By Rabbi Yerachmiel D. Fried

Rabbi Fried,
Research seems to indicate that fasting by pregnant women can lead to “smaller babies who will be more prone to learning disabilities in adulthood.”
Since these studies show that fasting can have permanent negative effects for the child and only makes pregnancy more difficult for the woman, why would the Torah require women to fast on Yom Kippur while pregnant?
— Marcie
Dear Marcie,
The research you cite, as well as many other studies, indeed show that for Muslim women to fast throughout the month of Ramadan could potentially have negative repercussions upon their pregnancies. This is mostly confined to reduced birth rates.
Most studies show that long-term intelligence is not usually affected; there are, however contradictory results of studies in that area. Be that as it may, these studies reflect only the effects of fasting every day for an entire month. They do not reveal any danger whatsoever for a mere one-day fast.
On the contrary, most studies show that there are no negative repercussions for a one-day fast, provided the mother takes certain precautions.
That being said, Jewish law requires that a pregnant woman fast provided she does not fall into a high-risk category. (This could include anemic women, women with a history of premature contractions, women on bed rest, abnormal blood pressure, an IVF pregnancy in its early stages [or later stages if the infertility doctor deems the pregnancy of risk], some fevers, angina or other infection, gestational diabetes, bleeding, any threat of early miscarriage, later stages of a twin pregnancy, dehydration due to vomiting and/or diarrhea, extreme weakness or fatigue).
In short, any condition the woman’s doctor deems as a risk should be discussed with her rabbi, who will, almost always, relax the obligation to fast.
This is not a loophole in the halachah, or Jewish law; rather it is the original intention of the law. Jewish law is not a rigid, all or nothing, black-and-white set of laws. It is, a fluid living body of laws that takes into consideration the human condition.
Under the circumstances stated above, the woman is not simply released from the obligation to fast and she may then choose as she wishes. It becomes forbidden to fast according to Jewish law!
The very act of fasting, which is a mitzvah for most, becomes a sin for her, should she decide to be extra punctilious; putting herself and her pregnancy at risk is not a religious feather in her cap, quite the opposite. That is how eclectic and heterogeneous the Torah laws can be in practice.
While we’re on the subject, there are a number of points every pregnant woman should know before Yom Kippur. Firstly, even though she feels completely healthy and strong, she should always discuss her fast with her doctor to be sure he or she has no concerns, and to pass it by her rabbi as well.
In addition, the pregnant woman should ease herself off caffeine a good few days before the fast, and drink water and other fluids. She should be very well hydrated before the onset of the fast.
If she will do a lot better by taking it easy and staying in bed, (this is not the year for going to shul, especially in Texas!), who will take care of the other kids? A hot flash to their husbands: unless you have made other acceptable arrangements, it becomes your job to stay home; it’s far more important to God that your wife makes it through the fast as easily as possible, than you be in shul for Yom Kippur!
Even with all that, if the woman experiences any danger signs during the fast, she should break the fast immediately and not wait until she can speak to a doctor or rabbi. This would include bleeding (even slight), contractions, a decrease in fetal movement, blurred vision or intense headache, extreme fatigue or weakness, or dizziness.
May God bless all the pregnant women with healthy pregnancies and births in the New Year ahead of us!
Rabbi Yerachmiel D. Fried, noted scholar and author of numerous works on Jewish law, philosophy and Talmud, is founder and dean of DATA, the Dallas Kollel. Questions can be sent to him at yfried@sbcglobal.net.