The title might not be perfect, but the idea is there.
How long does it take Eldar children to be born?
Reading the new eldar codex:
every year there are fewer of their number to take up arms against their enemies
This got me to thinking, why don't the Eldar just make babies all year round, constantly? Wouldn't this seem like a good plan
The gestation cycle of the Eldar is much longer than other races
As stated by the Lexicanum page for the Eldar there is fairly little known about the reproduction between Eldar. However it does state that they do reproduce in several stages.
However in the 7th edition Codex of the Dark Eldar this is noted:
The Eldar gestation cycle is much longer than that of many of the lesser races, and conventionally born children are rare symbols of status. Artificially grown Dark Eldar are far more commonplace.
Which says that it takes much longer for an Eldar child to be born than most other races and presumably also humans, so most likely a fair bit more than a year in contrast to a human's 9 months. And with the Eldar's very strict rules and other things like the Spirit Stones, it makes it quite a long and difficult process for a new Eldar Child to be born. I also don't think that the Craftworld Eldar grow their own children like the Haemonculi of the Dark Eldar do. at least I haven't found any evidence of it.
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.
I read this as meaning that there is a growing rise in the percentage of Eldar who are becoming sterile through either diseases, genetic degradation or simple ennui at the universe.
"make babies all year round" This alone makes me wonder at the age of the questioner, or at least their understanding of biology in general.
