CDC:
GUIDELINES FOR THE IDENTIFICATION AND MANAGEMENT OF LEAD EXPOSURE IN PREGNANT AND LACTATING WOMEN

Because there is no apparent threshold below which adverse effects of lead do not occur, CDC has not identified an allowable exposure level, level of concern, or any other bright line intended to connote a safe or unsafe level of exposure for either mother or fetus.

http://www.cdc.gov/nceh/lead/publications/LeadandPregnancy2010.pdf?source=govdelivery