First Trimester 12 Week Ultrasound
The First Trimester Ultrasound (NT) Scan
|At the first trimester scan they confirm that the fetus is alive, they assess the gestational age by measuring the crown-rump length and will look for any major problems. This is a routine ultrasound examination done at 10 to 14 weeks of gestation.|
During the examination, the fetus is seen by abdominal ultrasound. Occasionally the view is not clear and it may be necessary to perform a vaginal scan. You do not need to have a full bladder, a moderately filled bladder is needed for a proper examination.
Most parents look forward to their scan because it gives them the first glimpse of their baby. You will probably be give you a printout of your baby by the sonographer as a keepsake, it is important to remember the main purpose of the scan.
The first Trimester Scan, is not to provide the first photo for your baby album, or to find out your baby's sex. It is to check that your baby is growing and developing normally.
Examples of ultrasonography you may see used during your pregnancy.
What is actually done?Scans are usually performed by a doctors, midwives or radiographers who are specially trained in ultrasound, and are known as sonographers. The whole pregnancy will be assessed. The amount of liquor (amniotic fluid), the position and appearance of the placenta and a detailed scan of the baby itself. The baby will be measured, and the anatomy examined in detail.
It is recommended that all pregnant women have a scan in the first trimester at around 12 weeks of pregnancy to confirm their dates. First trimester ultrasonic scans may show 'soft' markers for chromosomal abnormalities, such as an increased fetal nuchal translucency (back of the neck) to enable detection of Down's syndrome fetuses.
When is the nuchal fold screening test carried out?The test is performed between 11 weeks and 13 weeks 6 days into your pregnancy. The test cannot be performed outside this time frame as certain features that need to be seen on the scan are not present. This test results in a 90–95% detection rate of pregnancies that may be at an increased risk of carrying a genetical problem such as Downs Syndrome.
If you are unsure of your period dates or have an irregular cycle, we suggest that you contact your doctor, midwife or health care provider as early as possible to arrange for a dating scan.
There is no hard and fast rule for the number of scans you should have during pregnancy.
A scan maybe ordered when an abnormality is suspected on clinical grounds. Otherwise a scan is generally booked in the first trimester to confirm pregnancy, exclude ectopic or molar pregnancies, confirm cardiac pulsation and measure the crown to rump length for dating.
What is the 12 week NT scan?
|The 12 week scan is a routine ultrasound examination carried out at 10 to 14 weeks of gestation. During the examination, the fetus is seen by abdominal ultrasound. Occasionally the view is not clear and it may be necessary to perform a vaginal scan. At the first trimester scan they confirm that the fetus is alive, they assess the gestational age by measuring the crown-rump length and will also look for any major problems.|
What is crown to rump length (CRL)?The crown rump length is a universally recognized term, very useful for measuring early pregnancies. The CRL (see image above between the two white + marked 1) is highly reproducible and is the single most accurate measure of gestational age. Charts have been developed for this purpose, but some simple rules of thumb can also be effectively used. From 6 to 11 weeks gestational age, the fetal CRL grows at a rate of about 1 mm per day.
Crown Rump Length:
Gestational age = 6 weeks plus (CRL x days). For example, a CRL of 23 mm would correspond to a gestational age of 9 weeks and two days (6 weeks plus 23 days = 9 weeks and 2 days). Accuracy of CRL after 12 weeks in predicting gestational age diminishes and is replaced by measurement of the width of the fetal head (biparietal diameter or BPD).
What is the Nuchal Translucency?The nuchal translucency (also spelled nucal translucency) is a collection of fluid beneath the fetal skin in the region of the fetal neck and this is present and seen in all fetuses in early pregnancy. The fluid collection is however increased in many fetuses with Down's syndrome and many other chromosomal abnormalities. It is called a 'translucency' because on ultrasound this appears as a black space beneath the fetal skin. It is this black space that you will see measured during the ultrasound scan.
Currently the most accurate non invasive test for detecting Down syndrome during pregnancy is the measurement of the nuchal translucency with an ultrasound between 11 to 14 weeks of pregnancy. This is normally less than 2.5mm and when seen increased (greater than 2.5mm, see image on the right) may indicate the baby has Down syndrome or may indicate another chromosomal abnormality. 95% of measurements will indicate a reduced risk.
Fetal Nasal Bone
|The nuchal translucency test will also check whether your baby has a visible nasal bone. In the past few years it has been seen that approximately 3 in 4 babies with Down's syndrome do not have a visible nasal bone at the time of the first trimester screening test. If the nasal bone is visible at the scan then this will reduce the chance of your baby having Down's syndrome. |
The view of the nasal bone should show three distinct lines:
• The first two lines, are horizontal and parallel to each other, resembling an 'equal sign'. The top line represents the skin and bottom one, represents the nasal bone.
• A third line, almost in continuity with the skin, but at a higher level, represents the tip of the nose.
Occasionally the nasal bone cannot be seen at 11 weeks as it is too early in the pregnancy.
If this is the case and this creates some concern then the scan can be repeated a week later.
However if the nasal bone cannot be seen clearly but the nuchal translucency and blood tests are predicting a low risk result it is sometimes not necessary to see the nasal bone in every case.
Examples of ultrasonography you may see used during your pregnancy.
What other markers are checked?
|The nuchal translucency test can also check whether your baby has all its limbs, that the head and brain appear to be developing normally, that the baby has a visible stomach and bladder and the umbilical cord is inserted correctly. From 12 weeks the spine can usually be seen clearly enough to rule out major cases of spina bifida. All of this information will hopefully provide important reassurance.|
Which markers are important?In large studies of thousands of pregnancies these following features were found to be the most important in changing the risk of Down syndrome:
The following features would also be looked for but are not so important as those listed above, and have been reported by some studies to give an increased risk of chromosomal problems.
The shape of the fetal head tends to be more round than oval; the fetal nasal bone is shorter; the hands tend to be broader with shorter fingers; the little finger can be shorter and bent (clinodactyly) with an absent middle bone; the hands in Edwards syndrome are clenched with overlapping fingers; the feet can be abnormal with so called 'rocker bottom”' appearance, the heel can be very prominent or there could be a wide gap between the big toe and the other toes (Sandal gap).
What is the first trimester biochemistry (blood test) for?
|As part of the NT Scan a small sample (about 5 mL) of blood is collected from the expectant mother. This is to perform two biochemical tests on hormones released by the body during pregnancy on a biochemistry analyser. These are namely PAPP–A (Pregnancy Associated Plasma Protein–A) and free beta–hCG (human chorionic gonadotropin hormone).|
The two hormones measured in your blood are both produced by the placenta and are unique to each pregnancy. If the two hormone levels are favourable they will reduce the chance of your baby having Down's syndrome. If they are less favourable they will increase the chance. Adding in the blood test to the NT scan, increases the overall accuracy of the test.
Ideally it is better if you can have the blood test at least one day before the scan so that they have the results back when you go for the scan. The overall risk can then be calculated immediately after the scan and the result discussed face to face. If you cannot get the blood test done before the scan it can be taken on the same day but the result will need to be discussed at a later date.
How is the overall risk calculated?
|Using a combination of ultrasound and the measurement of hormones in your blood the risk of your baby having Down's syndrome can be predicted. The ultrasound scan records your baby's nuchal translucency measurements, and can also examine your baby in some detail to check whether many of the main organs are developing normally.|
It is also important to note that a high risk screening result does not indicate with certainty that there is an abnormality and that most women with these results will still have normal healthy babies.
What other screening tests are available to me?
|Depending on where you live various types of screening tests will be offered, making it a bit confusing! Currently the most common one is a blood test offered around 15 to 16 weeks. This is taken to screen for Downs Syndrome and is known as the Double, Triple or Quadruple test depending on which of the hormones they are testing. This test also screens for Spina Bifida at the same time. This test offers a 60%-65% detection rate of picking up pregnancies with potential problems.|
Some Hospitals offer First Trimester bloods instead of the 15 to 16 weeks blood tests. These, in isolation of any scan, have a 60%-65% detection rate as well. Another test offered by some Hospitals is a Nuchal Translucency scan performed during a dating scan. This is when they measure the fluid at the back of the baby's neck. The more prominent the fluid space is, the greater the risk can be of the baby either having a potential genetical problem. It can be linked to a fetal heart problem. This test is more accurate than the blood test at 15-16 weeks, with an 80% potential detection rate if performed.
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