You're going to need a good reason why this species is able to survive being incapable of defending itself for significant periods of time, because that sort of glaring vulnerability tends to be quickly and ruthlessly removed by natural selection (meaning that those so vulnerable quickly get eaten by predators). I'm speaking from an evolutionary perspective here, so technology is irrelevant: it doesn't matter what your species is capable of with modern technology if predators hunt them to extinction before they get past fire-making.
I should note that my answer assumes your species to be of a relatively conventional sort from a human perspective; details like equal balance of males and females, not-ridiculous sexual dimorphism, the existence of family units, and so on (basically, like most mammals, which by observation are rather dominant on Earth). If your species is eusocial (picture a beehive or ant colony), some of my assumptions are instantly invalid.
For reference, the first part of this answer is a frame-challenge: it's pointing out that your premise as stated is likely not viable. I've edited in a possible solution to the problems after it, but I'm not sure it would be enough to explain the extremes you seem to be looking for.
One: Given your references to long gestation time and high brain development and so on, I'm assuming that the species you have in mind is meant to hit human-level intelligence or close to it. That is not conducive to rapid reproduction: flukes like twins or triplets might happen, but the great majority of cases will likely be one child per pregnancy, because the resources just aren't there to reliably supply larger pregnancies with the necessary nutrients, and the body isn't easily able to keep up with those demands.
This becomes a problem when you combine it with the fact that, in nature, the mortality rate among the young is appalling. Historically (pre-Industrial Revolution), less than half of all humans survived to the age of ten, despite our intelligence and technology and such medicine as we possessed: for most species, it's even worse. The result is that nature favors reproducing as often as is practical, to raise one's odds of having at least some offspring survive to reproduce themselves.
What this means is that the females of your species will have evolved to spend a significant portion of their adult lives pregnant. In any given community, then, you've got a lot of your people (my guess here is around a quarter of the total populace of reproductive age, because there is a necessary recovery time between pregnancies, and half will be males) unable to contribute meaningfully to the general cause of survival; many of the other females will be having to look after their children as they recover from the strain of pregnancy. You've also tied up more of your populace (a fair chunk of the males) with the need for supervising these pregnant individuals and protecting them, according to your concept, meaning they cannot themselves contribute to a lot of essential tasks. My best guess at the proportions is that you've got one adult trying to gather or hunt enough food for something in the vicinity of two full families; that is nowhere near sustainable in the general case.
Two: If pregnant individuals are this vulnerable and basically helpless, you impose severe limits on the species. Mobility is compromised; in the event of a forest fire or other natural disaster, pregnant individuals are unlikely to be able to escape, and their families are unlikely to abandon the mothers-to-be (or else you wouldn't have families to speak of). This also means migration is pretty much dead on arrival as an idea for your species, for the same reason.
If the pregnant can't even look out for themselves, you force communities with guards to protect them, but this circles right back to the first major problem, because you won't be able to get enough food for them all with so few hunters and gatherers. Your other sane choice is to find a safe hiding place for expecting mothers, but securing that from predators without meaningful technology beyond fire-making (evolution works slowly, and given the example of humans technology outpaces it too badly for significant evolutionary changes based on that technology) and doing so reliably is a notable challenge; not impossible, but definitely difficult.
Conclusion: Given all of this, your species is at a severe disadvantage compared to related species with less burdensome pregnancies (it's worth noting that human pregnancies are already at the extremely burdensome end, relative to other species on Earth), where the females are still able to work in at least a moderate capacity and move with the group. Your idea for a species will in all likelihood get pushed out and driven to extinction in favor of those other related species.
Possible Solution: You might have some luck if you set up your species as something close to pack hunters like wolves. The leaders are the only ones to breed; the rest of the pack is either their offspring, or else adoptees. For this purpose, you'd need a gap between when they become capable hunters and when they typically breed (not necessarily the same as sexual maturity: early maturity could be a safeguard if you need someone to start breeding early, or just if resources are abundant), probably of some years. This is to make sure that the offspring stick around to support the pack until the next few children can take their place, as opposed to rushing off to find mates and leaving their parent's pack vulnerable. To give you an outline, perhaps they tend to be decent hunters by 8 to 10, more or less fully grown by 13, but puberty only at 15 and typical breeding age not until 20+.
Your helpless pregnancy is covered by the rest of the family hunting for the one mother-to-be; in the case of newlyweds, they would likely stay with the pack of one of their parents initially, and only break away when their first children reach an age suitable to hunting (which might let you lower the typical breeding age if the pack is faring well enough to support two mothers at once). As intelligence (and length of childhood, most likely) grow, the idea of a "pack" would presumably expand to two families, then to three, and so on until you start getting villages, but the idea is basically that you mitigate my second key problem by minimizing the number of pregnant females at any given time.
Answering why the pregnancy is so debilitating is a little harder, but perhaps your species tends towards having litters of children; basically, imagine if triplets were typical for humans (and could go higher for your species, perhaps as many as five) instead of an oddity. This at once covers why it's so debilitating for the mother and why it's not necessary for everyone to be reproducing at once to keep up the numbers.