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American Journal of Epidemiology Advance Access originally published online on October 17, 2007
American Journal of Epidemiology 2008 167(2):164-168; doi:10.1093/aje/kwm276
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American Journal of Epidemiology © The Author 2007. Published by the Johns Hopkins Bloomberg School of Public Health. All rights reserved. For permissions, please e-mail: journals.permissions@oxfordjournals.org.

ORIGINAL CONTRIBUTIONS

Is Birth Weight Associated with Risk of Depressive Symptoms in Young Women? Evidence from the Southampton Women's Survey

Hazel M. Inskip1, Nick Dunn2, Keith M. Godfrey1,3, Cyrus Cooper1, Tony Kendrick2 and the Southampton Women's Survey Study Group

1 MRC Epidemiology Resource Centre, University of Southampton, Southampton, United Kingdom
2 Primary Medical Care Group, University of Southampton, Southampton, United Kingdom
3 Centre for Developmental Origins of Health and Disease, University of Southampton, Southampton, United Kingdom

Correspondence to Professor Hazel Inskip, MRC Epidemiology Resource Centre, University of Southampton, Southampton General Hospital, Southampton, SO16 6YD, United Kingdom (e-mail: hmi{at}mrc.soton.ac.uk).

Received for publication April 9, 2007. Accepted for publication August 24, 2007.


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 References
 
Although some studies have shown negative associations between birth weight and risk of depression, others have not. Studies also differ regarding the age and gender specificity of reported associations. In this paper, the authors report on a study of 5,830 women aged 20–34 years from the general population in Southampton, United Kingdom, interviewed in 2000–2002 that found no relation between birth weight and current depressive symptoms or past treatment for depression. Prevalence ratios for current symptoms and for past treatment, in relation to reported or recorded birth weights, were all remarkably close to 1.0, with narrow 95% confidence intervals. For example, the prevalence ratio from the fully adjusted model for current depressive symptoms in relation to a standard deviation increase in reported birth weight was 1.01 (95% confidence interval: 0.98, 1.05). Generally, the associations reported elsewhere are not strong. The authors found a weak, inverse association in exploratory analyses of duration of gestation at birth in relation to depressive symptoms, but this finding requires replication. Because birth weight and duration of gestation are relatively poor markers of fetal development, other markers of fetal and early development should be explored. However, data from this study do not support a major developmental contribution to the etiology of depression in women.

birth weight; depression; gestational age; women

Abbreviations: GHQ-12, 12-item General Health Questionnaire


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 References
 
Data on the relation between birth weight and depressive symptoms are inconsistent. Recently, Alati et al. (1) reported a study of adults aged 21 years and found a negative association for women but not for men, and the interaction with sex was strongly significant. Gale and Martyn (2) reported similarly discordant findings between the sexes at age 26 years, with a stronger negative association with birth weight for women than for men. In contrast, Thompson et al. (3), in a study of older adults aged 68 years, found a negative association for men but not for women. Neither of these latter two studies reported a formal test of the interaction with sex. In a study of more than 10,000 men followed prospectively, no association between birth weight and admission to a psychiatric ward for depression was found (4). Mixed findings have been reported elsewhere but without information on sex differences; a study of twins aged 8–17 years reported a small association (5), and borderline significant associations were found for adults aged approximately 50 years (6). Other studies have found negative associations between birth weight and more general psychological symptoms (7) or mood disorder (8) and between infant growth and suicide (9). In all but three of these studies (3, 4, 9), birth weights were adjusted for gestational age, but none of them reported on the relation between depressive symptoms and length of gestation. Recently, however, an association between depressive symptoms and duration of gestation, but not birth weight, was reported in a Finnish cohort (10).

To our knowledge, consistent evidence of a relation between birth weight and depression has not been found, nor whether there are differences between the sexes. Rates of depression differ between men and women, so the etiology may also differ between the two sexes. Because the highest rates of depression are seen among women of childbearing age (11), we examined the associations of birth weight and gestation at birth with past treatment for depression and current depressive symptoms in young women during the peak childbearing years of 20–34 years of age in Southampton, United Kingdom.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 References
 
The Southampton Women's Survey was established in 1998 to study young women in Southampton of childbearing age and follow them through their subsequent pregnancy. We aimed to assess the influence of maternal factors operating before and during pregnancy on the subsequent growth and development of the offspring. Between 1998 and 2002, all general practitioners in Southampton were asked to help recruit their female patients aged 20–34 years. Full details of the survey have been described elsewhere (12). The recruited women were representative of the population of Southampton, which is similar to that of England and Wales except that ethnic minority groups are underrepresented; approximately 94 percent of women in the study are White (12).

The information collected at baseline included sociodemographic factors and the women's own birth weights. When women were uncertain of their birth weight, they were asked to contact their parents for the information. They were also asked whether they were born early, late, or at term and, if not at term, how many days early or late. From this information, an estimate of gestational age at birth was derived. In addition to reported birth weight, birth weights and gestational ages recorded at the time of the birth were obtained from local hospital obstetric records for all those women who had been born in Southampton.

Beginning in March 2000, all Southampton Women's Survey participants were asked at the initial interview to complete the 12-item version of the General Health Questionnaire (GHQ-12) to assess current depressive symptoms (13). The women were also asked whether they had ever received treatment for depression, anxiety, or other mental health problem in the past. For each of the GHQ-12 questions, there are four response options. The two items indicating a lower likelihood of depressive symptoms were scored as 0 and the other two as 1 (0-0-1-1 scoring method). The scores were summed across the 12 questions, and women with a score of 3 or more were categorized as potentially depressed at baseline.

We derived standard deviation scores for birth weight as our main independent risk variable and assessed the relation between birth weight and 1) whether the woman reported ever having received treatment for depression and 2) whether she was classified as suffering from depressive symptoms according to the GHQ-12. Poisson regression with robust variance (14) was used to estimate prevalence ratios and 95 percent confidence intervals for our measures of depressive symptoms in relation to the factors of interest. An analysis using categories of birth weight was also conducted to assess whether there was any evidence of a threshold effect.

Adjustment for gestational age at birth was conducted in two ways. We derived birth weight standard deviation scores first by using the British 1990 growth reference data (15) and second by using polynomial regression.

Other factors known to influence depression or that might confound the relation with birth weight were assessed in adjusted models. The factors considered were age, education, social class, perceived financial strain, and low income as defined by whether or not the woman was receiving social security benefits.

All analyses were repeated for those women for whom birth weight as recorded at birth was available from the hospital records. Exploratory analyses were undertaken relating our measures of depressive symptoms to reported and recorded gestation at birth.

The study received approval from the Southampton and South West Hampshire Local Research Ethics Committee.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 References
 
Some 75 percent of those who were contacted about participation in the Southampton Women's Survey were interviewed. Beginning on March 2, 2000, 7,210 women participated in the survey; of these, 7,020 (97.4 percent) completed a GHQ-12 questionnaire. Figure 1 gives details of numbers of women with recalled and reported birth weights and gestational ages and the numbers who reported whether they had received past treatment for depression.


Figure 1
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  		FIGURE 1. Flow chart of the numbers of participants in each stage of the study of depressive symptoms among women in the Southampton Women's Survey (SWS) interviewed between March 2000 and November 2002, United Kingdom. GHQ-12, 12-item General Health Questionnaire.

 
The mean recalled birth weight was identical to the mean reported birth weight at 3.2 kg. The proportions of low birth weights (<2.5 kg) were 10.1 percent for the recalled values and 7.3 percent for those that were reported. Among the 1,729 women with both reported and recorded birth weights, Spearman's rank correlation coefficient for the two assessments was 0.87; that for gestational ages among the 1,518 women with reported and recorded assessments was 0.62. Bland-Altman plots of the associations between reported and recorded values are given in figures 2 and 3, and both indicate reasonable agreement.


Figure 2
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  		FIGURE 2. Bland-Altman plot showing agreement between reported and recorded birth weights for 1,729 participants in the Southampton Women's Survey interviewed between March 2000 and November 2002, United Kingdom.

 

Figure 3
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  		FIGURE 3. Bland-Altman plot showing agreement between reported and recorded length of gestation for 1,518 participants in the Southampton Women's Survey interviewed between March 2000 and November 2002, United Kingdom.

 
No relation was found between birth weight and depressive symptoms. Table 1 presents the prevalence rates and prevalence ratios for current depressive symptoms and past treatment for depression in relation to birth weight standard deviation scores. Adjustment for gestational age and potential confounding variables made a minimal difference in the findings, indicating that they were not exerting a confounding effect on the association of birth weight with depressive symptoms. Only the analyses using gestational age adjusted against the 1990 British growth standard are presented in table 1, but the alternative form of adjustment for gestational age gave rise to very similar findings. Using categories of birth weight led to similar conclusions, with no evidence of any threshold effect below a particular weight.


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  		TABLE 1. Prevalence rates and prevalence ratios for depressive symptoms in relation to birth weight in women interviewed in 2000–2002, Southampton Women's Survey, United Kingdom

 
We found weak relations between longer reported duration of gestation and lower prevalence ratios for current depressive symptoms and having ever been treated for depression (p = 0.040 and p = 0.028, respectively; prevalence ratio for both associations = 0.98 (95 percent confidence interval: 0.96, 1.00), n = 5,150). Recorded duration of gestation was available for fewer women; prevalence ratios for current depressive symptoms and past treatment for depression were 0.97 (95 percent confidence interval: 0.94, 1.00) (p = 0.08, n = 2,003) and 0.96 (95 percent confidence interval: 0.93, 0.99) (p = 0.006, n = 1,999), respectively. Adjustment for potential confounding variables made little difference in the associations with reported duration of gestation, but it had some effect on associations with recorded duration of gestation (adjusted p values = 0.05 and 0.08 for current depressive symptoms and having ever been treated for depression, respectively).


   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
References
 
We found no apparent relation between our measures of depression and birth weight, whether the latter was reported by the woman or recorded in the hospital records at the time of her birth. Ours is one of the largest studies of the relation between birth weight and depressive symptoms reported to date in women, so our statistical power is high and the 95 percent confidence intervals around our prevalence ratios are narrow. We used two different assessments of depressive symptoms, unlike other studies, and found no relation with birth weight. Although we did not have recorded birth weights and gestational ages for all women in the study, reported birth weights were improved by information provided by parents, and there was reasonable agreement between the reported birth weights and those recorded in hospital records among women for whom both were available. Nonetheless, there was error in reported birth weights and gestational ages, as the Bland-Altman plots in figures 2 and 3 show, and this error might have contributed to obscuring a relation with depressive symptoms. Strikingly though, no evidence of a relation was seen in the group whose birth weights were more accurate.

Depressive symptoms were assessed in two ways, but both methods have their weaknesses; studies using standardized clinical interviews would be needed to address this limitation, although none have been reported to date as far as we know. For our first measure, we considered those women who reported ever having been treated for depression, anxiety, or other mental health problems. This measure is broader than depression per se, although most of the treatments will have been for depression or for mixed anxiety and depression (16). It provides a summary of the lifetime experience of depression of a severity that led the woman to ask for help. Although it is possible that women might underreport past treatment for depression, the question was asked in a questionnaire completed by the woman at the end of an interview with a research nurse, when considerable rapport had developed.

Our second measure, the GHQ-12, assesses current symptoms of depression and anxiety, including depression that might be undiagnosed. However, this measure is also not specific for depression because it includes anxiety and other symptoms. As a screening instrument, the GHQ-12, compared with a standardized clinical interview, has a low threshold of severity for identifying possible "cases." It is also sensitive to recent changes in psychological well-being and will include "false positives" with mild transient psychological disturbance. The prevalence of cases in this study (32 percent) is similar to rates of 27 percent and 30 percent found among women in the annual British Household Panel Surveys (17, 18), but it is lower than the rate of 22 percent found in a British community survey using a standardized clinical interview (16). It is possible that we failed to find an association with birth weight because our measures of depressive symptoms also included other symptoms, such as anxiety, which are not related to birth weight. However, this seems unlikely to be the whole explanation because previous research using questions from the GHQ-12 did find an association with birth weight (6).

Our results contribute to the studies that have examined the relation between birth weight and depressive symptoms. Although a number of studies have reported significant associations, this finding is not universal, and no association was found in a particularly large study of men (4). Additionally, some studies have pointed to associations for women but not for men, or vice versa. Notably, our age group of 20–34 years is not dissimilar to that in the two studies that found associations for women but not for men in their twenties (1, 2). However, our study, which focused solely on women, could not address the question of whether associations between birth weight and depression differ for men and women.

These various studies are providing a confusing picture of whether a relation between birth weight and depressive symptoms exists and point to a need for a systematic review of the literature in this area. It is possible that positive publication bias has led to studies reporting an association to be overrepresented in the literature; if so, the relation might be weaker than the published studies indicate. Conversely, however, birth weight is a poor proxy summary of development in utero, and there may be detrimental aspects of fetal development that are only weakly summarized by birth weight but that contribute to the development of depression in adulthood. In exploratory analyses, we found some evidence for a weak association between a shorter duration of gestation at birth and later depressive symptoms, although the relation is not as strong as reported in an older Finnish cohort (11). Limitations in our gestational age data (either reported or abstracted from the clinician's assessment in the labor ward records) preclude detailed exploration of this association in our study, but we encourage its examination elsewhere. The issue of whether maternal depression in pregnancy or postnatally affects risk of depression in the offspring or alters the relation between birth weight and depression has been addressed, but no association was found (1). It appears that greater understanding of fetal development and early maternal and other influences on the fetus and child is needed. Studies recording information on such developmental influences are required before the early origins of depression can be assessed to a greater extent.


   ACKNOWLEDGMENTS
 
The Southampton Women's Survey is grateful for financial support from the UK Medical Research Council, the University of Southampton, and the Dunhill Medical Trust.

The authors are grateful to the general practitioners in Southampton who made this study possible and to the Southampton Women's Survey staff.

H. M. I., K. M. G., and C. C. designed and managed the Southampton Women's Survey. T. K. originated the idea for the depression work within the cohort, supported by N. D. H. M. I. conducted the analyses and wrote the first draft of the paper. All authors contributed to the interpretation of the findings and to the writing of the paper. All approved the final version of the manuscript.

Conflict of interest: none declared.


   References
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 References
 

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