Do not point out that because the MTB chose to have kids, she has no right to complain about being uncomfortable.
(I have had people do this, they ask how I am, i say my feet/back ache, have heartburn etc, and they say well, you chose to get pregnant)
My response would have been that I didn't actually choose, it just happened despite trying not too. The worst assumption made is the all pregnancies were planned.
Then there are people (me included) who were wrongly, as it happened, told by Doctors that it may well be "difficult to conceive" for a number of reasons. I believed doctors, as you do, and yes, we were careless back in January. But being that we had been together a long time by that point (5 years +) and were engaged, we knew that if "it" were to happen, we would be ecstatic to become parents. So, we weren't trying and we weren't not trying.
Whatever your circumstances, people should just not come out with "Well you chose to have children"...it's a dismissive thing to say, almost like "You asked for it!". Yes, there are negatives to being pregnant, and indeed to being a parent...but you take the good with the bad.
There are people who complain incessently about pregnancy, birth, being a parent, but that sort of person complains about everything. The majority of us suck it up, put it down to being "part of it" and carry on. Certainly when I do want a bit of a rant (after a week with not much sleep, believe me I need a let-out sometimes!), I choose someone who will not come out with that sort of comment.
As a final word, I had an argument with my mother last weekend. It was stupid, I didn't help myself and I apologised. But the damage had been done. I explained I was tired and her response was "What do you want, a medal? You chose to have him..." Yes, she took it back and apologised, but like I said, the damage had been done. I now choose either my BFF, my Chief Bridesmaid for our wedding or my DF.