3 weeks into a fast and being pregnant, will have caused absolutely no harm, but a great deal will depend on the quality of your food reserves and your reasons for fasting.
Most fasting supervisors would not endorse an extended fast, or a fast of any length unless it was absolutely essential for your own health.
I think this section entitled: FASTING DURING PREGNANCY will be of much help.
"A short fast, where one is necessary, or will be of benefit, should be entered upon without hesitancy by the pregnant woman suffering with a chronic "disease," but a long one should be avoided unless acute "disease" makes it necessary. Feeding in acute "disease" does not feed, anyway"...................... http://www.soilandhealth.org/02/0201hyglibcat/020127shelton.III/020127.ch26.htm