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.In another report, even among women with known LMP, neonatal age assessment differed markedly from that assigned by certain menstrual dates in 15%.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.Gestational age, synonymous with menstrual age, is defined in weeks beginning from the first day of the last menstrual period (LMP) prior to conception.Accurate determination of gestational age is fundamental to obstetric care and is important in a variety of situations.For example, antenatal test interpretation may be dependent on gestational age.
In the first trimester, gestational sac mean diameter and crown-rump length measurements have become the primary means of evaluating gestational age.
Fetal growth retardation or macrosomia may be missed or incorrectly diagnosed owing to errors in gestational age assignment.
Interpretation of antenatal biophysical testing (non-stress tests and biophysical profiles) may be subject to variation with gestational age as well.
Timed ovulation, either by basal body temperature recording or semiquantitative assessment of luteinizing hormone surge, predicts gestational age within ±4 to 6 days.
Ovulation induction with agents such as clomiphene citrate and Pergonal, also accurately predicts gestational age.