I have read that soy products interfere with birth control. Do I avoid soy and soy products? Or is it only in large amounts over a long period?

Volume. Soy products have phytoestrogens that can mimic the estrogen in your birth control pills. As long as you do not eat large volumes of soy prducts this should not be a problem. Make sure you evaluate other food items you eat for their estogen content, such as yams, licorice, and black cohosh.