Similarly, the magnitude of increased optical density above baseline at 450 nm (delta OD 450) by amniotic fluid spectrophotometric measurement is used to predict the severity of fetal hemolytic disease in pregnancies complicated by rhesus isoimmunization.
Test results are interpreted based on Liley's zones relative to gestational age.
Gestational age (GA) refers to the length of pregnancy after the first day of the last menstrual period (LMP) and is usually expressed in weeks and days. Conceptional age (CA) is the true fetal age and refers to the length of pregnancy from the time of conception.
This terminology does vary geographically and over time, and it may need clarification if not explicitly defined in relevant articles.
Specifically, the level of α-fetoprotein in both amniotic fluid and maternal serum is related to gestational age and when dates are inaccurate test results will be incorrect and misleading.
The date of the first documented positive pregnancy test and the beta-human chorionic gonadotropin (b HCG) level may help ascertain the minimum gestational age.
In women who conceived following assisted reproduction techniques, the date of embryo transfer is known and may date the pregnancy accurately.
The American College of Obstetricians and Gynecologists 409 12th Street, SW, PO Box 96920, Washington, DC 20090-6920 Ultrasound in pregnancy. The choice of transducer frequency is a trade-off between beam penetration and resolution.
To compare gestational age (GA) estimates in early pregnancy, determined by last menstrual period (LMP), human chorionic gonadotropin (h CG) concentration, ultrasound crown–rump length (Hadlock formula), and ovulation day (luteinizing hormone surge plus 1 day).