while I didn't go through ivf I did have q similar thing happen with my weight and fundal measurement. from about 35 weeks on my belly didn't grow at all. not 1cm, but having said that she was engaged from then on. I knew I was getting bigger, but for some reason didn't grow? I lost A LOT of weight (~25kg) in the first trimester and didn't start gaining weight until about 20 weeks.
I gave birth at 39+3 after spontaneous labour and DD was 3.24kg. not huge, but a nice size I had a scan about 3 days before I went into labour (hossie was worried I wasnt growing) and they predicted she was about 3.15kg or so, and thus within a normal range)

i guess what im trying to say is possibly if it feels like bub is growing, could it be that they have engaged at all?