The role of early allergen exposure has been a central focus in the effort to curtail the rising rates of allergic disease: Exposures in pregnancy have been of interest because the first signs of disease are often seen in early infancy. However, so far, strategies to avoid or reduce allergen exposure in pregnancy have had disappointing results in reducing disease. Although there is growing evidence that the fetus is exposed to allergens, this appears to be physiologic and not a risk factor for allergic disease: It is still not clear if this exposure has some role in the context of normal tolerance. Although infants who later develop allergic disease show some differences in neonatal allergen-specific responses; these are not consistent. Furthermore, the magnitude and frequency of responses do not correlate well with maternal allergen exposure, and it now seems likely that these responses do not reflect true immunologic "memory." Despite this; there is accumulating evidence that a number of other exposures in pregnancy have the capacity: to significantly influence immune development and alter the risk for allergic responses to allergens. This underscores the importance of further defining the events in this early period, to determine causal pathways and better strategies for prevention.