6 rules relative age dating
Fetal heart rate reactivity and fetal breathing develop with advancing gestational age; therefore, the absence of these biophysical parameters may be interpreted as abnormal for fetuses in whom the gestational age has been overestimated.
Obstetric management is also dependent on gestational age.
In the second and third trimesters, fetal head, body, and extremity measurements have been commonly used to assess gestational age.
Those parameters most commonly measured include biparietal diameter, In the first trimester, the gestational sac mean diameter and crown-rump length are used to establish fetal age.
Bimanual examination in the first trimester may be accurate within ±2 weeks; however, fundal height measurement, which is more commonly used to assess gestational age, is only accurate within ±4 to 6 weeks.In the past gestational age was established by a combination of the historical information and the physical examination.Reliance was placed on the menstrual history and the maternal sensation of fetal movement (“quickening”).Other factors include assessment of uterine size by bimanual examination in the first trimester, initial detection of fetal heart tones by Doppler (10–12 weeks) or auscultation (19–21 weeks), and uterine fundal height measurement.However, both the history and the findings on physical examination are fraught with error, even in the best of circumstances (Table 1).