The Left Hand of Darkness is a science fiction novel by Ursula K. Le Guin. One of the first major works of "feminist science fiction", it is the account of the efforts of Genly Ai, a representative from the galactic federation of worlds (the Ekumen) to bring the lost world of Gethen / Winter back into galactic society.
The inhabitants of Gethen are androgynes, biologically bisexual humans; for approximately three weeks of each month they are biologically neuter, and for the remaining week are male or female at random. Thus each individual can both sire and bear children. (The Gethenians were genetically engineered for this characteristic long ago, possibly to maximize reproductive success on the harsh glaciated world of Gethen, or because the ancient biological engineers were curious about what such people would be like.)
Le Guin developed this idea out of a desire to explore what remained basic to human nature when biological gender was no longer a factor.