At present, there is not scientific support for saying if amalgam is or isn't safe. A continued use of amalgam (as well as other materials) must be based on a calculation of risks and here a special concern has been given to mercury exposure to developing foetuses.
The Swedish National Board of Health and Welfare has recommended, as a precaution, that amalgam-work should be avoided, as far as possible, for pregnant women (Socialstyrelsen 1991).
Drasch (1994) has even suggested that the use of amalgam in women before, and in, childbearing age should be reconsidered.
The WHO Study Group writes (WHO 1991):

"Some studies have found miscarriages and abortions after occupational exposure to mercury, but other studies did not confirm these effects. The WHO Study Group in 1980 stated; "The exposure of women of child-bearing age to mercury vapour should be as low as possible. The Group was not in a position to recommend a specific value" (WHO 1980). This statement is still prudent and will remain so until new data become available"

M Berlin writes (Berlin in:Friberg 1986 page 402):

"In view of the fact that mercury vapor easily traverses the placenta, it cannot be precluded that exposure to mercury vapor during pregnancy gives rise to brain damage or results in inhibition of brain development of the fetus. However, experimental or clinical observations which allow conclusions concerning the level of exposure at which such damage may occur, are not available"

And NIH writes (NIH 1992 ):

"Estimating these effects of mercury from dental amalgam based on the current literature is difficult because of the following:... Susceptibility of the CNS can vary with critical periods of brain development. Experience with other agents damaging the nervous system (e.g., lead, radiation, alcohol, etc.) has demonstrated that we might expect a wide range of potential CNS dysfunction, depending on the stage of development at the time of exposure."

Also make sure to read these books: Poison in Your Teeth: Mercury Amalgam (Silver) Fillings...Hazardous to Your Health! and Mercury Detoxification by Tom McGuire