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blood or serum silicon or silicone concentrations in women with silicone breast implants, and elevations reported in two studies have been modest and have not been confirmed by subsequent studies (see below and Chapter 5 of this report).

The committee is not aware of any studies of reproductive or teratologic effects of silicone in humans. However, reproductive and fetal developmental effects of polydimethylsiloxane (PDMS) fluid have been evaluated in rats and rabbits, and mutagenic potential was evaluated in mice. Teratologic and mutagenic effects were not observed at the dose levels and in the species employed (Kennedy et al., 1976). Subcutaneously implanted silicone elastomer and silicone gel at several dose levels did not induce maternal or developmental toxicity before or during pregnancy and lactation, did not have adverse effects on parents or neonates, and did not impair reproductive performance in either male or female rats or pregnant female rabbits (Siddiqui et al., 1994a,b). These and other relevant studies are reviewed in Chapter 4. Evidence from these studies for toxic effects of silicone during or after pregnancy is lacking.

Effects During Lactation

Effect On Breast Milk

Many drugs and chemicals that appear in the maternal circulation may be detected in breast milk (Berlin, 1989). Characteristics that affect a compound's ability to traverse the mammary gland epithelium, appear in human breast milk, and become available to a nursing infant include its degree of ionization, molecular weight, lipid solubility, and protein-binding capacity. Except in the rare event of direct rupture of a deposit of silicone into a milk duct (Leibman et al., 1992; Shermis et al., 1990), to be transferred to breast milk, silicone must diffuse or be transported across a number of cell membranes. The evidence reviewed in Chapter 4 does not support diffusion or transport of silicone gel across membranes that presumably would exclude substances of high molecular weight. The evidence does suggest limited mobility of lower molecular weight linear or cyclic species, but these compounds are present in very low concentrations in breast implants, do not appear to be highly mobile in experimental distribution studies, and are subject to the body's clearance mechanisms.

Some proteins from maternal or external sources have been found in milk—for example, cows' milk proteins and maternal immunoglobulin G (IgG)—and these proteins can be found in the serum of breast-feeding infants. Transport of these proteins probably occurs through clefts between mammary alveolar cells (Berlin, 1989). Most other proteins do not

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