What is needed in a species for it to be particularly weak while pregnant?

You describe birds.

As opposed to internal gestation (pregnancy), birds have external gestation; they lay eggs. If one considers the bird/egg unit as the gestating state then this gestating unit is tremendously vulnerable. An egg cannot defend itself or flee at all. Neither can the mother move the egg from where it is. The mother bird is therefore herself much more vulnerable because fleeing the predator means abandoning the gestating state and the resources committed.

Once you have the entire repertoire of nesting birds to draw from, there are many amazing schemes you could use. Birds draw attention from the eggs by concealing the nest. They draw predators away so that the predators do not discover the gestating state and the defenseless eggs. Mother birds recruit help from the male bird and related juvenile birds to defend the nest. One species of hornbill walls the female up in a hole with her egg, and the male feeds her thru a tiny hole.

Having eggs is a baby step away from being pregnant, but is different enough that when in your fiction you have your internally gestating creatures recapitulate the adaptations of birds, your readers might not immediately recognize what you have done.

If your species is to be human-like, you can take some inspiration from rare complications during human pregnancies, and make that the norm for your species instead of the exception. Some examples include:

Relaxin In layman's terms: in order to be able to give birth, the relaxin hormone causes tissues to relax and become weaker, making them susceptible to injury. Severe cases of Symphysis pubis dysfunction or Sacroiliac joint dysfunction can render the pregnant individual bedridden.

Eclampsia In your species, the toll of pregnancy on an individual's heart and other systems can be so great that any slight physical exertion (e.g. walking) becomes life-threatening.

And inspiration from other human conditions:

Foreign tissue rejection (a la donor organ) The fetus is not the mother's own flesh. In order for the mother's body to not reject the foreign object, some aspect of the pregnancy can render the mother so immuno-compromised that she needs to live in isolation.

Don't make them weak. Make their predators strong, and numerous.

It's never gonna be evolutionary advantageous to be unnecessarily vulnerable while pregnant, because losing your offspring is a massive disadvantage. Instead, make it so that there is constant predation.

Their predators can smell their pregnancy and baby. The predators are hungry, and they're dangerous enough that only one of these beings in their peak could survive a fight.

Perhaps this elephant like creature can kick like a truck if someone gets close, but their predators are fast and numerous, and more often than not, a lone being will get eaten. A long gestation time and a huge pregnancy is needed to produce a strong, powerful offspring, but that means a lot of food to any predators.

By weak I mean physically weak

Not gonna be a thing. A powerful creature is gonna be powerful even with a massive weight. Physically lazy would be a thing though- anyone doing labour is risking a predator coming to bite at their chest, and they would rely on the males doing that.

not being able to defend against a predator

Strong and fast predators makes this true.

moving slowly (if at all)

This would be a massive evolutionary disadvantage. What about, not slow, but slower than their predators, and quick to tire? Perhaps they can sprint fast to escape an attack, but with their weight they can't outrun a pack.

not being able to do concentrated effort for a given period of time.

Sure, a lack of endurance in late pregnancy is common.

Make a dangerous world, not a weak pregnant person. Being pregnant does make you weaker in a fight. You need supervision, even if you are not the weakest.

(Inspired by a comment in Trioxidane's answer...)

I'm not sure how actually practical this would be, but I'm going to ignore that because of Rule of Cool...

Take a page from insects (or, perhaps more accurately, Xenomorphs): the fertile female doesn't carry her offspring like mammals or even reptiles. Instead they develop in an external and immobile (or at least, too big/awkward to drag around) organ which is attached to the female. The female may not necessarily be "weak", but she is definitely vulnerable by virtue of, at best, having to choose between near immobility (she can wave her limbs around but can't get up) or aborting her offspring.

This would work especially well if you have a small number of brooding females that gestate a large number of offspring.

Such an arrangement also negates many of the inherent problems with this arrangement; since few females are brooders (but produce a lot of offspring), the community as a whole can afford to feed and protect the brooders... who may, indeed, spend almost their whole lives as immobile baby factories.

Emergency mobility can be solved in a couple ways. One would be, as previously noted, that they can get up and move if they need to, but they lose their brood if they do. Another would be for them to just not care. Few things will make a species more obviously alien than a casual disregard for the brood queens' lives in an emergency; after all, there are plenty of drones that will escape and can become queens once a new home is found. (What's this "family" thing of which you speak?)

(It should go without saying that there is going to be some serious dimorphism going on here...)

Simultaneous Hibernation and Gestation

Many of Earth's more intelligent species adapted to subarctic climates follow a cycle of hibernation or semi-hibernation (bears, raccoons, etc). This is where they hide away and conserve their energy through the cold season because it is either too cold or food to scarce for being active to be advantageous to their survival. This conservation of energy leaves the animals quite weak from lethargy and vulnerable to predation if they do not hide themselves well.

These species' reproductive cycles tend to line up such that Gestation and Hibernation happen at the same time. This leaves the females of the species freed up to hunt and forage when food is most plentiful, and lets their body rest and focus on a less hazardous gestation when they are stuck inside all day anyway.

So, if you had a species adapted to a subarctic climates, but living in the tropical zone or climate controlled homes, the males of the species may not be triggered to go into hibernation since they would never get cold enough for that, but the hormones associated with pregnancy may still cause a vestigial hibernation response.

There's two variants I can think of.

Herd animals

Herd animals have more luxury to be vulnerable. They have more security from the group to protect them. Humans are a good example, as you can't get much more secure than us. We live in groups, have little enemies and our food supply is very stable (at least in parts of the world). Morning sickness, long pregnancies, pregnancy dementia, lower mobility and even after pregnancy depression among other things all make us weak. We can afford it, as our strength comes from numbers (and intellect).

Swarm animals

Queens of swarm animals aren't necessarily weak. They are larger and stronger than anyone outside their caste. Yet they are uniquely vulnerable when pregnant. They often move little and are hidden and protected as far away from danger as possible. Again, the strength flows from the rest of the hive/swarm, giving enough protection and food that they can afford a "weak" pregnancy.

Conclusion

Lessons learned from this you might find a species less relying on numbers, but still so secure they can afford "weak" pregnancies. Otherwise just grab from the above lessons. Advanced species are near certainly animals that rely on others. If more primitive, maybe they're just the highest in the food chain and a partner can already provide all other necessities. That way they'll still be able to afford a weak pregnancy.

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.