Pregnancy ICD-10 Coding Refresher: Chapter 15 Guidelines and Trimester Selection

Understanding Chapter 15: Pregnancy, Childbirth, and the Puerperium

Chapter 15 in ICD-10-CM covers the Pregnancy, Childbirth, and the Puerperium (O00-O9A) codes. It is important to code according to the guidelines to avoid denials.

All pregnancy cases should have a secondary code assigned to represent the weeks of gestation (Z3A.00-Z3A.49). Chapter 15 codes and weeks of gestation codes are only to be assigned on the maternal record.



Using the ICD-10-CM Tabular for Pregnancy Code Selection

Use the coding tabular to help determine what Chapter 15 code should go before the diagnosis. It lists what codes fall under each category. See example below:

Determining Trimester Based on Weeks of Gestation

Once you determine what category your diagnosis falls under, the last character in that code will be what trimester the patient is in during her pregnancy at the time of the visit. Trimesters can be figured out by the week of gestation. If the patient is 11 weeks and 2 days pregnant, code to the week of last completion, which would be 11 weeks in this case.

• 1st trimester: week 1-13
• 2nd trimester: week 14-27
• 3rd trimester: week 28-birth

Pregnancy Coding Example: Epilepsy in the Second Trimester

Example: A pregnant mom has epilepsy, and she is at 15 weeks of gestation.

The codes would be: O99.352, G40.909 and Z3A.15.

Explanation: O99.352 is used because the patient has a neurological disorder and is in the 2nd trimester. G40.909 is used to further describe the disorder – epilepsy. Z3A.15 describes the patient being 15 weeks pregnant. Most diagnoses will be coded as a pair for a pregnant patient – one code from Chapter 15 and one from another chapter to further describe the condition. Lastly, you will have the Z code for weeks of gestation.



Katie Brown, CCS 

Senior Consultant, Audit at UASI

Katie Brown, CCS, is an AHIMA-certified Senior Coding Consultant specializing in CPT/ICD-10 coding, compliance audits, and revenue cycle support. She’s worked across diverse outpatient and inpatient settings, including ER, surgery, observation, and specialty clinics, and enjoys training coders with practical, CDI-friendly tips.

Works Cited

Centers for Medicare & Medicaid Services. (2025). ICD-10-CM Official Guidelines for Coding and Reporting, FY 2026.

Healthy Blue Kansas. (2024). Pregnancy coding and billing guidance. Available at: https://www.healthybluekansas.com