Up until the 1930s bathing suits were made of real wool (and got real saggy and heavy when wet - ew). With the invention of yarn covered in rubber in 1931, swimsuits became stretchier, thinner and more comfortable to wear. 1930s swimsuits were generally made from cotton or rayon with built-in support. Outdoor leisure activities such as swimming became a popular past time for people of all economic classes, not just for those who lived near the beach. Enjoy these snaps of 1930s babes modelling the swimwear styles of the decade.
Take a look at this fascinating article from Holiday magazine (June 1950) titled "1950: A Good Round Figure". Apparently women were about to look more like "natural" women that Summer, instead of "being disguised as little boys or long distance swimmers". American designers were going for a more modest, "feminine" look than their French counterparts, who were veering towards bikinis. Isn't it interesting how fashion trends influence not only how we adorn ourselves, but also how we view and manipulate our bodies?