Teratogenic effects of Silastic intrauterine devices in the rat with or without added medroxyprogesterone acetate.

The teratogenicity of intrauterine devices (IUDs) made of silicone rubber (Silastic, Dow Corning Corporation, Midland, MI) with or without added medroxyprogesterone acetate (MPA) has been investigated in the rat. Small rod-shaped IUDs were inserted into the uterus, one between each embryo, on day 9 of pregnancy and left in place until the rats were killed just before term for examination of the fetuses. MPA exposure caused masculinization of the external genitalia of female fetuses and feminization of the external genitalia of male fetuses. There was no increase in other, nongenital malformations in MPA-exposed fetuses, compared with fetuses exposed to Silastic alone, but both Silastic-exposed groups had significantly more malformations than untreated control rats. In a second experiment, a significant increase in malformations in fetuses exposed to Silastic alone, compared with untreated control fetuses, was confirmed. The malformation rate in control rats that underwent sham operations was not significantly increased, compared with untreated control rats.