I'm not a doctor or any expert but from my studies the best thing a pregnant woman can do is to have her doctor order a blood test that says what she may be deficient in and then that is what she will need to boost. An occasional Omega 3 supplement can only boost things.
Omega-3 fatty acids reduce the risk of premature birth. A new review has found that increasing the intake of omega-3 long-chain polyunsaturated fatty acids (LCPUFA) during pregnancy reduces the risk of premature births. Source 7d.