I'd get some for every trimester, i think it's an awesome way to capture the memories of not only your growing bump but also your family life with your other kids during that time. It's amazing to see how much the older kids change from the start to end of the pregnancy. Obviously that depends on having a friend that could take the pics or a good priced shoot though. If i had to pay for a one off then i'd probably do them around 34 weeks .
I got mine done at 36 weeks and loved it.
You can take pictures at home to watch the "difference" for every week. But an actual professional shoot that you're going to pay for I say wait until between 34-36.