First trimester

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.

Ultrasound - obstetric

Article information

rID: 7358
System: Obstetrics
Synonyms or Alternate Spellings:
  • First trimesters
  • 1st trimesters
  • 1st trimester
  • First-trimester pregnancy

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Cases and figures

  • Case 1: normal first trimester pregnancy
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