Modifier TH for Antepartum Billing Began in September 2026: What Every OB-GYN Practice Must Do Now

What Is the Modifier TH Transition for OB-GYN Billing?

As of September 1, 2026, ACOG recommends that OB-GYN practices begin billing antepartum visits using individual Evaluation and Management (E/M) codes with the HCPCS modifier TH appended, rather than accumulating visits toward a global obstetric package. This transition prepares practices for the January 1, 2027 deletion of 17 maternity care CPT codes, including the four global packages, and their replacement with a new unbundled maternity care code set developed by the AMA and ACOG.

Why September 1: Patients who present for their first prenatal visit on or after September 1, 2026, will deliver in 2027 under the new code set, so billing their antepartum visits under E/M codes from the start avoids a mid-care code switch.

What is changing: The familiar global codes, including CPT 59400 (vaginal delivery package) and CPT 59510 (cesarean delivery package), will be deleted effective January 1, 2027, and replaced with individual codes for each phase of care.

What to do now: Append modifier TH to antepartum E/M claims for new OB patients whose first prenatal visit was on or after September 1, 2026, as ACOG recommends, and verify payer acceptance of this modifier with your top commercial payers, since payer requirements vary.

What Changed and Why

In September 2025, the AMA CPT Editorial Panel approved a comprehensive restructuring of maternity care coding, developed in collaboration with ACOG and multiple specialty societies. The restructuring deletes 17 maternity care codes, including the four global packages, and replaces them with a framework built around four distinct phases of care: antepartum, labor management, delivery, and postpartum. The new codes take effect January 1, 2027.

The global obstetric billing model has been in place for over 30 years. Under it, a single CPT code like 59400 or 59510 bundles all antepartum visits, delivery, and six weeks of postpartum care into one payment. The problem, as ACOG has documented, is that the global model pays the same amount regardless of how many antepartum visits a patient requires or how complex the pregnancy is. A straightforward pregnancy and a high-risk case requiring twice the visits and significantly more clinical work generate the same reimbursement.

The new system eliminates that bundling. Each antepartum visit is billed individually using standard E/M codes (99202 through 99215 for office visits), with modifier TH appended, as ACOG recommends (check each payer), to identify the visit as maternity care. Delivery is billed using new labor management and delivery codes. Postpartum visits are billed separately. This structure allows practices to be compensated based on the actual volume and complexity of care delivered.

One question we hear constantly from OB-GYN practice managers is why their maternity revenue does not reflect the intensity of care they deliver to high-risk patients. The global package model is the direct answer. The 2027 transition addresses that disconnect.

Who Needed to Act by September 1, 2026?

ACOG recommended that every OB-GYN practice that provides antepartum care begin transitioning by September 1, 2026. The date is an ACOG recommendation, not a regulatory deadline, but ACOG’s guidance is explicit: patients who present for their first prenatal visit on or after September 1, 2026, should have their antepartum visits billed under E/M codes with modifier TH from the start.

The reasoning is practical. A patient who begins prenatal care in September 2026 with an estimated due date in late spring or early summer 2027 will deliver under the new code set. If the practice bills the early visits under the old global package, it will need to unwind and rebill those visits when the global codes are deleted on January 1, 2027. Starting on E/M codes from the first visit eliminates that administrative problem.

Practices using CPT 59610 (VBAC after previous cesarean) face the same transition timeline. The VBAC global package is among the 17 codes being deleted. Practices that handle VBAC deliveries should verify modifier TH acceptance with their payers now, because VBAC patients often involve more antepartum visits and more complex documentation.

Before and After: How Billing Changes

The table below shows how the same patient encounter is billed under the current global model versus the new unbundled model taking effect in 2027.

Service	Current Global Model (through 2026)	New Unbundled Model (2027)
Antepartum visits	Bundled into global code (59400, 59510). No separate billing.	Each visit billed individually with E/M code + modifier TH (ACOG-recommended; check each payer).
Delivery	Included in global code.	Billed using new labor management and delivery codes.
Postpartum care	Bundled (6 weeks included).	Billed separately per visit.
High-risk add-on	Same payment regardless of visit count or complexity.	Each additional visit generates its own E/M claim.
Split care	Requires antepartum-only codes (59425, 59426) with manual visit counting.	Each provider bills their own E/M visits individually.

The practical effect is that practices with a high percentage of complex pregnancies, multi-provider teams, or telehealth-integrated prenatal care plans stand to see more accurate reimbursement under the new model. The risk is on the administrative side: billing teams that are not prepared for individual-visit claim submission will face a significant workflow change on January 1, 2027.

What to Do Before January 1, 2027

The transition requires both billing workflow changes and payer verification. Practices that start now avoid the claim rejections and revenue gaps that will hit practices that wait until January.

1. Verify modifier TH acceptance with each payer. Contact your top commercial payers and confirm they will accept E/M codes with modifier TH for antepartum visits. Not all payers have published transition guidance yet.
2. Update your billing software templates. Create new claim templates or superbills that default to E/M codes with modifier TH for antepartum encounters instead of routing to the global package.
3. Train coding and front-desk staff. Staff who have always billed under the global model need to understand that every prenatal visit now generates its own claim with its own documentation requirements.
4. Identify patients who straddle the transition. Patients currently in antepartum care whose due dates fall in 2027 will have visits billed under both systems. Use the ACOG transition timeline to determine when to switch.
5. Adjust your revenue cycle forecasting. Global packages generate one large payment at delivery. Unbundled billing generates multiple smaller payments throughout the pregnancy. Cash flow timing will change.
6. Document E/M level for every antepartum visit. Under the global model, visit-level documentation requirements were less rigorous. Under E/M billing, every visit must support the level billed through medical decision-making or time documentation.

CMS G-Codes and the Dual System Risk

Separate from the AMA/ACOG code restructuring, CMS floated 15 HCPCS G-codes in a comment solicitation in the CY 2027 Medicare Physician Fee Schedule proposed rule released on July 14, 2026. These G-codes would preserve the existing global payment structure for Medicare maternity patients in place of the new unbundled CPT codes, while other payers use the new CPT codes.

ACOG has publicly opposed this proposal. In a statement from ACOG President Camille A. Clare, MD, MPH, the organization warned that running two billing systems simultaneously would increase administrative burden, divide patients by insurer, and risk worsening obstetric care disparities. ACOG has urged CMS to make a clean break and adopt the new codes without preserving the global framework through G-codes.

As of August 2026, the CMS proposed rule is still in the public comment period. The final rule is expected later in 2026. Practices should prepare for the unbundled model regardless of the G-code outcome, because the new CPT codes will be the standard for all non-Medicare payers on January 1, 2027. In our experience matching OB-GYN practices with billing partners, the practices that prepare for the most complex scenario, which is operating under both systems, are the ones that avoid revenue disruption.

Common Mistakes During the Transition

Across the billing companies we vet for OB-GYN practices, the same transition errors keep surfacing as practices begin testing the new workflow.

Waiting until January 2027 to change anything. The code deletion happens on January 1. Practices that have not tested modifier TH acceptance, retrained staff, and updated templates by then will face claim rejections from day one.

Applying modifier TH incorrectly. The TH modifier goes on the E/M code, not on the diagnosis. It identifies the visit as maternity care so the payer does not process it as a standard office visit.

Assuming all payers will transition at the same pace. Some commercial payers have already published transition guidance. Others have not. Verify with each payer individually rather than assuming a blanket policy.

Underdocumenting antepartum visits. Under the global model, individual visit documentation did not drive reimbursement. Under E/M billing, it does. Every visit needs to support the level billed through either medical decision-making complexity or time.

Not adjusting revenue projections. The global package paid one sum at delivery. Unbundled billing spreads payments across multiple claims. Practices that do not update their forecasting models will misread their AR for months.

Frequently Asked Questions

Next Steps

Contact your top payers this week to verify modifier TH acceptance for antepartum E/M claims before the January 1, 2027 code change.

Review the ACOG Payment Advocacy for Obstetric Services page and the AMA maternity care coding transition resources for detailed code-by-code guidance.

If your billing team is not staffed to manage the transition alongside daily claim volume, get matched with an OB-GYN billing partner that specializes in obstetric coding and already has the new workflow in place.