Scan Markers found on ultrasound scan in the Second Trimester of pregnancy
Chromosomal abnormalities occur in 0.1% to 0.2% of live birth. Down’s syndrome is the most common and most important chromosomal abnormality (Trisomy21). Other chromosomal abnormalities detectable on ultrasound scan examination includes; Trisomy 13, 18, Turner’s syndrome X0 and Triploidy.
Second-trimester ultrasound scan detects 2 types of sonographic markers suggestive of chromosomal abnormalities:
A- Markers for major fetal structural abnormalities: this comprises:
1- Central Nervous System abnormalities include ventriculomegaly, holoprosencephaly, microcephaly and Dandy Walker Syndrome.
2- Musculo -skeletal abnormalities includes hand and feet abnormalities.
3- Face abnormalities includes Hare lip and cleft palate etc….
4- Neck abnormalities this includes cystic hygroma, etc...
5- Cardiac abnormalities include holes in the heart, Fallot’s tetralogy etc…
6- Gastrointestinal tract this includes atresia, diaphragmatic hernias, etc….
7- Genito urinary tract this included hydronephroses, absent kidney etc…
8- Miscellaneous conditions include intrauterine growth restriction and hydropes fetalis
B- The second type of markers is known as “soft markers” of chromosomal abnormalities. These latter markers are nonspecific, often transient, and can be readily detected during the second-trimester ultrasound. The most commonly studied soft markers include:
1- Increased Nuchal Translucency (Down’s syndrome) or Increased nuchal fold in 2nd trimester.
2- Shortened femur length or short long bones
3- Choroids plexus cysts
4- Intracardiac echogenic Foci
5- Echogenic bowels
6- Single umbilical artery i.e. 2 vessels cord
7- Dilated renal pelvis
There is a great deal of interest in the ultrasound detection of chromosomal abnormalities, however evaluating the significance of the soft markers varies widely and show contradictory results. Major abnormalities are observed in fewer than 25% of affected fetuses in most studies, whereas 1 or more soft markers may be observed in at least 50% of cases. Ultrasound examination in the second trimester of pregnancy currently diagnoses 50% to 70% of cases of Down’s syndrome, 70% to 100% trisomy 18, and 90% to 100% trisomy 13 approximately.
In general ultrasound scan in pregnancy does not diagnose Down’s syndrome. Even if one of these soft markers are seen during the ultrasound examination, It is important to remember that most babies are found to be perfectly healthy normal babies with no underlying chromosomal abnormalities
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