Catov et al. Respond to “Troubling Trends in Birth Weight”

There are now at least 5 reports in addition to ours (1) that indicate that infant birth weight might be decreasing in developed countries. This reverses a 50-year trend of increasing birth weight. We examined whether these trends are similar for both infants born to black and to white women and whether they persist after accounting for maternal conditions linked to abnormal fetal growth. The results were, indeed, troubling, as noted in the commentary by Dr. Weinberg (2). She agrees with the potential importance of our findings but suggests alternative explanations, which encourages a dialog about how future studies can resolve these questions.

An important and creative suggestion posed by Dr. Weinberg is designing sibling studies to resolve problems with population studies. This approach, however, raises other key considerations. Our results indicated that birth weights among second-order births were higher than those among first births, as expected, but that second-born children had lower birth weights in later years of the study than in earlier years. First-born children are one of the few groups in whom temporal increases in birth weight were seen. We interpreted this to mean that women accumulate factors with each pregnancy that might influence fetal growth (e.g., increased age, weight, environmental exposures, or stress). These time-course associations might be masked by the overall increase in birth weight associated with parity, and thoughtful approaches are needed to disentangle competing risks.

Studies of birth-weight trends in diverse settings are also needed. The results from our large hospital registry study are remarkably consistent with those from a study in which US vital statistics data were used (3) and those from a single-state study (4). Similar results have been reported in Japan (5), France (6), and Germany (7); results from births in China yielded a different trend (8). Changes in dating of gestational ages, prepregnancy body mass index, gestational weight gain, nutritional status, access to health care, and environmental exposures could all contribute in competing ways to fetal growth as measured by birth weight. Further studies in different economic, environmental, and health care settings will be necessary to separate the discrete effects of each.

We are challenged to seek factors with sufficient impact to affect average birth weight. Increasing body mass index and the concomitant nutritional factors involved in this metabolic transition may be profound enough to be reflected in population-level birth weight trends. This possibility must be explored further, and other potential sources with large effects on fetal growth must be considered.

Biologic mechanisms that might be involved also warrant study. Can telomere biology and epigenetic effects play a role? It will also be important to study other markers of fetal growth and nutrient transport, such as those that can be determined via ultrasound, and emerging technologies to evaluate placental function in vivo.

We also need to explore, as Dr. Weinberg insightfully reminds us, whether trends in newborn health are changing over time. Large longitudinal childhood studies in different time epochs are needed to know how much birth weight reflects a trajectory of growth that is linked to improved or worsening lifelong health status. Lastly, we need large studies in racially diverse populations to determine whether the race differences in birth weight trends that we detected are real.

As noted by Dr. Weinberg, birth weight has been studied for almost 100 years because it is consistently reported and measured well. It is, however, a proxy for underlying processes. That is both its power and its challenge. Changes in birth weight can guide our efforts to understand contributors to health and disease. The explosion of developmental origins of disease research is perhaps the most prominent example of this. We thank Dr. Weinberg for her reminder to interpret our results cautiously and look forward to future studies that contribute new insight into how newborn and childhood health may or may not be linked to trends in birth weight.