Are pregnancy cravings real?

My mother claims during her past pregnancies she's had cravings for stilton cheese, pickled eggs and cockles. I've also heard many stories of women craving nonfood stuff like coal or soil.
Talking to people I've noticed some items seem to be more popular at a given time in history which leads me to think women are simply conforming to an idea rather than instinctively knowing an item contains an element they need.

So are cravings real or just women jumping on a bandwagon
Are pregnancy cravings real?
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