This study is to examine the change of fasting insulin, glucose, insulin sensitivity and related traits in response to the 14 wk treatment of omega-3 fatty acids, including fish oil (n=25) and flaxseed oil(n=25), in Chinese gestational diabetic patients. Corn oil(n=25), rich in omega-6 fatty acids, will be selected as a controlled oil. The intervention will start from the third trimester of these pregnant women and finish at the 4wk after the birth of their children. The investigators hypothesize that omega-3 fatty acids could improve insulin sensitivity and glucose metabolism in these patients.