Vegetarians have an advantage over non-vegetarians since a vegetarian diet is usually very high in folic acid.
Foods that have a lot of folic acid are:Citrus FruitsGreen Leafy Vegetables

Calcium supplements can interfere with zinc absorption. The best time to take a calcium supplement is between meals.

Beans and tofu
Whole grain
Peas
Broccoli
Blackstrap molasses
Eggs

Fortified meat and milk substitutes (read the labels for amount of B12 added)
Dairy
Eggs
Red Star Nutritional yeast

Tofu made with calcium
Almonds

Sesame seeds
Oranges
Dried figs
Leafy green vegetables
Broccoli
Dairy
Fortified dairy substitutes
The nutritional composition of breast milk is much the same no matter what kind of diet the mothers have. However, nursing mothers need adequate calories and liquid to produce enough milk, and must eat well to stay healthy while they are nursing. Drinking enough liquids and taking a good prenatal supplement during pregnancy and breastfeeding is important too.
Plan for adequate nutrition during pregnancy, from "Becoming Vegetarian", Vesanto Melina, R.D. et al:
9 servings of grains
8 servings of fruits and vegetables
6 Servings of milk or foods that are equivalent
3 servings of beans or foods that are equivalent
2 servings of good sources of omega-3 fatty acids (pumpkin and flax seeds, walnuts, soybeans, and dark green vegetables)

This may seem like an overwhelming amount of food until you consider that many foods fall in more than one category. For instance walnuts fall into the fatty acid and bean category. And you often don't have to eat that much for to make a serving. For instance a serving of vegetables is ½ cup cooked or 1 cup raw. 1 slice of bread, or ½ c. of cooked grains is a serving. So if you eat 2 slices of toast and a cup of oatmeal for breakfast, that's four grain servings.