Any exercise where your head is lower than your heart should be practiced in moderation after the first trimester because additional blood volume can lead to nausea and/or loss of breath. Downward dog can be done in moderation, but don't hold the position for extended periods of time.
If you decide to do the pose throughout your pregnancy, listen to your body and stop if you feel uncomfortable in any way. Also, modify the pose by keeping your knees bent and your heels lifted off the floor to reduce stress on your lower spine. Your arms should also be positioned a little further apart than in your regular practice. The modification means your body weight will be supported predominantly by your upper body to reduce compression of your abdomen.
Exercises that give similar benefits to downward dog are the cat-cow yoga pose and standing hamstring stretches with your foot resting on a chair.
Tracey Mallett is a certified personal trainer and sports nutritionist. She is the author of Sexy in 6: Sculpt Your Body with the 6-Minute Quick Blast Workout, as well as the creator and star of the 3-In-1 Pregnancy System for pre- and post-natal mothers. Her newest videos are Renew You and Super Body Boot Camp. A proud mother of two, Tracey lives in Los Angeles, California. (www.TraceyMallett.com)