Can low body weight prevent a 21 year old woman from getting pregnant?

Yes: If her body weight is low enough to cause menstrual abnormality, then her chance of getting pregnancy will decrease.

Sure.: If you are too thin or too heavy for your height, your fertility rate is decreased.

Yes: A certain amount of body fat is required in order to establish normal ovulation in most women. Being much less than your idea weight may hamper normal ovulation.