Cravings for unusual and special types of food are very common during pregnancy. In fact, most pregnant women probably experience one type of craving or another at least at some point during the pregnancy.
The exact reason that cravings happen is not known. Some researchers have hypothesized that cravings may be related to the body seeking out some specific nutritional need. So, for example, a woman whose body is deficient in calcium may crave ice cream. However, most cravings have no clear relationship to any specific nutritional need, so this may not actually be what is going on.
Another possibility is that women crave foods as part of a more generalized phenomenon of being in tune with their bodies during the pregnancy. They pay close attention to every physical stimulus, including hunger, and this may lead to a desire for specific types of foods.
Finally, there is probably a very strong cultural factor. We all grow up learning that craving foods is something that pregnant women do. Therefore, we learn that this is a socially expected part of being pregnant, and this subconscious learning goes on to affect our future behavior.
Regardless of the cause, food cravings are very common and are not dangerous!