A pregnancy charm is a charm, worn around the neck to prevent conception. They are sold by healers almost everywhere, although the quality varies depending on the healer and her magic. The charms are also more or less effective depending on the material, which is often gold or silver.
Women known to use pregnancy charms are Alanna of Pirate's Swoop and Olau, when she was still unmarried; Keladry of Mindelan (though its actual application, i.e. sex, is never confirmed within the series), Alianne of Pirate's Swoop, and Rebakah Cooper.