Walif Chbeir Radiology Notes 2016- 12- 05.
Attribution of Gestational Age ( GA) by UltraSound
A- On initial UltraSound :
* Early From 5 to 6 weeks gestation, two methods are used:
1-measurement of mean sac diameter (MSD)
It increases from 2 mm at 5 weeks to 10 mm at 6 weeks.
2- Sonographic identification of gestational sac contents.
the identification of a gestational sac in the uterine cavity is the first distinguishable sign of an intrauterine pregnancy at 5.0 weeks,
the yolk sac at 5.5 weeks ,
the embryo and heartbeat at 6.0 weeks.
At 6,3 weeks and beyond, an embryo of 5 mm or greater in length will be discernible and heartbeat must always then be seen if the embryo is alive.
* From 6 weeks until the end of the first trimester (12W), GA is tightly linked to the crown-rump length (CRL) of the embryo or fetus.
2-Second Trimester (13W-24W)
* If OFD measurable: BPDc or HC are the best predictors of gestational age. Corrected-BPD and HC, are equivalent in accuracy and more accurate than the BPD throughout gestation because they take head shape into account
* If OFD not measurable: BPD or FL
3-Third Trimester (25W- 40 W )
* If OFD measurable, BPDc or HC or FL or composite age formulas: all provide gestational age with comparable accuracy. A potential disadvantage of using Composite age formulas is that the gestational age will be underestimated in a fetus with a skeletal dysplasia.
* If OFD non measurable: FL
4 – When the initial scan occurs late in pregnancy, the better is to use ultrasound criteria up to 24 weeks of gestation and the LMP (if available) after 24 W.
B– The age at any time later in pregnancy (2ème US) , should be estimated on the basis of the initial Ultrasound examination (GA at the time of the first scan + number of weeks since until now).
On following examinations, fetal measurements (BPD, OFD, AC, and FL) should be obtained and should be compared to the normal standards for the GA to determine the weight of the fetus.
Fetal Measurements: Normal and Abnormal Fetal Growth, Carol B. Benson and Peter M. Doubilet, in DIAGNOSTIC ULTRASOUND 4th EDITION, Carol M. Rumack ISBN: 978-0-323-05397-6
NB: Pregnancy is considered at term when gestation has lasted between 37 and 42 weeks.
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