Hi wishing, like El1en said, they mark on your notes out of 5ths, usually that is the amount of the head they can feel above pelvis but I know some midwifes do the other way round (as in how much has gone in to pelvis) so can be confusing. This is my first too and she keeps going 4/5 (forehead in) then free, thing is her head dims are quite large now so dont think she could fit until during a contraction which will squeeze the head through encouraging the overlapping of the skull to take place...also baby needs to line their head up right for engagement i.e. either left or right facing is best, back to back or back to belly makes it harder for head to engage as dims are larger...
also despite being told 36-38weeks as average for ftms, its actually more common 39-42...dont worry if they dont til labour though as thats quite common. by the scans we have had think our lil girl likes stroking her hair (yes she has 1.5cm length hair already!) and touching their face/sucking thumb etc a little too much hence why she has bobbed in and back out again lol... so long as they do engage during labour you'll be fine...