In the 1940s corset manufacturers saw a gap in the undergarments market. Corsetry was losing ground, but the new more revealing swimsuits really needed experts to design garments that hid faults in a woman's shape. Manufacturers achieved this by adding stretch tummy control panels to hold in the stomach. Most also used bra cups and boning to give bust support.
Fashion swimming costumes could then be worn either strapless or with small straps that buttoned onto the inside or worn halter style as in this example. Even then women were not keen on lighter body marks that may have made a strapless dance dress look less attractive.
Women still continued to wear all in one swimwear in the 1950s, rarely wearing a daring two piece bikini. These interesting studio portrait photos reveal some super sexy women's swimsuits in this decade.