Maternally mediated exposure of the fetus: In utero exposure to lead and other toxins

The purposes of this paper have been to point out the potential significance of nonconcurrent - specifically pregestational - exposure of the mother for the subsequent exposure of the embryo or fetus to toxic chemicals. In evaluating the risks of developmental toxins, it is customary to focus attention on those exposures, which are concurrent with gestation or, in the case of mutagens, to exposures which may affect the germ cells of either parent. However, the presence of long-lived chemicals or metals in stable depots in the mother points to the potential for uterine exposures to occur to these chemicals, which were taken up by the mother long before gestation began. Several variations in physiological function may mobilize stored chemicals: changes in nutritional status; significant loss of body weight or depletion of fat stores; hormonal modulation; age; administration of chelating agents and certain drugs; and illness. The changes associated with pregnancy itself may result in mobilization of certain stored substances, such as lead stored in bone. These observations raise significant questions for further consideration. First, the assessment of exposure in terms of evaluating potential risk for the developing organism may not be accurate unless compartments are measured which summate exposure, such as bone or teeth for lead or fat for long-lived halogenated hydrocarbons. Second, strategies which are based upon keeping levels of chemicals in rapidly turning over compartments (such as blood) below a certain level may not adequately protect against such risks. In this respect, the standard policy of medical removal for occupations with lead exposure is based upon keeping workers' blood lead levels below a certain level 930 or 40 mcg/100 ml). This does not protect against the accumulation of lead in bone under conditions of elevated but 'acceptable' exposures. Third, and most significantly, the possibility that pregestational exposures of the mother may affect her fetus many years after the cessation of external exposure raises issues of balancing optimal protection of the fetus against reducing barriers to employment for women.