Kes experiencing premature elogium
The elogium is the physiological process in which a female Ocampa becomes sexually mature, similar to puberty in most humanoids. The elogium happens only once in a lifetime (between the ages of 4 and 5 years) and during this time, she must conceive if she is to have a child.
In 2372 as the USS Voyager encountered a swarm of spaceborne aliens, Kes went through her elogium prematurely. In order to prepare herself for bearing a child, she went through the relessin, where one of the woman's parents massage her feet until her tongue swells (though in Kes case the relessin was performed by The Doctor due to the fact that none of her parents were present on Voyager). But after the ritual in which the entire process took place, Voyager got out of the swarm and Kes's elogium stopped. (VOY: "Elogium")
Notes
It was speculated by the Doctor that, since Kes' elogium was triggered by an outside source, she may go through it naturally when the usual time came. Although she departed Voyager before this could take place, in the alternate timeline she witnesses in "Before and After", she indeed entered the elogium of her own accord, and, with Tom Paris as the father, she had a daughter called Linnis Paris.