How does a counseling and chapter 8 work for a pregnant Army Reserve Soldier?

TPU Soldiers DO NOT have the option to seperate from service obligation due to pregnancy.
AR 135-178 (Enlisted Administrative Separations) references AR 135-91 which states:
A woman who becomes pregnant after completing IADT will be counseled using the Pregnancy Counseling Checklist. She will be advised that she has the following options:
(1) Transfer and/or reassignment to the Retired Reserve (fig 4–5, para 2d) or to the IRR (fig 4–5, para 2g) if eligible.
(2) Transfer to the ING until pregnancy is no longer a factor (fig 4–5, para 2c).
AR 135–91 • 14 March 2016 29
(3) Continue membership in a unit and be granted maternity leave per paragraph 4–28 (fig 4–5, para 2e).

Chapter 8, per AR 635-200, this chapter applies to RA Soldiers, and ARNGUS and USAR Soldiers ordered to active duty, except for ARNGUS and USAR Soldiers found to be pregnant upon entry on IADT, to whom paragraph 5–1 applies. You are more than likely going to fall under Chapter 6 of AR 135-178.

If the doctor that wrote your diagnosis and treatment plan is NOT a military medical provider, then you need to take that documentation and have a military medical provider write you an actual Army profile. Until that happens, legally and per regulations, the unit can still require you to attend BTA. That military profile is the key component that you need to have in hand and in the system of MEDPROS. If it isn't there, then you are between a rock and a hard place.

You need to read Army Regulation 635 – 200 Active Duty Enlisted Administrative Separations Chapter 8. It will tell you everything you need to know, especially in relation to the counseling you should receive.

https://armypubs.army.mil/epubs/DR_pubs/DR_a/ARN30496-AR_635-200-000-WEB-4.pdf