Allostatic load, or the “wear and tear” on the body due to stress, is thought to have a negative impact on length of pregnancy and contribute to health disparities in preterm birth. However, the magnitude of the effect on birth outcomes is unknown, in part due to questions of timing of measurement of allostatic load during pregnancy. This study used linear regression analysis of data from 156 pregnant women to test whether allostatic load is a predictor of length of gestation in the study sample, finding that third trimester allostatic load predicted length of gestation among women with full-term births. The study also compared allostatic load in each trimester to determine an optimal time of measurement for prediction of preterm birth. Findings were inconclusive because regardless of trimester of measurement, allostatic load was not a significant predictor of gestational length in the sample. Finally, the study compared allostatic load with scores on the Everyday Stressors Index, a psychosocial measure, to understand the relative benefits of allostatic load measurement during pregnancy. Neither was found to be a statistically significant predictor of preterm birth, so direct comparisons were not possible. Implications and suggestions for future research are discussed.