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
    • conception usually occurs ~2 weeks after a woman's last menstrual period (LMP) and the gestational sac is usually too small to be seen for ~3 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:

Practical points

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).


Ultrasound - obstetric
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Article Information

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

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