18 weeks pregnant. i haven't exercised at all this pregnancy and have been sedentary the whole time. how much exercise should i do that is safe ?

See below: If your pregnancy has been uncomplicated then brisk walking or even more agressive aerobic exercise such as running, elliptical training, or even swimming may be beneficial. It largely depends on how your pregnancy is faring and what types of exercise you engaged in before pregnancy. If you were aerobically fit before pregnancy then you may go after more aggressive levels of exercise during pregna.