Pregnancy Articles:

12 - 13 weeks scan


 

 

 

12-13 Weeks Ultrasound Scan

Can the scan tell exactly how many weeks pregnant I am?

If you have an irregular cycle, or if you have been taking the contraceptive pill recently, working out the date from your LMP is not a very accurate method. Scans are more accurate than using the LMP and, if the scan date differs by more than five days from your period dates, the scan date will be used.

How will the scan be done?

If the scan is done through your tummy, you will need a full bladder. The sonographer will put some (usually very cold!) gel on your tummy and will move a small hand-held transducer over your skin to get views of the baby.

 

What can be seen on a scan in the first trimester?

Your baby is measured from head to bottom -- this is called the Crown Rump Length (CRL). The CRL is very accurate in the first trimester, but after 13 weeks the baby can curl up and stretch out, so measuring the length becomes less accurate. The width of the head (biparietal diameter or BPD) then becomes the best way to measure the baby.

 

What else can be seen in the first trimester?

Some major abnormalities can be seen by 12 weeks, but it is recommended you also have a detailed scan at 20 weeks.

Nuchal translucency can be measured from 11-14 weeks to estimate the risk of Down's syndrome and other chromosomal abnormalities.

 

What if there is something wrong?

The majority of pregnancies are successful and if when you have a scan the findings are inconclusive, you will probably find all is well when you go back for a second scan. The one or two weeks' wait will probably seem a very long time and you will naturally be worried during this time.

 

What is nuchal translucency?

This is a collection of fluid under the skin at the back of a baby's neck at 10-14 weeks that can be measured using ultrasound. All babies have some fluid, but in many babies with Down's syndrome, the nuchal translucency (NT) is increased.

 

An NT scan is a method of assessing whether your baby is likely to have Down's syndrome. It's a screening test. A screening test can only estimate the risk of your baby having Down's, as opposed to a diagnostic test, such as CVS oramniocentesis, which will give you a definite diagnosis (but also carries a small risk of miscarriage).

 

The NT scan can't tell for certain whether your baby is affected. However, it can help you decide whether or not to have a diagnostic test.

 

When can I have the test?

NT scans are usually performed from 11 to 13 weeks + 6 days of pregnancy. Before 11 weeks the scan is technically difficult because the baby is so tiny and, after 14 weeks, excess fluid may be absorbed by the baby's developing lymphatic system.

 

How is it done?

The main part of the test involves an ultrasound scan. The scan is usually done through your tummy, but depending on the position of your baby and your womb, sometimes it's necessary to perform a vaginal scan, which will give better views. There is no risk to you or your baby and it should not be too uncomfortable.

 

To accurately date your pregnancy, the sonographer will measure your baby from the top of its head to the bottom of the spine. She will then measure the width of the NT. The skin will appear as a white line, and the fluid under the skin will look black. Your baby fits nicely on the screen at this stage, and you will be able to see the head and spine, limbs, hands and feet. Some major abnormalities may be excluded at this scan, but it's recommended you also have a detailed scan at 20 weeks.

 

What is a normal measurement?

An NT measurement of up to 2.0mm is normal at about 11 weeks, and up to about 2.8mm by 13 weeks and 6 days. This is because the NT normally grows in proportion to the growth of the baby. Image 1 shows a baby with a normal NT of 1.3mm; this baby will be low risk.

 

1. Normal nuchal translucency

If the NT is increased, it does not mean there is definitely a problem. Some normal babies have increased fluid, too. Image 2 shows a baby with an NT of 2.9mm, which is at the upper limit of the normal range. Though the risk will be higher than the baby's in the first image, nine out of 10 babies with a measurement between 2.5mm and 3.5mm will be completely normal.

 

2. Borderline nuchal translucency

As the NT increases, so does the risk. The baby in image 3 has an NT of 6mm and will be very high risk for Down's, as well as other chromosomal and genetic syndromes and heart problems. Fortunately, not many babies have as much fluid as this.

 

3. Increased nuchal translucency

 

How is the risk calculated?

Every woman has a risk of giving birth to a baby with Down's syndrome, and this risk increases with age. First the database will show you the risk that applies to all women of your age, known as the background risk. Then, using the measurements from the scan, a new risk will be generated which is your own individual risk for this pregnancy. This may be higher or lower than your background risk.

 

When will I get the results?

The results should be available immediately.

 

What do the results mean?

The results will express the probability of your baby having Down's syndrome.

Low risk is a result where the risk is less than one in 300. On your results print-out this will be expressed as a ratio, for example, 1:800. This means that out of 800 women who have this risk, one will have a baby with Down's syndrome.

High risk is a result where the risk is greater than one in 300, for example, 1:150. This means that out of every 150 women who have this risk, one will have a baby with Down's syndrome. 

Make sure that you thoroughly understand the information that the specialist gives you. Ask her to go through things again if you don't understand. If you would like to know more about Down's syndrome, contact the Down's Syndrome Association.

 

What should I do if I am high-risk?

About one in 20 women will be given a high risk, and the vast majority go on to have a normal baby. Even with a risk as high as 1:5, the baby has four out of five chances that he does NOT have Down's. Nevertheless, once your pregnancy has been labelled high-risk, you will probably feel anxious and perhaps unsure of what to do.

 

The only way to know for certain if your baby has Down's or another chromosomal abnormality is to have a diagnostic test such as CVS or amniocentesis. This decision can be very difficult, but you do not have to decide in a hurry. One advantage of the NT scan is that it is done early in your pregnancy, and it is possible to have a CVS and get the result while you are still in the first trimester. If you are unsure what to do, you can wait until 16 weeks and have an amniocentesis then if you wish. Your midwife or doctor will provide more information if you need it.

 

How reliable is a nuchal translucency scan?

A large research study involving over 100,000 pregnancies found that about 75 per cent of babies with Down's syndrome were correctly identified using this method. When combined with a blood test, the detection rate improves to about 90 per cent. However, to achieve these detection rates, it is important that the NT is measured accurately. For this reason, strict standards have been devised so that every sonographer measures in exactly the same way. To use the database to calculate the risk, sonographers must first undergo training by the Fetal Medicine Foundation and, once they are considered competent, their technique and results are reviewed every year.

 

 


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