Some facts about European underwear, 1700 - 1900, and its
relationship to what women used for menstruation
Only men wore pants as outer clothing, a symbol of their authority (in English we still say "so-and-so wears the pants in the family," as do the Germans in their language) although women would sometimes wear versions of them next to their body when riding or when the weather was cold. Later, with the French Revolution and afterwards, women started to wear long-legged underpants to shield themselves under diaphanous dresses, but it took decades for such pants-like underwear to gain wide acceptance among the upper classes and even longer among the common people. They continued to wear only the chemise under their clothing for most of the 19th century. Women who wore traditional regional costume in Germany (and I bet elsewhere) sometimes wore no underpants until the 1950s.
In 1757 a German doctor gave another reason why women shouldn't wear pants or closed underwear: her genitals needed air to allow moisture to evaporate, which could otherwise cause them to decay (German, "vermodern") and "stink." But he conceded that women could wear them in cold weather and to protect against insects. (Christian T. E. Reinhard, in his Satyrische Abhandlung von denen Krankheiten der Frauenspersonen . . . Teil 2, Berlin/Leipzig, 1757, quoted in Zur Geschichte der Unterwäsche 1700-1960.)