Why is this? I can understand if the cc hair on a sim is not set to appear for maternity, but I thought 0x0000137F was pretty much setting any hair to every category there is?..This is very annoying.

I had this problem too recently. If you change the sim's appearance, do you have the option to give them their original hair, even whilst they are pregnant? I found that, for some of my sims, their hairstyles would change at the first pregnancy pop but if I changed their appearance using a mirror I could select the original hairstyle and then it would change back, because the hair was set to be available during pregnancy but for some reason it was changing......

This is the first thing I tried of course, but as you know, maternity category does not appear on the mirror. My sim displayed wearing her original hair, not the one she changed into. there was..no changes to be made.

Hmmmm. Somehow I managed to change my sims' hairstyles. Did I use the mirror? Perhaps I used Christianlov's Gussy Up mod? I can't remember! Did you try using the mirror, changing the sim's Everyday style and then changing it back to the original hair before closing the change appearance window?

I think that I followed the instructions here.

I used the mirror..I could try blender, or gussy up I guess. I also saw that link, just before I posted actually! But thinking my sim was already showing up with the proper hair, I didn't bother too much.
So..I'll try a few things. Thank you!
Well, it worked! I used the mirror again, just to see if the trick works without resorting to a mod. I simply selected another hair, then reselect the one i want her to have. Done.
So thanks again.