I was recommended by my doctors to take the Enfamil Expecta DHA.(but this may be due to them getting paid to promote the product? I don't know. They always give out samples.) Anyway, I've taken it with all my pregnancies and it doens't upset my stomach like regular fish oil caps do. Also, you don't have to worry about mercury. It's a little on the expensive side, varying from $10 - $15/mo. I ordered mine on Amazon where it was only $20 for a 2 month supply.
i don't remember my research on this fully, but i do recall that plant derived ALA's are not biologically equivalent to marine derived DHA. i take fish oil supplementation for DHA. the only concern there is impurities like mercury present in the fish oil - there are brands that make DHA capsules that have been purified for this purpose. some brands will even go as far to source their oils from the purest fish and highest standards, so that purification isn't even necessary.
i take this one. it is made specifically for pregnancy:
http://www.vitaminshoppe.com/store/en/browse/sku_detail.jsp?id=YE-1021
hope this helps!