I read studies about mercury and the effects on a pregnant woman and her baby. Essentially they said that mercury crosses the placental barrier to the baby, but once it does so, it stays in the baby and cannot cross back to the mother. So it builds up in the baby and the baby can have higher levels of mercury than the mother.
I would be extremely cautious about anything that displaces mercury in pregnancy. Mercury is very toxic, especially to a developing baby. Definitely keep the dose low.