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11 weeks to 14 weeks scan
1- Verify and document site, number and viability of pregnancy
2- Verify gestational age and pregnancy dating
3- Correctly obtain nuchal thickness measurement
4- Correctly visualize fetal nasal bone
5- Suspect evident major abnormalities
6- Comment on fetal activity & growth.
Apart from visualizing a normal fetal structure (head – chest – abdomen – limbs) this scan is very important to screen for chromosomal anomalies mainly Down syndrome (sometimes this is termed Down screening scan) this is done by measuring nuchal thickness and visualizing the fetal nasal bone;
Tip: Examination MUST be done with full urinary bladder.
Nuchal thickness is measured 11 to 14 weeks, practically, NT less than 2 mm carries low risk of chromosomal anomalies. Many criteria are put forward to "accurately" measure NT with the fetus in sagittal section and a neutral position of the fetal head neither hyperflexed nor extended. The fetal image is enlarged to fill 75 of the screen, and the maximum thickness is measured, from leading edge to leading edge. You may not be able to fulfill all the criteria but the most important is not to be "fooled" by the fetal amnion.
Illustration of Nuchal thickness mesurement, much more images and clips of 11 to 14 weeks anomaly scan are available in OBGYN Atas and Gallery
Nasal bone Fetal nasal bone is seen in the saggital view at 11 to 14 weeks gestation as it is easier to view at this gestational age. Absent nasal bone (flat face) may reflect chromosomal anomaly. I find this when associated with increased NT is an indication to perform free fetal DNA testing in maternal plasma.
Illustration of Fetal Nasal bone visulization, much more images and clips of 11 to 14 weeks anomaly scan are available in OBGYN Atas and Gallery
Illustration to compare normal feindings with increased nuchal thickness and absent fetal nasal bone, much more images and clips of 11 to 14 weeks anomaly scan are available in OBGYN Atas and Gallery
BASIC STRUCTURAL SCAN
WILL BE REVIEWED ALONG THE RELEVANT SPECIFIC SECTIONS
CRL; crown-rump length; between 7 and 13 weeks of gestation CRL is very important in order to correctly determine EDD (Expected Delivery Date). It is the most accurate measurement for dating pregnancy and can be used later in the management of FGR (Fetal growth restriction).
It is measured as the largest dimension of embryo/fetus, excluding the yolk sac and extremities.
Gestational age estimation is most accurate by CRL measurement, it carries less than 5 days falacy while 2nd trimester BPD carries about 7 days falacy, third trimester scan is not accurate to estimate gestational age.
Illustration of Crown-rump length and Biparietal diameter measurement, much more images and clips of 11 to 14 weeks anomaly scan are available in OBGYN Atas and Gallery
Cervical Measurement: Only with a full urinary bladder the cervical length and internal os diameter are measured and documented. These measurements are correlated with the patient’s obstetric history.
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