Why would I have announced I would have done less work because I was pregnant? I don't work at a small company, and I do a desk job, so there was no reason to assume that I would have physical limitations.

Congratulation of being lucky enough to have a desk job which is not the case for the vast majority.

why does it matter that I took it all at once?

It does matter, at least in most cases

Lets say i take a week off, this means that for a week my job is understaffed. Some of what i should have done will be done by others, this is called compensation. The problem is that its not really possible to compensate for everything so once i comeback i will have a little extra work for the first few days and then things will be back to normal.

You might be thinking that compensating a whole month is just simply like adding up the compensation for 4 separate weeks but its something else entirely because of deadlines.There is also the fact that spending a whole month outside of the office environment will have an effect on the quality of your work while short 4-5 days break won't.

There is also the problem of having multiple people taking a break at the same time. The shorter the vacation the less likely it is to have multiple people off at the same time for things. If i take a month long vacation then i can be almost certain someone will fall sick or will have to take time off leaving the company even more understaffed.

Finally there is the problem of forcing your employer hand, planning work months ahead already stressful as it is without getting told that you will lose an employee for a month and that there is nothing you can do about it. By taking a month off you are also preventing anyone else from taking a vacation during that time.