DHA supplement?

Do you take dha supplements during pregnancy? I have read many places that dha is important before and during pregnancy (much like folic acid), however it doesnt seem to be included in many prenatals and my doctor never mentioned it. I have been taking a prenatal that of course contains folic acid, but no dha. I picked up a new prenatal that includes dha and i currently plan to rotate that with the prenatal i already had. I am a little worried that i wasnt getting dha before pregnancy or the first 6 weeks. If its so important why doesnt it seem to be included in so many prenatals?