2026 ICD-10-CM Diagnosis Code O99.350Diseases of the nervous system complicating pregnancy, unspecified trimester

ICD-10-CM Codes›O00-O9A›O94-O9A›O99

ICD-10-CM O99.350
CMSSource: CMS FY 2026 ICD-10-CM dataset · Effective Oct 1, 2025 – Sep 30, 2026

O99.350 is a billable ICD-10-CM diagnosis code for diseases of the nervous system complicating pregnancy, unspecified trimester. It is valid on HIPAA claims for fiscal year 2026 (October 1, 2025 through September 30, 2026) and groups to MS-DRG 817 through 819, 831 through 833. The code is restricted by the Medicare Code Editor to maternity cases (age 9 through 64). Coders also document this condition as chorea gravidarum. In AHRQ's Clinical Classifications Software (CCSR), this diagnosis falls under Other specified complications in pregnancy.

Code Identity

ICD-10-CM Code
O99.350
Billable Status
Yes — Valid for Submission
Code Describes
Diseases of the nervous system complicating pregnancy, unspecified trimester
Short Description
Diseases of the nervous sys comp pregnancy, unsp trimester
Parent Code
O99.35
Diseases of the nervous system complicating pregnancy, childbirth, and the puerperium

Code Classification

ChapterO00-O9APregnancy, childbirth and the puerperium
SectionO94-O9AOther obstetric conditions, not elsewhere classified
CategoryO99Other maternal diseases classifiable elsewhere but complicating pregnancy, childbirth and the puerperium
This CodeO99.350Diseases of the nervous system complicating pregnancy, unspecified trimester

Code EditsBilling

Medicare Code Editor checks that affect claim validity for O99.350.

Maternity diagnoses
The Medicare Code Editor detects inconsistencies in maternity cases by checking a patient's age and any diagnosis on the patient's record. The maternity code edits apply to patients age ange is 9–64 years inclusive (e.g., diabetes in pregnancy, antepartum pulmonary complication).

Approximate SynonymsGuidance

Alternate terms and clinical phrases that map to this code.

  • Chorea gravidarum
  • Epilepsy in mother complicating pregnancy
  • Lymphocytic hypophysitis of pregnancy
  • Lymphocytic hypopituitarism
  • Maternal epilepsy due to perinatal stroke
  • Post-cerebrovascular accident epilepsy
  • Posttraumatic seizure
  • Restless leg syndrome in pregnancy
  • Restless legs syndrome
  • Sleep related movement disorder

Clinical ClassificationClinical

AHRQ’s CCSR groups this code into broader clinical categories.

CCSR PRG028
Other specified complications in pregnancy
Default principal diagnosis: inpatient Yes · outpatient Yes

Clinical InformationClinical

  • Chorea Gravidarum

    a rare movement disorder developed during pregnancy, characterized by involuntary jerky motion (chorea) and inability to maintain stable position of body parts (athetosis). rheumatic fever and collagen vascular disorders are frequently associated with this disease. chorea may vary from mild to severe and occurs in approximately 1 per 2,000 to 3,000 pregnancies. (from md med j 1997 sep;46(8):436-9)

  • Restless Legs Syndrome

    a disorder characterized by aching or burning sensations in the lower and rarely the upper extremities that occur prior to sleep or may awaken the patient from sleep.

  • Autoimmune Hypophysitis|Lymphocytic Hypophysitis

    an autoimmune condition affecting the pituitary gland, characterized by lymphocytic infiltration, commonly presenting with pituitary hormone deficiencies.

  • Grade 1 Hypophysitis, CTCAE|Grade 1 Hypophysitis

    asymptomatic or mild symptoms; clinical or diagnostic observations only; intervention not indicated

  • Grade 2 Hypophysitis, CTCAE|Grade 2 Hypophysitis

    moderate; minimal, local or noninvasive intervention indicated; limiting age-appropriate instrumental adl

  • Grade 3 Hypophysitis, CTCAE|Grade 3 Hypophysitis

    severe or medically significant but not immediately life-threatening; hospitalization or prolongation of existing hospitalization indicated; limiting self care adl

  • Grade 4 Hypophysitis, CTCAE|Grade 4 Hypophysitis

    life-threatening consequences; urgent intervention indicated

  • Grade 5 Hypophysitis, CTCAE|Grade 5 Hypophysitis

    death

  • Hypophysitis

    an inflammatory process in the pituitary gland.

  • Hypophysitis, CTCAE|Hypophysitis

    a disorder characterized by inflammation and cellular infiltration of the pituitary gland.

  • Lymphocytic Neurohypophysitis

    an autoimmune condition affecting the posterior pituitary gland, which is characterized by lymphocytic infiltration, and which often presents as diabetes insipidus.

Patient EducationClinical

Health Problems in Pregnancy

A health problem in pregnancy is any disease or condition that could affect your health or the health of your fetus. Some health problems may make it more likely that you will have a high-risk pregnancy. A high-risk pregnancy is one in which you, your fetus, or both are at higher risk for health problems than in a typical pregnancy.

The full article covers:

  • What are health problems in pregnancy?
  • What raises my risk for health problems during pregnancy?
  • Can chronic health conditions cause problems in pregnancy?
  • Can health problems in pregnancy be prevented?

Read the full article at MedlinePlus

Courtesy of MedlinePlus, a service of the U.S. National Library of Medicine.

Convert O99.350 to ICD-9-CMHistory

The closest ICD-9-CM equivalents under the General Equivalence Mappings.

ICD-9-CM
646.80 Preg compl NEC-unspec
Approximate The match is approximate rather than exact.

Code HistoryHistory

FY 2016AddedAdded to the ICD-10-CM code setEffective October 1, 2015, the first year of ICD-10-CM.
FY 2017–2025No changes
FY 2026CurrentCurrent code set, no changesEffective October 1, 2025 through September 30, 2026.

Questions About O99.350Overview

Is O99.350 a billable code?

Yes. This is a billable ICD-10-CM code, specific enough to report diseases of the nervous system complicating pregnancy, unspecified trimester on HIPAA-covered claims from October 1, 2025 through September 30, 2026.

What MS-DRG does O99.350 group to?

When diseases of the nervous system complicating pregnancy, unspecified trimester is the principal diagnosis on an inpatient stay, it groups to MS-DRG 817, 818, 819, 831, 832, 833, with relative weights from 0.5229 to 1.6880 depending on complications. Higher weights mean higher Medicare reimbursement.

Who can O99.350 be reported for?

The Medicare Code Editor checks diseases of the nervous system complicating pregnancy, unspecified trimester against patient demographics: this code is intended for maternity cases (age 9 through 64). Claims outside these limits are flagged as inconsistent.

What is the ICD-9 equivalent of O99.350?

Under the General Equivalence Mappings, diseases of the nervous system complicating pregnancy, unspecified trimester converts to ICD-9-CM 646.80 (preg compl NEC-unspec). The mapping is approximate, so confirm the match fits the documentation.

Footnotes

[1] Not chronic - A diagnosis code that does not fit the criteria for chronic condition (duration, ongoing medical treatment, and limitations) is considered not chronic. Some codes designated as not chronic are acute conditions. Other diagnosis codes that indicate a possible chronic condition, but for which the duration of the illness is not specified in the code description (i.e., we do not know the condition has lasted 12 months or longer) also are considered not chronic.