When does each trimester begin?

The duration of pregnancy is divided in to three time periods called trimesters . The definitions of trimesters according to the 10th revision of the InternationalStatistical Classification of Diseases and Related Health Problems are:

• First Trimester Less than 14 weeks, 0 days

• Second Trimester 14 weeks, 0 days through 27 weeks and 6 days

• Third Trimester 28 weeks through delivery

REFERENCE

The Centers for Medicare & Medicaid Services, CMS,
https://www.cms.gov/Medicare/Coding/ICD10/Downloads/ICD10ClinicalConceptsOBGYN1.pdf. Accessed 11/21/2015