Pregnant and nursing women are obligated to fast on Yom Kippur (Pesaḥim 54b; SA 617:1). They are even obligated to fast on Tisha Be-Av, which is a rabbinic requirement, so certainly they are obligated to fast on Yom Kippur, which is a Torah requirement.
In recent times, some rabbis have been allowing pregnant women to drink le-shi’urim on Yom Kippur, because they believe that women are weaker nowadays and may miscarry if they fast. However, studies in Israel and abroad show that fasting does not increase the risk of miscarriage. In rare cases, fasting during the ninth month may induce labor, but this is not life-threatening. There is also no basis for the claim that people today are weaker than they used to be. On the contrary, people are healthier than they have ever been, whether due to the abundance and variety of available food or due to advances in medicine. Life expectancy has increased by decades. Therefore, there is no reason to be more lenient today than in the past, and the law still applies: pregnant and nursing women are required to fast (Tzitz Eliezer 17:20:4; Nishmat Avraham 617:1).
This means that even pregnant women who throw up, have slightly elevated blood pressure, low hemoglobin, or other normal discomforts associated with pregnancy are obligated to fast on Yom Kippur and are not permitted to drink le-shi’urim. Consulting a God-fearing doctor is only necessary if a woman is in the first few weeks of pregnancy following IVF or going through an especially difficult or high-risk pregnancy. If the doctor says that there is possible danger to the life of the mother or the fetus, then she may drink, preferably le-shi’urim. In contrast, a woman experiencing a normal pregnancy with normal symptoms (even if this includes throwing up) must fast. There is no reason to even ask a rabbi about it. Nevertheless, if a pregnant woman who is fasting feels that her situation has become dangerous, she should eat and drink as needed.