Identifying Trimester in ICD-10
In ICD-9, the episode of care (antepartum, delivered, postpartum) is identified using a 5th digit of 0-4. In ICD-10, however, the trimester must be specified.
For services provided in an outpatient setting, the ICD-10 trimester character is assigned based on the gestational age at the time of the encounter.
The same principle applies to inpatient services, when the entire inpatient stay occurs within the same trimester.
According to ICD-10 guidelines, for inpatient stays which span more than one trimester, if the condition developed prior to the current admission, or is pre-existing, assign the character for the trimester at time of admission. If the condition developed during the admission, code the trimester in which the condition developed.
For example:
A patient is admitted at 27 weeks, 2 days with pre-existing essential hypertension and pre-eclampsia. It is now 2 weeks later (29 weeks, 2 days), and she remains in the hospital. The ICD-10 codes for the visit at 29 weeks, 2 days would be:
O11.2 Pre-existing hypertension w/Pre-eclampsia, 2nd trimester
O10.012 Pre-existing essential hypertension, 2nd trimester
Z3A.29 29 weeks gestation
In the example, although the patient is currently in her 3rd trimester, the codes for the condition have a 2nd trimester character, since the condition was present at the time of admission (during the 2nd trimester).
Also note that code Z3A.XX, which specifies the weeks gestation, is coded as 29 weeks. According to ICD-10, code Z3A. XX should always reflect the actual completed weeks gestation.