The American College of Obstetricians and Gynecologists has updated the guidelines first released in 2002. For aerobics, it's advising pregnant women to walk about 11 hours per week or spend about five hours a week swimming or using a stationary bike.
When strength training, women are advised to use light weights or resistance bands, and never lift while on your back. Also, avoid walking lunges to prevent pelvic injuries.
Of course, you should always consult with your doctor before begin any exercise routine.
