Straining won't lead to miscarriage. Not written as a cause in any of the books. But constipation in itself is not good. For one to pass motions well, there has to be enough roughage (fibre) in the diet. She should eat one banana everyday, and two other complete fruits daily. Salad should form a part of daily diet. And leafy vegetables also should be consumed at least 3 times a week. This along with 6-8 big glasses of water should be enough to relieve constipation . Along with all this take fybogel or isabgul ( husk) for a few days.slowly she will be able to stop the laxative and continue with the diet rich in fibre.