First trimester

Last revised by Andrew Murphy on 31 Jul 2022

The first trimester is defined as the first 13 weeks of pregnancy following the last normal menstrual period (some authors refer to early pregnancy as 0-10 weeks). It can be divided into a number of phases, each of which has typical clinical issues. These phases are:

  1. conceptus phase: 3-5 weeks
  2. embryonic phase: 6-10 weeks
  3. fetal phase: 10-12 weeks

During the first trimester, the pregnancy progresses from a tiny gestational sac with no visible embryo, to an ~80 mm fetus with identifiable features and internal organs. For a more detailed look at the stages of the first trimester see: ultrasound findings in early pregnancy

Ultrasound during this period is predominantly concerned with the following clinical issues:

The earlier in pregnancy a scan is performed, the more accurate the age assignment from crown-rump length. The initial age assignment should not be revised on subsequent scans 5.

Overall, the accuracy of sonographic dating in the first trimester is ~5 days (95% confidence range).

Trimester was first seen in English in 1821, a direct borrowing from the French word trimestre, itself introduced from the Latin word 'trimestris' meaning three months, which is derived from the Latin roots, 'tri-' meaning three and 'mensis' meaning month 6.

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