Flax seed is a rich source of omega 3 fatty acids (alpha-linolenic acid, ALA) and lignans. Some of the health benefits of omega 3 fatty acids are decrease in low density lipoprotein, LDL, cholesterol, and total cholesterol, lower risks of incidence of cardiovascular diseases and inflammation.

Omega-3 fatty acids may also reduce risks of menstrual syndrome and postmenopausal hot flushes.

Flaxseed consumption during pregnancy could be beneficial for fetus brain development.

Flaxseed provides lignans, phytoestrogen, which are estrogen-like phytochemicals. Therefore, flax seed may alter hormonal balance during pregancy. And since women during pregnancy and fetus are in their hormone-sensitive period, caution is advised on the dosage of flax seed intake.

Most information on flax seed (and flax seed oil) and its effect on pregnant women is inferred from studies on animals. In animal studies, intakes of high dosages of flax seed have caused reproductive complications.

On the other hand, foods high in omega-3 fatty acids, to which flax seed is a major source, are recommended during pregnancy and during breastfeeding, forexample to enhance the baby's brain development.

Other food sources of omega 3 fatty acids are fish (examples: salmon and shrimp), Neptune krill oil, cod liver oil, chicken, egg, meat and vegetables such as walnuts, chia, purslane and hemp.

Research on the role of flax seed on prevention or treatment of breast cancer in postmenopausal women is not conclusive. However, there are indications that flax seed lignans may prevent breast cancer in postmenopausal women. Flax seed oil also has been shown to reduce mammary tumor growth.

Flax seed lignans also affect estrogen metablosm.

References:
Jackson, A. A. and Robinson, S. M. 2001. Dietary guidelines for pregnancy: a review of current evidence. Public Health Nutrition: 4(2B), 625-630

Bourre JM. 2007. Dietary omega-3 fatty acids for women. Biomed Pharmacother. 61(2-3):105-12.