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:

  • 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:

    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).

    Siehe auch:
    und weiter: