Cement

The materials most frequently used for direct dental restoration are amalgam, composite resin (plastic and glass), the glass ionomer (glass + organic acid) and the cement ionomer resin (resin + glass + organic acid).
Those used for indirect dental restoration (inlays, crowns, bridges and frames) mostly are porcelain pottery, porcelain fused to metal, gold alloys and copper and nickel alloys, cobalt and chromium.

Resins are biocompatible and safe. Glass cements and other dental materials (porcelains, gold alloys, nickel, cobalt and chromium) are nontoxic and safe.

Amalgams contain mercury (Hg), silver, tin and copper. Hg vapors are continually shed from fillings, mostly when chewing (especially chewing-gum) and drinking very hot liquids, increasing Hg concentrations in plasma and milk of nursing mothers.

There is controversy about how much mercury from amalgams actually passes into blood and breast milk (Roberts 2009).
For some authors it would be excessive with a daily intake of Hg higher than recommended by FAO / WHO (Norouzi 2012, da Costa 2005).
For others, it would be very small, even much smaller than that passing through eating fish and far below than that occurring through placenta during pregnancy, so that the levels of mercury found in cord blood which drop gradually in infants, whether breastfed or not, during the first weeks after birth (Drexler 1998 Drasch 1998).

Wearing of amalgam dental fillings does not contraindicate breastfeeding (Drasch 1998).
It is not convenient to place or remove amalgam fillings during pregnancy and lactation, otherwise, do so with proper precautions, because mercury vapors are released (FDA 2015, Ekstrand 1998) and may pass through inhalation into blood and breast milk.

Mercury exposure, both chronic and excessive, may have adverse health effects, but there is much controversy about the effects of minimum exposures such as amalgam fillings. In some countries of northern Europe and the USA an informed consent is required about the material to be used for dental filling procedures (Edlich 2007).

Despite the alarmism originated by some, there is no scientific basis for pointing out to such fillings as a cause of any disease (Mitchell 2005, Brownawell 2005, Ekstrand 1998), however, it is a goal of WHO the reduction and elimination of mercury exposure worldwide.
Dental amalgams fall within that policy of progressively mercury elimination: those amalgam fillings that are still in good shape must be left in place, whenever a replacement is needed, a resin or cement must be used.

The benefits of breastfeeding outweigh the risk that would be presented in low levels of environmental contaminants in human milk, which in many cases are lower than those appearing in cow's milk based formulas and other foods (WHO).