(covers information from several alternate timelines)
The elogium is the physiological process in which a female Ocampa becomes sexually mature, similar to puberty in Humans. The elogium happens only once in a lifetime - usually between the ages of four and five years. An Ocampa female must conceive during this time if she is to have a child. In order to prepare the female for conception, a mitral sac - where the offspring will grow - starts developing in the posterior part of the neck. Some side effects of this condition include high fever, hypertension, a dangerously elevated heartbeat, and delirium with elements of anxiety and paranoia. A significant increase in appetite for strange food sources also develops. Perspiration, changes in eye color, and discoloration of the skin surrounding the eyes may also be evident.
An Ocampan ritual to prepare a woman before conceiving is the rolissisin, in which the feet are massaged until the tongue swells. This task is usually performed by the woman's father. The rolissisin is performed soon after the appearance of the ipasaphor. This bonding ritual is a time where parent and child move into a new kind of relationship, for when the child has her own child, the parent must acknowledge her true adulthood. After the ipasaphor appears, there is only fifty hours to begin the mating process, which consists of the female remaining bonded to her mate for six days to insure conception.
It is never actually explained how the Ocampa can survive, as it is explicitly stated that Kes can have a child, and the possibility of twins are never even mentioned, their population would drop by half each generation.
In 2371, when the USS Voyager encountered a swarm of space-dwelling lifeforms, Kes' metabolic activities accelerated and she went through her elogium prematurely. In Kes' case, the rolissisin was performed by The Doctor because neither of her parents were present on Voyager. She wanted to mate with her partner, Neelix, who was initially somewhat unsure about becoming a father. However, when Voyager got out of the swarm, Kes' elogium stopped. The Doctor theorized that since it was brought on by an outside source, she would still be able to conceive at the proper age. (VOY: "Elogium")
In an alternate timeline in which Kes was transporting through time to various stages of her life, she entered the elogium at the right time - conceiving a daughter, Linnis, with Tom Paris. (VOY: "Before and After")