Copyright © 2004 by the Johns Hopkins Bloomberg School of Public Health
LETTERS TO THE EDITOR |
FOUR AUTHORS REPLY
1 Department of Preventive Medicine and Biometrics, University of Colorado Health Sciences Center, Denver, CO 80262
2 Center for the Study of Complementary and Alternative Therapies, University of Virginia Health System, Charlottesville, VA 22908-0905
We welcome this opportunity to respond to the comments of Drs. Harder and Plagemann (1) on our study (2). These authors argue that 1) adjustment for prepregnancy body mass index resulted in incorrect estimates of the association between maternal birth weight and risk of pregnancy-induced hypertension (PIH); and 2) we gave insufficient attention to the importance of high birth weight as a risk factor for PIH. We disagree.
Harder and Plagemann point out that because high birth weight is itself a predictor of large adult body size, adjustment for prepregnancy body mass index may constitute adjustment for a causal intermediate factor (1). We concur. In our paper (2), we explicitly stated in the Abstract, in the Results section, and in the Discussion that we found a U-shaped association between birth weight and PIH risk in unadjusted analyses, with the greatest risks being associated with very low and very high birth weights. We presented both unadjusted and adjusted odds ratios in the tables. We did not assert or imply that the association between PIH risk and high birth weight was spurious. However, we did conclude that this relation was explained by high prepregnancy body mass index. Thus, in our paper, we implicitly agreed with the conclusion of Harder and Plagemann that prepregnancy body mass index appears to be on the causal pathway between high birth weight and PIH risk.
Harder and Plagemann would have us place exclusive emphasis on the unadjusted risk estimates associated with maternal birth weight (1). However, that analysis, if presented alone, could well be criticized by others as having not adequately separated the effects of early life factors from the effects of later life factors. Maternal birth weight is a relatively novel risk factor for PIH, in contrast to adult obesity, which is a well-established risk factor. Therefore, we believe it is important to distinguish the effects of birth weight from the effects of adult body mass index, especially since these two factors have often (but not always) been linked. Moreover, while prepregnancy body mass index appeared to mediate the effect of high maternal birth weight on PIH risk in our study (2), it did not explain the influence of low birth weight. This and other interesting findings, which may be informative as to causal pathways, require consideration of both the unadjusted and adjusted risk estimates.
Finally, while we agree that very high birth weights pose potential clinical and public health concerns, using high birth weight as a clinical marker of increased PIH risk would be of limited practical utility. If, as our study suggests, the positive association between high birth weight and PIH risk is due almost entirely to its association with high prepregnancy body mass index, the better and more direct risk marker would be adult body mass index per se. In addition, although high maternal birth weight may affect a woman’s risk of PIH by predisposing her to obesity, high birth weight is only one of many factors that contribute to adult obesity, most of which are more easily modifiable. Also important is our observation that high birth weight carried no excess risk for PIH among women who were not overweight.
We maintain that the analyses presented in our paper are appropriate and informative from both an epidemiologic and a clinical standpoint. We are especially gratified that the risk estimates we presented supported a correct conclusion by Drs. Harder and Plagemann about the interplay among high birth weight, prepregnancy body mass index, and risk of PIH.
REFERENCES
- Harder T, Plagemann A. Re: "Association of a woman’s own birth weight with her subsequent risk for pregnancy-induced hypertension." (Letter). Am J Epidemiol 2004;160:507–8.
[Free Full Text] - Innes KE, Byers TE, Marshall JA, et al. Association of a woman’s own birth weight with her subsequent risk for pregnancy-induced hypertension. Am J Epidemiol 2003;158:861–70.
[Abstract/Free Full Text]
CiteULike 
Connotea 
Del.icio.us What's this?
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||