first trimester

Radiopaedia • CC-by-nc-sa 3.0 • de

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

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

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)
- early features supportive of an intrauterine pregnancy
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 .

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