Sadly, it is not much known about Eldar biology, bu it is not the gestation period that would be the problem, but getting pregnant in the first place:
To successfully conceive, woman requires not one, but several sexual intercourse, probably in quite specific moments of her ovulation cycle - this makes accidental pregnancies near impossible but, funny enough, suggests that Eldars can have multiple fathers.
Well "whats the problem with trying all the time then?" you could ask? There is one: Slaneesh or, as Eldars know him "She Who Thirsts". Every time Eldars get too... excited, they risk seriously bringing attention of the Chaos god and losing their souls. That's why Craftword Eldars choose to follow Paths, that teach them to master their emotions: Path of the Artist will teach them how to express themselves, Path of the Warrior how to deal with anger, Path of the Servant - how to be ready for self-sacrifice. Maybe following the last one mentioned, Eldar can learn how to be parents, but then they don't change Paths on a whim and tend to stay for many, many years (sometimes forever) on a same one.
On top of that there is the obvious issue with longevity: long life means long childhood and even longer period of restless youth - in "The Eldar Path" book a 45 year old woman says that she is too young to even start thinking about sex and relationship (but then she might be lying to her "friendzoned" companion)
Of course Eldars could start cloning, but besides the moral issues it would bring more problems than gains: Who would rise those kids and put them on the right Path? How twisted could they possibly be without the mother guiding psychic influence during pregnancy? Would there be enough of the precious Spirit Stones to give to those children to protect them?
Since Dark Eldars have no such issues (since they are constantly replenish their souls preying on others), they basically mass produce their children: Haemonculi - a horrific Flesh Sculptors perfected ways of controlling reproduction and produce adults (since childhood is a waste) in their hundreds. The catch? They let their offspring loose on the streets, when they die in dozens over the first few hours, unable to control their emotions or simply killed for fun. Those smart enough that survive this culling are later accepted into society.