Fish oil is a better choice. See the button on the left menu of this site that says "Omega-3 Fish Oil" for a comparison of why fish oil is better for pregnancy than flaxseed oil.
Be very careful and ask your doctor's advice before taking flax seed oil when you are pregnant. Flax might have an effect upon the body's estrogen metabolism. That might be good if you are in menopause but your hormones might not something to tinker with when you are pregnant.
The UC Berkeley Wellness Letter website says "Plant estrogens, like human hormones, are not always benign."
The United States government website of the U.S. National Library of Medicine and the National Institutes of Health says:
"The use of flaxseed or flaxseed oil during pregnancy and breastfeeding is not recommended. Animal studies show possible harmful effects, and there is little information in humans. Flaxseed may stimulate menstruation or have other hormonal effects and could be harmful to pregnancy."