Yttri are an odd race. Like many species on their planet they can selectively position themselves along the r/K reproductive strategy spectrum. r species are those (like some species of frog, fish or insect) that throw out thousands of 'expendable' but cheap children in the hope that enough of them will survive to adulthood. K species are those that instead focus lots of resources into a small handful of offspring, so they stand a much higher chance of surviving. The Yttri have evolved so that in times of dire need they can adopt an r style strategy, while in times of plenty they instead opt for a K style strategy.
Yttri females, when first impregnated, have hundreds of foetuses in their womb. They then gestate these for a minimum of two months, after which they can opt to give birth to a whole brood of small, weak, ravenous children that are capable, but ultimately expendable. Or they can delay the birth. The still-gestating children will begin to consume each other over time (in ways facilitated by the mother and also quite literally eating each other). At any point up to about a year (when only one child remains and preventing the birth is no longer an option) the mother can release the children in to the world.
Now, the children that stay in the womb are demonstrably larger, stronger and smarter than those that are born earlier. A brood of a hundred children that were born after two months and then had a year to develop will never physically or mentally match up to a child that gestated for a year and then had two months (though they will eat more). This is an acceptable tradeoff for almost every species on the Yttri's home planet, as it lets them deal with a wide variety of seasonal changes. Well fed creatures (or those in positions of security) have fewer children, while those living in uncertain times or climates 'fire and forget' whole broods in the hope that some children will survive the lean times and make it through to the other side alive.
But the Yttri are (for want of a better word) industrialised. They have advanced medicine, agriculture, engineering, robotics, science etc. They are very much at a technological and social level equivalent to that of a modern day first world country. In such a world there is no need (unlike in their early industrial period, where more children meant bigger earning potential) for Yttri parents to need or even want large broods of dumb, weak children when they could instead give birth to one or two smart, strong ones. There is a strong social stigma attached to abandoning one's children, so 'fire and forget' is even less of a sensible option.
Given that the mother can always choose to delay the birth, why would a Yttri parent still opt to go down the r reproductive route rather than sticking with K?
