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The 20-week ultrasound is also known as the anomaly scan. The main objective of the scanis to determine whether the development of the child’s structures and organsare taking place normally. This is a detailed scan typically done between 18weeks and 20 weeks of gestation during which each part of the fetal anatomy is examined to see if the baby is developing normally. Special attention is paid to the brain, face, spine, heart, stomach, bowel, kidneys and limbs. Another observation that would be made is whether there is sufficient amniotic fluid.
Examples of physical defects that can be revealed by a 20-week ultrasound include: spinal cord defects, improper formation of the skull, accumulation of fluid in the brain, heart defects, a hole or rift in the diaphragm, a hole or rift in the abdominal wall, missing or malformed kidneys, missing or malformed bones, malformed arms and legs.
The anomaly scan is done between 18 and 20 weeks (second trimester). In addition to this scan, theobstetrician may ask more scans if:
While a majority of babies are born healthy, a small percentage stands the risk of developing complications. Even a normal anomaly ultrasound scan is no guarantee of a healthy child as some defects will not show up on the ultrasound. The procedure by itself does not pose any risk to the mother or child.
The specialistwill examine all the baby's organs and take measurements. These include:
The placenta will be described as low if it reaches down to or covers the mouth of the uterus (the cervix). If the placenta is lying low in the uterus, the patient will have another scan in the third trimester to check its position. By then, it's likely the placenta will have moved away from the cervix.
It is possible to count the three blood vessels (two arteries and a single vein) in the umbilical cord, but it is not routine procedure.
The level of amniotic fluid for the baby to move freely will be checked.
Also parts of thebaby's body will be measured, to gauge the growth. The measurements include:
The measurements should match up to what's expected for thebaby, depending on when the due date is.
Fetal medicine specialists have a list of conditions to look out for. In ‘mid trimester anomaly scan’ (18 to 22 weeks as per ISUOG guidelines), only the fetal parts mentioned in the scan report have been evaluated. Parts of fetus not mentioned in the report are not evaluated in a routine scan. For eg. counting of fingers/toes, assessment of external ears, palate etc.
Abnormal conditions are either very serious, which may mean that a baby can't survive, or they may be treatable conditions, once the baby is born.
If the condition is treatable, the fetal medicine specialist will collaborate with the obstetrician to enable adequate care once the child is born.
This scan can also identify 50% to 60% of cases of Down syndrome, but the First Trimester Screening (FTS) test is definitely better for this.
Because a portion of cases of Down syndrome appear normal on ultrasound, only an amniocentesis (testing the fluid surrounding the baby for its chromosomes) can give this information for certain.
Sometimes babies with chromosomal abnormalities have signs called ultrasound markers. These include in order of importance thick skin behind the neck (nuchal fold), absent nasal bone, mild fluid within the ventricles of the brain, aberrant subclavian artery in the neck, occasionally short arms or legs.
While some babies with chromosomal abnormalities have these markers, it is important to remember that many normal babies also have these signs. The only way to diagnose or exclude a chromosomal problem for certain is to have an amniocentesis.
If the 20-week ultrasound reveals any abnormalities, the consequences to thechild may not yet be clear. The patientwill be offered the option of follow-up testing. Any defect(s) revealed during the original test will then be examined in further depth. The patient may also be offered the option of undergoing an amniocentesis procedure.
Unlike thefirst trimester scans, patientsdo not need a full bladder for this scan. Thebaby is now big enough and high enough in theabdomen to be seen very clearly by a scan conducted via the abdomen.