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 16 global obstetric CPT codes 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: Begin appending modifier TH to all antepartum E/M claims for new OB patients starting September 1, 2026, and verify payer acceptance of this modifier with your top commercial payers before the transition date.
What Changed and Why
In April 2026, 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 16 global obstetric codes 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 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 Needs to Act by September 1, 2026?
Every OB-GYN practice that provides antepartum care must begin transitioning by September 1, 2026. The deadline is not regulatory in the sense that a fine applies, 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 16 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. |
| 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.
The modifier TH transition is six days away. If your billing team is not ready to shift from global OB packages to individual E/M claims, a specialized OB-GYN billing partner can manage the transition while protecting your revenue through the code change. Get matched with vetted billing companies that specialize in obstetric and gynecologic billing.
What to Do Before September 1
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.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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 published 15 proposed HCPCS G-codes in the CY 2027 Medicare Physician Fee Schedule proposed rule on July 14, 2026. These G-codes would preserve the existing global payment structure for Medicare maternity patients alongside the new unbundled 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
What is modifier TH in OB-GYN billing?
Modifier TH is a HCPCS modifier appended to E/M codes to identify a visit as maternity care. Starting September 1, 2026, ACOG recommends appending TH to all antepartum E/M claims for new OB patients so payers can distinguish maternity visits from standard office encounters during the transition to unbundled obstetric billing.
Are the global OB codes being deleted?
Yes. Sixteen global obstetric CPT codes, including 59400 (vaginal delivery package), 59510 (cesarean delivery package), 59610 (VBAC package), and their component codes, are scheduled for deletion effective January 1, 2027. They are being replaced by individual codes for antepartum, labor management, delivery, and postpartum care.
What happens to patients already in antepartum care?
Patients whose first prenatal visit occurred before September 1, 2026, and who will deliver in 2026 can continue under the global package. Patients whose care extends into 2027 will have visits split across both code sets, using global codes for 2026 visits and E/M codes for 2027 visits, following the ACOG transition timeline.
Will this increase or decrease OB-GYN reimbursement?
For practices with a high volume of complex pregnancies, unbundled billing is expected to increase reimbursement because each additional visit generates its own claim. For straightforward pregnancies with a standard visit count, reimbursement should be roughly equivalent. The net effect depends on practice mix and payer contracts.
Do I need a new billing system for this transition?
Not necessarily. Most modern practice management systems can handle E/M coding with modifiers. The changes are to claim templates, superbills, and workflows, not to the billing platform itself. However, practices should verify that their system can append modifier TH and test claim submission before September 1.
What are the CMS G-codes and should I worry about them?
CMS proposed 15 HCPCS G-codes that would preserve the global OB payment structure for Medicare patients alongside the new CPT codes. ACOG opposes the proposal. The final rule is pending. Practices should prepare for the unbundled model regardless, since it will be the standard for non-Medicare payers starting January 1, 2027.
Next Steps
Contact your top payers this week to verify modifier TH acceptance for antepartum E/M claims before the September 1, 2026 transition date.
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.
The biggest OB-GYN billing change in 30 years hits January 1, 2027, and the preparation window starts September 1, 2026. Do not let the transition create denials, revenue gaps, or documentation failures. Get matched with vetted billing companies that specialize in obstetric and gynecologic billing and already understand the modifier TH workflow.
