The two piece beach costume developed from trunks and suntop.
In Europe it would have been totally unacceptable for a woman to show her navel even on the beach until the 1950s. In the USA this taboo extended to the 1960s.
Swimming costumes with large side and back cutouts appeared during the late thirties for both men and women.
This woman has rolled her suntop down and almost invented the bikini.She would not have done this in public or allowed a photo to be taken if it was not common practice.
I have long suspected that the man (probably French) who claimed to have invented the bikini was an imposter.

The two piece which started as a fashion at this time spread during wartime when new swim and beach wear were deemed non essential items. Clothing coupons could not be spared for leisure items but old costumes could be adapted and altered.

• Date about 1938

It was about this time that men began adopt trunks for swimming