Fiber helps prevents constipation and gestational diabetes, two very common issues during pregnancy. High-fiber foods are digested slowly and have a more gradual effect on blood sugar and insulin. Studies have found that high insulin levels appear to inhibit ovulation. Fiber also lowers blood pressure and reduces the risk of preeclampsia. Best Sources: Beans, lentils, dark leafy greens, vegetables, berries, pears, apples, avocados, oats, quinoa, chia seeds, flax seeds.