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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:
- conceptus phase: 3-5 weeks
- embryonic phase: 6-10 weeks
- 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)
- confirming the number of pregnancies
- dating of the pregnancy
- assessment of suspected early pregnancy failure
- assessment of suspected ectopic pregnancy
- antenatal screening for aneuploidy (nuchal lucency measurement)
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.
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