The timing hypothesis and body proportionality of the intra-uterine growthretarded infant

Intra-uterine growth retardation (IUGR) is a heterogeneous designation, as seen in the fact that neonatal morbidity and mortality differ for IUGR infa nts that are disproportionate vs proportionate based on the ponderal index (meight/heights). Much less is known, however, concerning the etiology of t hese two forms. This study tests the hypothesis that the form of IUGR (prop ortionate vs disproportionate) is related to the timing of stress during ge station, specifically, that linear growth is compromised by second trimeste r stress and the ponderal index is compromised by third trimester stress. T his hypothesis is tested using data on 755 full-term mother-infant pairs st udied prospectively in rural northern Malawi where the seasonal stress of t he pre-harvest rainy season (PHRS) is a regular occurrence. The results ind icate that exposure to PHRS in the second trimester is not associated with weight, length, or the ponderal index at birth. Exposure to PHRS in the thi rd trimester is associated with diminished birth weight and length, but not ponderal index. The results do not support the timing hypothesis, at least as previously framed in the literature, and suggest the possibility that l inear growth may precede or be more sensitive to prenatal stress than the p onderal index. (C) 1999 Wiley-Liss, Inc.