Dean Lerner's Reply:
As yoga teachers, we need to show sensitivity and common sense in recognizing conditions or problems our students have. It is our responsibility to know, to the best of our ability, what they can and should do, and what is contraindicated.
In any school of yoga, there will be do's and don'ts for pregnant students. However, the first chapter of Hatha Yoga Pradipika states, "The young, the old, the extremely aged, even the sick and infirm obtain perfection in yoga by constant practice." In other words, everyone can practice yoga. The fact is that yoga is fantastic for pregnant women, and it can be a wonderful and powerful means to help them through pregnancy and the birth process.
Rather than excluding a pregnant student from your class, develop your capacity to accommodate her. Practically speaking, you should have the pregnant student in your class do what she can appropriately do, and when the class is practicing poses that are contraindicated for her, have her do appropriate alternatives. It may take time for you, as a teacher, to develop this capacity. In the meantime you should simply tell the truth to the student: Due to the nature of your class, or your inexperience in working with pregnant students, it would be best for her to practice on her own or attend a class specifically for pregnant women. No doubt she would appreciate this forthrightness.
An excellent resource for everyone who practices yoga, and an essential book for teachers, is Yoga: A Gem for Women, by Geeta S. Iyengar. This book describes the three milestones in a woman's life—menstruation, pregnancy, and menopause—and discusses how to adjust a woman's yoga practice accordingly for optimum health, balance, and well-being.
Regarding the specifics you mentioned in your question, kneeling (going into Downward Dog, for example) is OK during pregnancy. However, a prone position would not be appropriate. You can find details in Iyengar's book, as well as in Linda Sparrowe and Patricia Walden's excellent Women's Book of Yoga and Health. Once you have read these books, and perhaps received assistance from senior teachers, you will know what alternative poses to give to pregnant students in your class.
Certified Advanced Iyengar instructor Dean Lerner is co-director of the Center for Well-being in Lemont, Pennsylvania and teaches workshop across the United States. He is a longtime student of B.K.S. Iyengar and served a four-year term as president of the Iyengar National Association of the United States. Known for his ability to teach yoga with clarity and precision, as well as warmth and humor, Dean has conducted teacher training classes at Feathered Pipe Ranch in Montana and other locations.