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:

• confirming intrauterine pregnancy (IUP)
  • early features supportive of an intrauterine pregnancy
    • double decidual sac sign
    • intradecidual sign
    • double bleb sign
• confirming the number of pregnancies
• dating of the pregnancy
  • crown-rump length (CRL) measurement is most accurate, however if no fetal pole is seen, the mean sac diameter (MSD) may be used.
• assessment of suspected early pregnancy failure
• assessment of suspected ectopic pregnancy
• antenatal screening for aneuploidy (nuchal lucency measurement)

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).

History and etymology

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.

See also

• second trimester ultrasound scan
• second trimester
• third trimester