I see what youre saying now. So in the hospital I worked for, we don't code admitting diagnosis. So as you know, the admitting diagnosis will usually not be the same as the principal. Your admitting diagnosis will be what she came in with, and when you are saying patient is coming in strong labor pains, I wouldnt use an O code for that, as its really not a complication of the pregnancy.
It anything, you have 2 options. You can use the normal pregnancy code O80 since at the time of admission its technically a normal pregnancy until complications start showing up (nuchal cord, decels, anemia, etc) or you could use a Z code for supervision of pregnancy third trimester. At that point it will be more of what your facility wants to do for those scenarios.
Also, you mentioned about SROM, im not sure if you mean PROM or PPROM since there is no code for SROM since its just that the water broke and is also normal process as well, when its PROM or PPROM then thats when you will find a code for it.
hopefully this information clarifies your concern.