Jogging is OK for pregnant women who are acclimated to the activity. High impact exercise can be tough on the knees and ankles, and since the hormones of pregnancy soften ligaments, I don't recommend starting this type of routine during pregnancy.
But certainly, aerobic exercise should form the foundation of prenatal fitness in the first half of pregnancy. Pregnant women who engage in moderate level aerobic exercise grow bigger placentas, have stronger immune systems, gain less fat, and have babies that have stronger cardiovascular sysytems. Animal studies show that regular aerobic maternal exercise increases the density of neurons in the cerebral cortex (the thinking brain) so aerobics may well increase your baby's IQ!
Good chioces for prenatal aerobics include power walking, treadmill, elliptical trainer, stationary bike, low impact aerobic classes, water aerobics, and easy dance-based aerobic classes. Aim for a minimum of 30 minutes of aerobic activity on most days of the week. If you are a regular exerciser already, then you can workout for longer periods.
When performing aerobic exercise, monitor your heart rate and your precieved exertion. ACOG, the American College of Obstetricians and Gynecologists revised their recommendations in 2002, and no longer advise pregnant women to keep their heart rates below 140 BPM. But keeping track of your aerobic heart rate, and correlating it to your rate of perceived exertion is a good idea. Always stay at an intensity that you feel is "somewhat difficult."
As your pregnancy progresses, your rate of perceived exertion will rise during a given activity and you will need to taper off your exercise intensity, especially after 20 weeks. A sudden increase in your usual heart rate or perceived exertion is a sign that you should slow down, and may be an indicator of over-training, an immune system challenge, or other stress.
Basic strength training and regular stretching will round out your prenatal fitness program and keep both you and your developing baby in top health.