I'm pretty sure this is completely random. My experience coincides with that of HamUnDo.
What's even "worse" is that sometimes you can see Trixie to be pregnant in her personal description while she is totally non-pregnant and cheerfully working without showing any belly at the pub.
Most other girls' descriptions are consistent with their showing in-game. Trixie is the first (and until now) only one where I noticed the discrepancy.