If a woman of child-bearing age suddenly develops a craving for pickles, many people think the case is already clear: she must be pregnant. Some even have an explanation ready. Pickles supposedly contain especially large amounts of boron, and boron is needed for cell division.
Pregnancy undoubtedly involves major changes in hormonal balance, and appetite and food preferences can change. The need for nutrients also changes, and genuine deficiencies can be medically relevant. But those general facts do not prove the specific folk belief that pregnant women characteristically eat more pickles.
On 16 March 2006, a DPA report described a Dutch study according to which pregnant women did not consume more pickles than non-pregnant women. Their consumption was reported to be similar or even lower. About one thousand women were said to have been examined.
The little story is useful because it illustrates a common reasoning pattern: first we accept a stereotype, then we invent a plausible biological explanation for it. A mechanism that sounds scientific does not establish that the underlying observation is true.