Before and during pregnancy. You need more of certain nutrients than usual to support your health and your baby’s development. These nutrients include protein, calcium, iron, and folic acid. Many doctors recommend prenatal vitamins or a folic acid supplement during this time. Many health insurance plans also cover folic acid supplements prescribed by your doctor during pregnancy. You also need to avoid some foods, such as certain kinds of fish. Learn more about healthy eating during pregnancy in our Pregnancy section.
A spokesperson for optimal nutritional intake is the well-known biochemist Bruce Ames, who proposed the "triage theory of nutrients," in which enzymes responsible for cell maintenance functions evolved to have lower affinity for the essential vitamin and mineral cofactors than the enzymes responsible for short-term survival, to preserve life during times of famine. 
On a related note, if you’re thinking “I’ll just put less food on my plate” … it’s not that simple. The picture below explains why. When you eat a small portion off of a large plate, your mind feels unsatisfied. Meanwhile, the same portion will feel more filling when eaten off of a small plate. The circles in the image below are the same size, but your brain (and stomach) doesn’t view them that way.
^ Dwyer, J. T; Wiemer, K. L; Dary, O; Keen, C. L; King, J. C; Miller, K. B; Philbert, M. A; Tarasuk, V; Taylor, C. L; Gaine, P. C; Jarvis, A. B; Bailey, R. L (2015). "Fortification and Health: Challenges and Opportunities". Advances in Nutrition: An International Review Journal. 6 (1): 124–131. doi:10.3945/an.114.007443. PMC 4288271. PMID 25593151.
A better approach is to make a few small changes at a time. Keeping your goals modest can help you achieve more in the long term without feeling deprived or overwhelmed by a major diet overhaul. Think of planning a healthy diet as a number of small, manageable steps—like adding a salad to your diet once a day. As your small changes become habit, you can continue to add more healthy choices.
This study secretly took place in the hospital cafeteria and helped thousands of people develop healthy eating habits without changing their willpower or motivation in the slightest way. Thorndike and her team utilized a concept known as “choice architecture.” Choice architecture is just a fancy word for changing the way the food and drinks are displayed, but, as it turns out, it makes a big difference.
In addition to diet, exercise and other lifestyle factors can also play an important role in bone health. Smoking and drinking too much alcohol can increase your chances of developing osteoporosis, while weight-bearing exercise (such as walking, dancing, yoga, or lifting weights) can lower your risk. Strength or resistance training—using machines, free weights, elastic bands, or your own body weight—can be especially effective in helping to prevent loss of bone mass as you age.
A healthy vegetarian diet falls within the guidelines offered by the USDA. However, meat, fish and poultry are major sources of iron, zinc and B vitamins, so pay special attention to these nutrients. Vegans (those who eat only plant-based food) may want to consider vitamin and mineral supplements; make sure you consume sufficient quantities of protein, vitamin B12, vitamin D and calcium. You can obtain what you need from non-animal sources. For instance:
Part of the reason why so many women fail to get the amount of iron they need is because one of the best sources of iron is red meat (especially liver) which also contains high levels of saturated fat. While leafy green vegetables and beans are also good sources of iron—and don’t contain high levels saturated fat—the iron from plant foods is different to the iron from animal sources, and not absorbed as well by the body. Other foods rich in iron include poultry, seafood, dried fruit such as raisins and apricots, and iron-fortified cereals, breads, and pastas.