الفهرس 
Normal Anatomy of the Fetus by Ultrasound ExaminationOnly 14 pages are availabe for public view
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Abstract
An abnormal fetal nuchal finding in the early 2nd trimester may serve as a marker of fetal chromosomal anomaly. Fetal karyotyping and detailed sonographic examination is advocated in all these cases.
First-trimester combined screening, either alone or as part of sequential or integrated tests, is widely accepted and effective in detecting Down syndrome and other chromosomal abnormalities. Before the advent of first-trimester aneuploidy screening, second-trimester multiple-serum marker testing was the standard Down syndrome screening test.
Second-trimester sonography reliably modifies the serum-based trisomy 21 risk, improving overall screening efficiency. However, there is a paucity of data on adjusting a combined screen’s trisomy 21 risk with second-trimester sonography. Therefore, it is better to evaluate the trisomy 21 screening performance of the first trimester combined test followed by second-trimester sonography.
At 11th to 13th week an intracardiac echogenic focus, or hydronephrosis or hyperechogenic bowel was observed. Consequently, first-trimester screening using these sonographic markers would be associated with a substantially lower detection rate and higher false positive rate than with screening by fetal NT thickness and maternal serum free β-hCG.
Fast MR imaging is a valuable complement to US in various obstetric settings when results of US are inconclusive. It can provide important information that may influence a patient’s treatment.
The role of MRI in fetal imaging is that it provides around 10% additional findings, MRI mostly done in late second or third trimester however its limitations are as follows : High cost, Limited availability, Limited expertise, High Potential for false positive and it is of questionable value at < 24 weeks.
In conclusion , routine ultrasound done in first and second trimesters found to be useful in detection of anomalies associated with Down Syndrome , however MRI was found to have an upper hand upon sonography in case of cerebral anomalies as well as having better tissue characterization than sonography especially during second and third trimesters of pregnancy.