Ob sonogram for dating
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.
Abdominal circumference (AC) has a wide variation of 2 to 3 weeks because it involves measurement of soft tissue rather than bone and is also most affected by fetal growth.
These include an estimation of gestational age - optimally by 12 weeks, evaluation of fetal growth, assessment of vaginal bleeding, determination of fetal presentation , evaluation of a suspected multiple gestation, evaluation of a suspected ectopic pregnancy, evaluation of a suspected gestational trophoblastic neoplasia (molar pregnancy), confirmation of cardiac activity, fetal echocardiography, amniotic fluid index, biophysical profile, and many more.
Gestional Age Gestational age can be estimated based on measurements of specific structures.
Between 14 and 26 weeks, biparietal diameter (BPD) is usually most accurate, with a variation of 7 to 10 days.
These measurements vary depending on when the scan is performed, as does the accuracy of the estimation.
Transvaginal sonography has dramatically changed our ability to evaluate early pregnancy and early fetal growth and development.