WHAT IS THE NUCHAL TRANSLUCENCY?
The nuchal translucency is the fluid found at the back of your baby’s head and neck, just beneath the skin. The thickness of this fluid can be precisely measured and this is called the nuchal translucency (or NT) measurement. Normally the amount of fluid is small, producing a thin NT measurement.
We know that the amount of fluid can increase in the presence of certain conditions, producing a thicker NT measurement. Conditions associated with an increased NT measurement include some chromosome abnormalities (for example, trisomy 13, 18 and 21) and some structural problems (for example, heart abnormalities). An increased NT measurement does not always mean the baby has a problem but it does increase the risk.
There are established and strict criteria for the accurate measurement of the nuchal translucency. It is important for the measurement to be done properly to ensure an accurate result. If your baby is not co-operating and the NT cannot be measured accurately, we may ask you to return for more images later in the day. Occasionally, patients need to return on another day if the baby remains in an unhelpful position.
HOW IS A NUCHAL TRANSLUCENCY ULTRASOUND PERFORMED?
A nuchal translucency ultrasound is performed with a transabdominal ultrasound.
Transabdominal ultrasound involves scanning through your lower abdomen. A small amount of ultrasound gel is put on the skin of the lower abdomen, with the ultrasound probe then scanning through this gel. The gel helps improve contact between the probe and your skin.
Occasionally a transvaginal ultrasound is also needed. We may need a closer look at your baby, ovaries or cervix. Transvaginal ultrasound during all stages of pregnancy, including the first trimester, is safe and will not harm either you or your baby. Your sonographer will be experienced at performing these ultrasounds during pregnancy.
Transvaginal ultrasound is an internal ultrasound. It involves scanning with the ultrasound probe lying in the vagina. The transvaginal ultrasound probe is thin, about 2cm diameter. The probe is covered with a disposable protective sheath. A small amount of ultrasound gel is placed on the end of this probe. The probe is then gently inserted a short distance into the vagina by the sonographer. All transvaginal probes have been cleaned and sterilised according to recommended protocols.
Performing the transvaginal ultrasound usually causes less discomfort than a pap smear. No analgesia is required for this ultrasound.
Your privacy will always be respected during your ultrasound, especially the transvaginal examination. You will have a large towel covering your lower body, in addition to wearing a gown during the transvaginal ultrasound.
You will always have a choice about whether transvaginal ultrasound is performed. If you have concerns about transvaginal ultrasound, please discuss this with your sonographer.
DO I NEED A FULL BLADDER FOR THIS ULTRASOUND?
We usually get better images during transabdominal ultrasound if the bladder is partially filled, so to help your examination we ask you to drink water prior to the assessment. Please empty your bladder 1 hour before your appointment, drink 2 glasses of water and try not to empty your bladder again until after your appointment.
A full bladder moves bowel out from the pelvis into the abdomen, helping visualisation of the pregnancy, uterus and cervix.
Your bladder should not be so full that it causes pain. If your bladder is very full and painful, you should empty a small amount so you are more comfortable.
You will be able to empty your bladder after the transabdominal ultrasound is completed and before the transvaginal ultrasound begins (if transvaginal ultrasound is required).
WHY SHOULD I HAVE A NUCHAL TRANSLUCENCY ULTRASOUND?
All pregnant women, regardless of age, should be offered first trimester screening and consider if this test would be beneficial for them.
When considering first trimester screening, it may be helpful for you and your partner to consider what you would do if your baby had a chromosome abnormality such as Down syndrome.
Not all women choose to have first trimester screening.
Women of all ages are at risk of having a baby with a chromosome abnormality however the risk of chromosome abnormalities does increase with maternal age. The risks increase with each advancing year.
If you are under 35 years old, you are termed “low risk” (using age alone) for a chromosomal abnormality in your pregnancy. While it is true that most babies born to younger women have normal chromosomes, there is still always a chance of a chromosomal problem. Low risk means that your risk is lower but the risk is not zero.
If you are over 35 years old, you are termed “high risk” (using age alone) for a chromosomal abnormality in your pregnancy. While it is true that most babies born to older women will have normal chromosomes, there is an increased risk of a chromosomal problem. High risk means that your risk is higher but it is not 100%.
HOW ACCURATE IS FIRST TRIMESTER SCREENING?
First trimester screening offers a more accurate way of assessing your individual risk of chromosome abnormalities in pregnancy than just using your age alone.
Using the age of a mother alone will detect only 30% of babies with Down syndrome. It is the least accurate screening test for Down syndrome.
Using the nuchal translucency ultrasound alone will detect 70-80% of babies with Down syndrome.
Using combined first trimester screening (ultrasound and specific blood tests) will detect 80-90% of babies with Down syndrome.
WHAT ARE CHROMOSOMES?
Each cell in our body contains chromosomes. Chromosomes contain DNA which determines our genetic makeup. Each cell contains 46 chromosomes, made up of 23 individual pairs – one pair comes from the mother and one pair comes from the father.
Most babies will have normal chromosomes, but we know that sometimes chromosome problems can occur. This is called “aneuploidy”. Changes in the number, structure and arrangement of chromosomes is determined at conception and cannot be altered. There may be fewer chromosomes than normal or more chromosomes than normal. There can be parts of chromosomes missing (called “deletions”) or rearranged (called “translocations”). Changes in the chromosomes are usually significant and can cause miscarriage or birth defects.
First trimester screening assesses the risk for your baby having the following chromosome abnormalities:
- Trisomy 21 (also known as Down syndrome)
These babies have an extra chromosome 21 in each cell of the body. Down syndrome is one of the most common chromosome abnormalities, affecting 1 in 600 live births. It is associated with intellectual delay and physical problems such as heart and intestinal defects.
- Trisomy 18 (also known as Edward syndrome)
These babies have an extra chromosome 18 in each cell of the body. Trisomy 18 is associated with multiple and severe birth defects. Babies with this condition usually die during the pregnancy or shortly after birth.
- Trisomy 13 (also known as Patau syndrome)
These babies have an extra chromosome 13 in each cell of the body. Trisomy 13 is associated with multiple and severe birth defects. Babies with this condition usually die during the pregnancy or shortly after birth.
WHAT ARE THE OTHER ADVANTAGES OF THIS ULTRASOUND?
There are other advantages to having an ultrasound at this stage of the pregnancy. These include:
- Confirmation of the baby’s heart beat.
- Assessment of the dating of the pregnancy. The sonographer will measure the length of your baby from top to bottom, called the crown-rump-length (shortened to “CRL”). We will also measure the baby’s head (usually the biparietal diameter, or “BPD”). Such measurements provide an assessment of the size, or gestation, of your baby.
- Assessment of the basic structure of your baby. Many parts of your baby can already be examined, even at this early gestation. Most parents are amazed at the detail of this scan. We will look at your baby’s basic structure including the head, arms and legs, and abdominal wall. While some major abnormalities can be detected on this ultrasound, the 18-20 week morphology ultrasound is a more detailed and thorough examination of your baby’s anatomy.
- Assessment of the number of babies. Multiple pregnancies (such as twins) can be diagnosed on this ultrasound. The type of twins can be established by looking at the appearance of the placentae and amniotic sacs. This is important for management of twins, and it becomes more difficult to reliably assess as the pregnancy progresses.
- Review of the uterus and pelvis. We will be looking for such conditions as uterine fibroids and large ovarian cysts.
WHAT IS THE BLOOD TEST USED IN FIRST TRIMESTER SCREENING?
The blood test used in combined first trimester screening currently incorporates two serum proteins or markers:
- free BhCG (beta human chorionic gonadotrophin)
- PAPP-A (pregnancy associated plasma protein A).
The level of these proteins is known to change with certain conditions including chromosome abnormalities.
At present, these blood tests cannot be used on their own to calculate risk, but must be combined with the nuchal translucency ultrasound.
The best time to have the blood test is 10 weeks gestation, however it can be performed at any time between 10 and 13 weeks 6 days gestation.
It is ideal (but not essential) to have the blood test results available at your nuchal translucency ultrasound. This means you receive your final results on the day of the ultrasound, making counselling about your results easier.
You should speak with your referring doctor about having these special blood tests before your nuchal translucency ultrasound. It usually takes 1-2 days for blood test results to become available after the sample has been taken (the exact time will depend on the laboratory used for testing).
A number of factors can affect the results of the blood test. The two most significant variables affecting the accuracy of the blood test are:
- The gestation of your pregnancy.
- Your weight.
Less significant variables potentially affecting these blood tests include smoking status, ethnic group and the presence of diabetes.
Providing details of these variables will make your results more accurate.
WHO CAN PERFORM THIS TESTING?
It is important that you have your first trimester screening performed at an accredited practice. This ensures the practice undergoes regular audits of its performance with RANZCOG (Royal Australian and New Zealand College of Obstetricians and Gynaecologists). Advanced Women’s Imaging is an accredited practice for first trimester screening.
HOW IS MY RISK CALCULATED?
All risk assessments are calculated using a special computer programme, based on data from the Fetal Medicine Foundation. It is not appropriate, as occurs in some practices, for a nuchal translucency measurement to be taken in isolation, without reference to other variables and without calculating an adjusted risk in this special programme.
The main variables used to calculate risk are:
- The mother’s age.
- The nuchal translucency measurement of your baby.
- The gestational age of your baby.
- The blood tests.
- The fetal nasal bone
Your report will give a “background risk” and an “adjusted risk”. Your risks will be calculated for trisomy 13, 18 and 21.
The “background risk” is the starting point for calculating the individual risk for this particular baby. It is calculated mainly from the mother’s age and the gestational age of the baby. If you have a previous baby with a chromosome abnormality, your background risk may be increased in subsequent pregnancies.
The background risk will be presented as a “1 in ….” risk.
For example, your background risk for trisomy 21 may be “1 in 500”. This means you have a 1 in 500 chance of having a baby with trisomy 21, and 499 in 500 chance of having a baby without trisomy 21.
The “adjusted risk” is the risk for this particular baby and will be presented as a “1 in ….” risk. It is calculated using the nuchal translucency measurement and the special blood tests.
Your adjusted risk will be termed “low risk” if the risk is less than 1 in 300. For example, 1 in 650, 1 in 1500, 1 in 3000.
“Low risk” does not mean “no” risk. A low risk result is reassuring that your baby is healthy. Women with low risk results sometimes still have babies with chromosome abnormalities such as trisomy 13, 18 and 21.
Your adjusted risk will be termed “high risk” if the risk is greater than 1 in 300. For example, 1 in 270, 1 in 125, 1 in 30.
“High risk” does not mean that your baby definitely has a chromosome problem. It means your risk is increased and further definitive testing (such as CVS or amniocentesis) should be considered.
It is important to realise that 5% of women (or 1 in 20 women) will have a high risk result after combined first trimester screening.
Most babies with “high risk” results do not actually have a chromosome problem and fortunately most women that proceed to diagnostic testing (CVS or amniocentesis) will come back with normal results (the baby’s chromosomes are normal). We understand that waiting for these final results causes much anxiety but for most women the end result will be reassuring.
WHAT IS THE FETAL NASAL BONE?
Using the fetal nasal bone can significantly alter the management and outcomes for your pregnancy.
Using the fetal nasal bone may make all the difference for your pregnancy. It may mean the difference between having a chromosome problem detected or missed. It may mean the difference between having invasive prenatal diagnostic testing (CVS or amniocentesis) or not.
Fetal nasal bone accreditation is available through The Royal Australian and New Zealand College of Obstetricians and Gynaecologists (RANZCOG) to users of the Nuchal Translucency computer software programme. Strict criteria must be followed in order to gain accreditation for use of the fetal nasal bone. Accurate assessment of the nasal bone requires experience and care to ensure this area is optimally viewed.
Accreditation means the presence or absence of the fetal nasal bone can now be formally incorporated into the risk assessment of aneuploidy in your pregnancy.
Prior to the introduction of this service in 2010, the presence or absence of the fetal nasal bone was simply noted, but could not be used in the actual calculation of risk.
Most practices will report whether the nasal bone is present or absent, but they will not be able to incorporate the nasal bone into the risk assessment.
Use of the fetal nasal bone improves the accuracy of screening for trisomy 21. It will improve detection of trisomy 21 from 90% to 93%, thus detecting more cases of trisomy 21.
Use of the fetal nasal bone reduces the false positive rate of screening for trisomy 21. It will reduce the false positive rate for trisomy 21 from 3% to 2.5%, thus reducing the need for invasive testing in women with euploid fetuses. This means fewer women will be subjected to the risks of prenatal diagnostic testing (both chorionic villous sampling and amniocentesis are associated with a procedure-related risk of miscarriage, so we do not want to perform CVS or amniocentesis unless it is necessary).
The relationship of the fetal nasal bone to risk assessment is not straight forward, thus the need for computer software to formally calculate risks. The presence of a normal fetal nasal bone usually reduces the risk of aneuploidy if the risk is between 1 in 50 and 1 in 1000. The presence of a normal fetal nasal bone does not lead to additional change if the risk is already very high (ie. greater than 1 in 50).
The absence of the fetal nasal bone always increases risk. An absent fetal nasal bone is seen in 60% of trisomy 21, and 40-50% of trisomies 13 and 18.
An absent nasal bone occurs in 1-3% of normal fetuses, especially if the gestation is earlier, the nuchal translucency is increased, and in certain ethnic groups.