The timing hypothesis and body proportionality of the intra-uterine growth retarded infant.

Intra-uterine growth retardation (IUGR) is a heterogeneous designation, as seen in the fact that neonatal morbidity and mortality differ for IUGR infants that are disproportionate vs proportionate based on the ponderal index (weight/height(3)). Much less is known, however, concerning the etiology of these two forms. This study tests the hypothesis that the form of IUGR (proportionate vs disproportionate) is related to the timing of stress during gestation, specifically, that linear growth is compromised by second trimester stress and the ponderal index is compromised by third trimester stress. This hypothesis is tested using data on 755 full-term mother-infant pairs studied prospectively in rural northern Malawi where the seasonal stress of the pre-harvest rainy season (PHRS) is a regular occurrence. The results indicate 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 third 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 linear growth may precede or be more sensitive to prenatal stress than the ponderal index. Am. J. Hum. Biol. 11:638-646, 1999. Copyright 1999 Wiley-Liss, Inc.