresponders.

A surprising result of this study is that education and income were not asso- ciated with BMI among black women, as they have been among white women in this and other studies. The lack of association among black women is apparently not due to greater homogeneity among black women than white women, as the coefficients of variation for income and education are comparable in both groups. Nor is lack of association explained by sample size differences since the correla- tions for BMI and education and for BMI and income were much lower among black women than among white women.

One explanation may be that, though variation in education and income were equal among black and white women, the distributions of income and education in black women are centered at the lower end of the scale, and in white women the distributions are centered higher on the scale. It is possible that the negative association between BMI and income and education, reflecting social norms, is stronger at the upper ends of the income and education scales. Clearly, further studies are needed to clarify this relationship.

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