I used to think it was just some quaint local custom, but I've got enough kids to fill half the seats in the senate, so at this point I can't really deny there's something to it.
—Benjamin Lambert, regarding the fertility chant.
The Yhjanda fertility chant, known simply as the fertility chant by its users, was a form of ancient Sith magic developed some time after the Great Hyperspace War by the surviving members of Darth Taer's Sith forces to maintain the dwindling number of Force-sensitives within their ranks after they were marooned on Farrow. It continued to be used by their descendants, the Yhjanda tribe, though without the Force its usage become purely symbolic.
Taking the form of a spoken spell, the chant was recited by a Force user – either male or female, though it was often spoken by both in unison – during sex to increase the chances of a prospective mother bearing a twins, triplets or quadruplets, or a single child, if the woman in question had difficulty conceiving naturally. Initially this was to combat the increasingly small pool of Force-sensitives being born in comparison to the population boom taking place among those who served them, but after their followers rebelled and drove the Sith to extinction it was maintained under the mistaken belief that it could still prove effective, eventually becoming a tribal custom that was more akin to a prayer, with its Sith origins long forgotten. Large families remained desirable to the tribes of Farrow, however, as the larger a family's contribution to the tribe the greater their collective standing.
The language of the chant was a variation of the Sith language unique to the Yhjanda tribe, though all of Farrow's tribes were said to have their own variation of the fertility chant. It had no effect when recited by those who could not use the Force or whose ancestry could not be traced back to the ancient Sith who crashed on Farrow, but for those who qualified on both counts it was capable of manipulating the biology of the prospective mother, drastically improving her chances of conceiving multiple children.
By the time of the Great Galactic War, there was some debate amongst the tribe's members as to when in the process the chant should be recited, though the debate was noted as having been ongoing for centuries by the time of the Eternal Empire's rule. Aaellu, an exiled tribeswoman, successfully used the chant while stranded on Hoth with her partner Benjamin Lambert in 28 ATC, and though he doubted it was anything more than tribal superstition Aaellu eventually gave birth to quintuplets in 29 ATC, giving Benjamin reason to suspect the chant may have had some effect.
Aaellu used the chant again later that year, and in 30 ATC she once again gave birth to quintuplets, prompting Benjamin to convince her to submit to a battery of medical tests which confirmed her fertility – while good – would not imply she had a higher than average chance of conceiving twins. Aaellu's doctor could not explain her situation, and had to conclude that she was simply "one in a billion", though Aaellu remained adamant the chant was the reason and, ultimately, Benjamin conceded that it was the only explanation.
In 34 ATC, Aaellu gave birth to triplets after using the chant yet again, though in 39 ATC she gave birth to a single child, causing her to speculate whether the chant was becoming less effective the more she used it or whether she was just becoming too old to bear so many children.