Omega 3 polyunsaturated fatty acids (PUFAs) play a vital role in human department and health. There are 4 important omega -3 PUFAs. Of these DHA is very important for baby- growth per say, prevention of preterm delivery, brain growth and for the eye-retina. Fish and fish oils are rich in omega 3- especially EPA and DHA. Some other omega 3 are present in leafy vegetables, walnuts and soyabeans. Numerous organizations recommend that intake of omega 3 should be supplemented in diet, especially in pregnant and lactating women and those who do not eat fish.
The International Society for the Study of Fat and Lipids states the adult requirement of DHA is 220 mg per day and 300 mg prer day for pregnant and lactating women. There a number of proprietary brands selling omega 3 in multivitamin tablets. Please consult your obstetrician for one such brand.
There are numerous causes for not gaining adequate weight during pregnancy- most important being deficient diet and malabsorption. Your doctor will investigate this and guide you.