What kind of prenatal vitamin is she taking? If it's a prescription brand, then it is quite likely that it is way too high in iron, which could be constipating her. I recommend that she switch to the "Whole Foods" brand of prenatal vitamin (which requires taking 3 tablets a day, preferably with meals).
Prescription prenatal vitamins are also notoriously low in B-vitamins, in spite of the fact that as soon as conception happens the mother's need for the B-vitamins skyrockets. Constipation can also be caused by a B-vitamin deficiency. The "Whole Foods" prenatals have more healthy levels of the Bs.
She can also try cutting out all caffeine from her diet, because caffeine destroys B-vitamins, contributing to a B-vitamin deficiency. You can see more about B-vitamins in pregnancy by reading this page (scroll down)...
http://home.mindspring.com/~djsnjones/id1.html
Is she on diuretics, either prescription or herbal? They can cause constipation and also are very hazardous in pregnancy...
http://home.mindspring.com/~djsnjones/id86.html
She can also try taking a little extra magnesium, or make sure that her calcium supplement has magnesium in it. She should make sure that she not take too much, or she could get diarrhea.
She can also try eating 1/2 cup of yogurt with each meal. This will soften her BMs.
Is she in a high-stress situation? High levels of stress can deplete one's body of B-vitamins, and potassium, and low potassium can help to cause constipation. Foods that can help with this potassium deficiency are dried apricots, avocados, cantaloupe, dried dates, orange juice, papaya,raisins, prunes, prune juice,rhubarb, and watermelon.
She can try eating foods that are high in pectin, like apples, carrots, beets, bananas, cabbage, citrus fruits, dried peas, and okra.
She can also work at avoiding all refined and processed foods and focus only on foods that she cooks herself from scratch, so that they still contain optimal levels of fiber and nutrients. She can buy whole-grain breads and cereals, checking to see if their fiber content is at least 4 grams per serving, or at least the highest that she can find (not white bread).
She can try cutting down on her added-sugar intake, especially from sodas (drinking milk and vegetable juices instead).
She can also work on eliminating fried foods from her diet.
She can try eating something every hour instead of three big meals a day. Each hourly snack should include one protein item and one raw or dried fruit or vegetable -- like 1 c milk and a handful of raisins, or a handful of almonds and a handful of dried dates, or an egg and a handful of baby carrots, or a handful of pumpkin seeds and a handful of dried apricots, or a handful of sunflower seeds and a handful of prunes, or a handful of almonds and a couple of slices of cantaloupe, or a peanut butter sandwich on 100% whole grain bread with a couple of slices of watermelon. She can eat larger servings of these foods if she is more hungry.
She can try taking a 20-min walk every day, which helps the food move through the bowels better.
She can take 250-500 mg of Vitamin C with each meal (or 3 times a day with food).
If her birth attendant wants her to take some kind of iron supplement, she can switch away from the iron salts that s/he may be giving her and instead use Floradix (available at Whole Foods stores)...
http://www.florahealth.com/flora/home/usa/products/r64771...
Best wishes,
J.