1. Get connected with your doctor: The first and foremost thing you have do is to consult with your doctor before taking up the fast. There is no as such study that proves fasting during pregnancy is dangerous but it all depends upon your health and how smooth your pregnancy is. Also, Karvachauth fast is without water so; this might lead to dehydration, further causing a lot of physical discomfort and eating nothing the entire day might distress the baby too. Thus, before taking a decision to fast or not, it is always advisable to consult your gynecologist.
2. Make moderations: Eating and drinking nothing the entire day is much difficult for pregnant ladies so it is better that you can make moderations in you fast by eating fruits once in a day and sipping water through out so that you can continue with your fast without discomforting the baby inside.
3. Choose your morning sargi carefully: First of all make sure you drink a big glass of milk, also eat nuts, fruits, a parantha so that you do not feel
hungry for a long time. Secondly avoid taking too many sweets in the morning as they are just a storehouse of fats and calories instead pick something healthy and light. 
4. Take ample rest: Do not over exert your body this day. Try taking proper sleep and rest throughout the day so that you can save some energy and your body too feels relaxed.
5. Distract your mind: Busy yourself as more you sit idle more the hunger pangs would increase so try to distract your mind in doing things like reading, listening to music etc but stay away from strenuous physical work.
So, all the expecting mothers should keep in mind that during this time, your first and supreme priority is your baby’s health and well-being. If you have strength, willpower and the consent of your doctor, only then keep the fast. Happy fasting ladies!