There are 5 prohibitions on Yom Kippur. All coming from the Biblical verse in Vayiqra 23:27: On the 10th of Tishri "ve'initem et nafshotekhem" you should deprive your bodies.
The five prohibitions are:
(1) Eating and drinking, (2) washing our body (3) anointing our body (4) having marital relations and (5) wearing leather shoes.
Everyone has to fast on Yom Kippur, except the following:
1. Minors: a boy or a girl younger than nine years old should not make any type of fast even for a couple of hours. When children reach nine they should fast for a couple of hours if they are in good health. Once they are nine every year the parents should encourage them to fast for a little longer so they will slowly get used to the idea of a complete day of fasting. When the son or daughter reaches eleven years old they should try to fast the whole day if they are in good health. A healthy girl from the age of twelve and a healthy boy from the age of thirteen are obligated to fast.
2. Mothers: After childbirth, during the first three days, a mother should not fast. The same rule applies for the 72 hours after a miscarriage. After the first 72 hours from the day of childbirth until the 7th day from childbirth, if the mother says that she needs to eat, she should eat. After the 7th day she has to fast unless otherwise indicated by a reliable doctor. A woman in labor cannot fast.
3. Sickness: A person who is sick or an elder who is weak because of his age or someone who is getting some type of important medication should get the advice of a reliable physician to see if he needs to eat and/or take the medication. After talking to a physician a Rabbi should be consulted to establish, based on the doctor's advice, how to eat his food. In a case of a chronic illness, such as diabetes, and especially in a case where there is a risk (safek) of some kind of danger to someone's life, one should break the fast, even if one was not able to consult with a physician.
4. According to Jewish Law, if a chole -a sick person- asserts that he or she needs to eat he or she should be fed even without asking a Doctor.
Pregnancy and Yom Kippur
The following information is intended as an educational guideline for normal and healthy pregnancies. You should not rely on this information as a substitute for personal medical advice.
A pregnant woman should observe the fast normally provided she and the baby are in good health. Nevertheless, if during Yom Kippur she feels sick, and particularly if she is vomiting or having other signs of dehydration, she should break the fast and eat immediately (See below: "How to eat"). However, mild dizziness and nausea that can be coped by lying down on a couch or a bed are considered normal and should be endured.
VERY IMPORTANT
Dr. Jessica Jacob O.B. explains that according to research done on healthy pregnant women, there is no evidence that fasting would cause any complication to the mother or her baby. Four years ago, Dr Jacob gave a very detailed lecture in our community on 'Pregnancy and Yom Kippur'. It is highly recommended for every pregnant women to listen to her lecture where she explains the studies that were done on this subject. She also explains why would a doctor advise a pregnant woman against fasting; how to cope with discomfort; the circumstances in which pregnant women should break the fast, etc.
Click here to listen to Dr. Jacob's lecture
HOW TO EAT?
When you were instructed by your doctor to eat on Yom Kippur, proceed as follows
1. Eat a portion of food that weights less than 1 oz. (an ounce is 30 grams). After you finished the first portion you should wait ten minutes and eat a second portion of less than 1 oz. You could do this as many time as you need until you recover.
2. For drinking, you should drink up to a little more than 1 oz. Then wait 5 minutes and drink 1 oz. again and so forth.
Once you feel better, you should continue the fast.
To measure "1 ounce" it is recommended to use the plastic 1 oz. liquor shot cups (Kiddush little plastic cups).
When you eat or drink on Yom Kippur for medical reasons, you do not need to fast another day.