(P-555) The Utility of Non-specific ICD-10 Z3A Codes in Estimating Gestational Age

Background: With the introduction of new International Classification of Diseases, 10th revision (ICD-10) Z3A diagnosis codes denoting weeks of gestation, billing codes now include more detail related to timing of pregnancy. The additional timing detail allows for more accurate estimation of last menstrual period (LMP, or pregnancy start date) within healthcare administrative claims databases than with previous methods that backdated LMP date based on the pregnancy outcome date (e.g. 39 weeks of gestation for full term births). While codes ranging from Z3A.08 through Z3A.42 denote exact weeks of gestation, three Z3A codes are non-specific: Z3A.00 (unspecified weeks gestation), Z3A.01 ( <8 weeks gestation), and Z3A.49 (>42 weeks gestation). The utility of these non-specific codes in estimating LMP date remains to be determined.

Objectives: To investigate whether and how non-specific Z3A codes may be utilized to refine estimated LMP.

Methods: We used Z3A codes that denote the week of gestation to identify pregnancies in the Optum Research Database between October 2015 and September 2020. The LMP for each pregnancy was estimated using the median date of all specific Z3A codes observed during the pregnancy period. The presence of non-specific Z3A codes relative to the specific Z3A codes was described.

Results: Among 791,027 women with ≥ 1 Z3A code, 7% had non-specific Z3A codes only, 59% specific Z3A codes only, and 35% a combination of specific and non-specific Z3A codes. Among 59,785 women without any specific Z3A codes within 42 weeks of a non-specific Z3A code, 39% had ≥ 1 Z3A.00 code, 81% ≥ 1 Z3A.01 code, and 0.1% ≥ 1 Z3A.49 code. Among 267,147 women with specific and non-specific Z3A codes within 42 weeks of each other, these percentages were 63%, 53%, and 0.3%, respectively. Among pregnancies where both specific and nonspecific Z3A codes were present, Z3A.00 codes were most commonly observed at week 40 of gestation, but they were often observed in the first and second trimester. Z3A.01 codes were typically observed between weeks 5 and 9 of gestation with week 7 being most frequent. Z3A.42 codes were observed rarely and were not associated with a specific time point in pregnancy.

Conclusions: The non-specific ICD-10 code Z3A.01 was most commonly observed at 7 weeks gestation, providing value in estimating LMP for pregnancies that do not have more specific Z3A codes. Z3A.00 and Z3A.49 do not appear to be useful in estimating LMP.