I am evangelical about fish oils (!) If you are going to take a supplement it's worth paying for the best you can get. Although I'll sometimes pick up vitamins from Boots when I'm saving money, certainly in pregnancy I go for really good brands like Solgar (they do a great vitamin - Prenatal Multivitamin and mineral Formula for Pregnant and Lactating Women), Higher Nature and Bio-care.
Have tried a few brands and my fave and according to several nutrition experts I have spoken to, one of the best, is Higher Nature.
It's the highest purity fish oil taken from the body, not the liver of the fish. It has the key Omega 3 fatty acids, EPA and DHA, important for a healthy heart and arteries, the brain, eyes and flexible joints. It's also scrupulously screened for pollutants. They're 1000mg and the dose is up to three, I tend to take two.
I take these anyway, especially when I am stressed or run down, but especially in pregnancy to boost the baby's brain too.
You can shop around online and have them delivered within a few days, online pharmacies do varying deals, the bigger the jar the better value.
HTH