First trimester dating ultrasound accuracy
Repetitive arrays of ultrasound beams scan the fetus in thin slices and are reflected back onto the same transducer.
The information obtained from different reflections are recomposed back into a picture on the monitor screen (a sonogram, or ultrasonogram).
Fetal body measurements reflect the gestational age of the fetus. In patients with uncertain last menstrual periods, such measurements must be made as early as possible in pregnancy to arrive at a correct dating for the patient. In the latter part of pregnancy measuring body parameters will allow assessment of the size and growth of the fetus and will greatly assist in the diagnosis and management of intrauterine growth retardation (IUGR).
a) The Crown-rump length (CRL) This measurement can be made between 7 to 13 weeks and gives very accurate estimation of the gestational age.
A short history of the development of ultrasound in pregnancy can be found in the History pages.
Ultrasound scan is currently considered to be a safe, non-invasive, accurate and cost-effective investigation in the fetus. The gestational sac can be visualized as early as four and a half weeks of gestation and the yolk sac at about five weeks.
At 5-8 weeks a bradycardia (less than 90 bpm) is associated with a high risk of miscarriage.
(Chart and further comments) Similar to the BPD, dating using the FL should be done as early as is feasible.Currently used equipments are known as real-time scanners, with which a continous picture of the moving fetus can be depicted on a monitor screen.Very high frequency sound waves of between 3.5 to 7.0 megahertz (i.e.A positive pregnancy test 3 weeks previously for example, would indicate a gestational age of at least 7 weeks.Such information would be useful against the interpretation of the scans. In the presence of first trimester bleeding, ultrasonography is also indispensible in the early diagnosis of ectopic pregnancies and molar pregnancies. Determination of gestational age and assessment of fetal size.