When does each trimester begin?
Pregnancy is divided into three time periods called trimesters. Based on the 10th revision of the International Statistical Classification of Diseases (ICD-10), the trimester definitions are:
- First Trimester: Less than 14 weeks, 0 days
- Second Trimester: 14 weeks, 0 days through 27 weeks, 6 days
- Third Trimester: 28 weeks through delivery
The Centers for Medicare & Medicaid Services (CMS). https://www.cms.gov/Medicare/Coding/ICD10/Downloads/ICD10ClinicalConceptsOBGYN1.pdf . Accessed 11/21/2015.