Why might a vampiric race have low birthrates?

This pure-blood vampiric race is not immortal, but their lifespans are about five times that of mortals. This race has lived in a vast city beneath a small mountain range for thousands of years (the world outside is populated by humans) Living with them, though apart in a specific section of the city, is a colony of humans whose purpose is to act as sustenance for the vampiric race. These vampires cannot imbibe anything other than blood.

Though their city is vast, it is still a finite space and the antagonist of my story will eventually use this as a point of argument to back up his belief that they should spread from the mountain and take over human lands. He's not going to win this argument, but I can't help but acknowledge that he has a good point, so I had a thought that maybe the female pure-bloods usually only have one child during their lifetimes; two is not unheard of, but is extremely rare and usually kills the mother. I'd like to have a well thought out reason for this, don't really want to just hand wave it, so can anyone lead me in the right direction? Many thanks, in advance.

In addition, though, there are a rare few newborn vampires mixed in with population, my focus for this question isn't really on them. My focus is the pure-bloods, descendants of several generations. The newborns are notably different from the pure-bloods, they crave blood after being turned, but do not experience the true pain of thirst that blooded and pure bloods do until after their first drop of blood. Also, before their first drop of blood, these newborns can sustain themselves on human food and drink, though only very small amounts of the former, but after, their bodies immediately reject it. I was thinking that perhaps I would make the newborns' birthrates higher, so maybe the low birthrate of the pure-bloods has to do with how long they've sustained themselves on blood?

Update

Also, female newborns would give birth to pure-bloods if their partner was also a vampire.

posted about 5 years ago

P. M. B.‭

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