This thought about not wearing jewelry while pregnant stems from both practical and superstitious beliefs. Someone already mentioned that the body will swell or gain weight, making rings on fingers tight. Same with tight bracelets and chokers. That pinches capillaries and restricts blood flow which is not good for the fingers and hands.
Any sort of allergic reaction from metals for those with sensitive skin places minor stress on the body, too. It has been reported in the medical community that some women who have not experienced metal allergies can develop contact dermatitis (especially to gold) during pregnancy due to fluctuations in hormone levels and antibodies.
One must be especially aware of what type of metal is in their jewelry, especially when manufactured overseas. Some "silver" or costume jewelry contains lead which is highly toxic. Beware of children's jewelry and charms. The CA gov just cited an LA jewelry seller for mislabeling jewelry as "lead free" when, in fact, it was not. A child's death was reported due to lead poisoning.
Healthystuff.org just announced that cheap jewelry is filled with toxic chemicals and that pregnant women should avoid wearing them: "The chemicals tested were lead, cadmium, arsenic, chlorine, PVC, bromine, chromium, nickel and mercury – toxins which have been linked to cancer, liver toxicity, impaired learning, birth defects, allergies, delayed conception, and more."
Unless you know your jewelry source and quality, some pregnant women find it safer to go jewelry free during gestation.