system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do pregnant women get specific and unusual cravings? Do they serve a biological purpose?
gpt
There are two thoughts on why it happens, both are probably somewhat correct. One is that hormone production changes during pregnancy and that stimulates the desire for certain things. For example, as a woman's insulin levels drop or rise their body wants certain things that will adjust those levels. Some women report that they experience changes in tastes or desire for food in relation to menstruation, another time that hormones fluctuate.

The second reason is that pregnancy is very physically demanding and it's necessary to increase both blood volume and nutrients. The brain is a very greedy organ and initiates cravings to not only supply the body and baby with nutrition, but to meet its own needs.