i just found this
Omega-3 and Pregnancy
Pregnant women are often told to take fish oil capsules but must be careful to get the best quality, instead of cheap, generic stuff. The cheap fish oil may come from fish with contamination and may not be free of pollutants. Women should be careful about the types of fish and fish oil they consume when they are pregnant, nursing, or even when experiencing pregnancy symptoms.
Flaxseed oil does have omega-3 in it and is preferred by some people however some doctors say flaxseed oil should not be taken by pregnant women.
The reason is that flax seed, like soy or even more so, seems to have an effect on estrogen and hormones and might interfere with your body during pregnancy. Ask your doctor.
One study on this was: Brooks JD, Ward WE, Lewis JE, et al. Supplementation with flaxseed alters estrogen metabolism in postmenopausal women to a greater extent than does supplementation with an equal amount of soy. Am J Clin Nutr 2004;Feb, 79(2):318-325.
so id ask your gp or midwife