What number of 6 oz servings of swordfish would a mother have to eat to have high enough blood levels of mercury to cause developmental problems?

Nobody knows: Nobody has been able to demonstrate an effect of mercury in ordinary fish-eating even in countries where a lot is consumed (the Faroes, the Seychelles, Australia). The EPA advice against all women and children eating any swordfish is political -- and no surprise from an agency that once declared sandbox sand a cause of cancer. I find no realistic basis for fear.