The prenatal genetic screen tells you the chance of your baby having one of three conditions: Down syndrome, trisomy 18, or an open neural tube defect.

Down Syndrome

Down syndrome is a genetic condition in which a baby has 47 chromosomes (three copies of chromosome 21 instead of two) instead of 46. Down syndrome is the most common chromosome problem. Approximately one in 700 babies is born with Down syndrome.

People with Down syndrome have mild to moderate intellectual delays and a higher chance of some health problems. Each person with Down syndrome is different and there is no way to predict how serious any problems will be. People with Down syndrome usually live into their 50s.

For more information on Down syndrome, please see the websites listed on our Resources page.

Trisomy 18

Trisomy 18 is a genetic condition in which a baby has an extra copy of chromosome 18. About one in 7,000 babies is born with trisomy 18. Many pregnancies with trisomy 18 will miscarry, and babies that are born rarely live past the first few days or months because of serious heart and brain defects and poor growth before and after birth. 

For more information on trisomy 18, please see the websites listed on our Resources page.

Open Neural Tube Defect (ONTD)

An open neural tube defect occurs when the brain or spinal cord does not form properly. In BC, approximately 1 in 1,000 babies has an open neural tube defect.

Spina bifida is an open neural tube defect in which the spine does not completely close around the spinal cord. This is the most common form of open neural tube defect. People with spina bifida may have both physical and mental disabilities.

Anencephaly is an open neural tube defect involving the brain. A baby with anencephaly will be stillborn or die shortly after birth.

For more information on open neural tube defects, please see the websites listed on our Resources page.

What is the chance that my baby will have Down syndrome or trisomy 18?

All women have some chance of having a baby with Down syndrome or trisomy 18. As a woman ages, the chance of having a baby with a chromosome problem such as Down syndrome or trisomy 18 increases.

What is the chance that my baby will have an open neural tube defect? 

The chance of having a baby with an open neural tube defect does not depend on the age of the mother. The chance is approximately 1 in 1,000 for all ages.

Prenatal screening consists of one or more blood tests and sometimes a special nuchal translucency (NT) ultrasound.

Blood Tests

• Blood test #1 can be done between 9 and just under 14 weeks of pregnancy.
• Blood test #2 can be done between 14 and just under 21 weeks of pregnancy, but preferably before 16 weeks to provide you with earlier results.

The blood tests may be done at any laboratory or hospital. You do not need to make an appointment, but remember to bring the requisition your doctor or midwife gave you.

Use our pregnancy dating tool to find out when to have your blood drawn. If you miss the first blood test, you may still have the second blood test. It is best to have both when possible. Having both improves the accuracy of the screen result.

Nuchal Translucency (NT) Ultrasound

A nuchal translucency (NT) ultrasound measures a fluid space at the back of your baby’s neck. The NT ultrasound is done between 11 and just under 14 weeks of pregnancy. 

Depending on your age, medical or obstetrical history, you may be offered a NT ultrasound along with the blood tests.

The NT ultrasound must be done at an ultrasound clinic by a specially trained ultrasound technician or doctor. See the list of NT ultrasound site locations.

Not every woman will choose to have prenatal screening. The decision is a personal one. 

In addition, you may want to think about some of the following questions when making your decision:

• Do you want to know if your baby has Down syndrome, trisomy 18, or an open neural tube defect before the baby is born?
  
• What would you do if diagnostic tests showed that your baby had one of these conditions? Would you continue the pregnancy? Would you want to know in order to prepare for the birth of a child with special needs? Would you consider ending the pregnancy? Would you consider  an adoption plan for your baby?
  
• How will this information affect your feelings throughout your pregnancy? Would it cause you too much worry? Would you feel too worried for the rest of your pregnancy if you don't follow up with more diagnostic testing such as the amniocentesis?
  

Remember, you can choose whether or not you will have prenatal screening.

Here are some other things you may want to think about:

Most women have a prenatal screen result showing chances are low for Down syndrome, trisomy 18, or an open neural tube defect.

• Although some women will have a screen positive result, most of them will not have a baby with Down syndrome, trisomy 18 or an open neural tube defect.
  
  
• Prenatal screening will detect most babies with these conditions, but not all of them.
  
  
• The chance of having a screen positive result for Down syndrome or trisomy 18 increases as a woman ages.
  
  
• Occasionally, prenatal screening may detect other medical conditions in your baby.
  
  
• No test can detect every type of physical or mental complication.
  
  
• Most babies are born healthy. However, every woman, no matter what her age, has a 2-3% chance of having a baby with some kind of complication found at birth. Examples are heart defect, cleft lip or extra toes.

There are different kinds of prenatal genetic screens:

• Serum Integrated Prenatal Screen (SIPS): 2 blood tests
• Integrated Prenatal Screen (IPS): 2 blood tests plus an NT ultrasound
• Quad Screen: 1 blood test

The prenatal screen you are offered depends on how far along you are in your pregnancy, your age, your health, family history, and whether or not you are carrying more than one baby. The earlier you see your health care provider, the more options you will have. The following table shows the different screening options available based on how far along you are in your pregnancy:

The above screening tests are available to all pregnant women in BC through the BC Prenatal Genetic Screening Program and are paid for by BC's Medical Services Plan (MSP). 

Comparing provincially funded and privately paid screening tests can sometimes help women decide which screening test is best for them.

A prenatal screen result tells you the chance of your baby having Down syndrome, trisomy 18, or an open neural tube defect.

How is my result calculated?

The levels of proteins made by the baby and the placenta are measured in your blood. If you have a nuchal translucency ultrasound, the amount of fluid normally found at the back of the baby's neck is measured. A baby with Down syndrome, trisomy 18 or an open neural tube defect may have different amounts of these proteins or fluid. These differences are used to estimate the chances for your pregnancy. Other factors that affect the results include the dating of your pregnancy, your age, weight, ethnic origin and whether it is a twin pregnancy. 

As part of your screen result, you will be given a number that estimates the chance your baby has one of these conditions. This number is compared to the screen cut-off. The screen cut-off is the chance above which you will be offered diagnostic testing. If your chance is lower than the screen cut-off, this is called a screen negative result. If this number is higher than the screen cut-off, this is called a screen positive result.

What does a screen negative result mean?

If your result is screen negative, the chance that the baby has Down syndrome, trisomy 18 or an open neural tube defect is low. You can be more than 99.9% certain that your baby will not have any of these conditions.

What does a screen positive result mean?

If your result is screen positive it does not mean that your baby has the condition. In fact, most women with this result do not have a baby with one of these conditions. The result means that the chance of Down syndrome, trisomy 18 or an open neural tube defect is high enough that you will be offered diagnostic testing or another blood test called Non-Invasive Prenatal Testing (NIPT). Diagnostic testing tells you for sure if your baby has any of these three conditions. NIPT is highly accurate in detecting Down syndrome and trisomy 18.

What if my result is screen positive for Down syndrome or trisomy 18?

If your result is screen positive for Down syndrome or trisomy 18, your health care provider may need to confirm how far along you are in your pregnancy. If the dates are correct, you will be offered amniocentesis or Non-Invasive Prenatal Testing (NIPT). Amniocentesis will tell you if your baby has either of these conditions. NIPT will give you a new risk (very low or very high) to have a baby with Down syndrome or trisomy 18. If your risk is found to be very high on NIPT, an amniocentesis would then be needed to confirm. It is your choice whether you have the amniocentesis or not.

What if my result is screen positive for an open neural tube defect?

If your result is screen positive for an open neural tube defect, you will be offered a detailed ultrasound and an appointment with a maternal fetal medicine doctor or a genetic counsellor at one of the medical genetics clinics (in Vancouver or Victoria). If your baby does have an open neural tube defect this will usually be seen on the ultrasound scan.

Note: The information in this section is also available in a pamphlet that you can download: A Screen Positive Result: What Does It Mean and What Do I Do Now?

You have learned that the result of your prenatal genetic screen is "screen positive." This result does not mean your baby has Down syndrome, trisomy 18, or an open neural tube defect. In fact, most women with this result do not have a baby with one of these conditions. The result means that the chance of your baby having Down syndrome, trisomy 18, or an open neural tube defect is high enough that diagnostic testing or another screening blood test called non-invasive prenatal testing (NIPT) is offered. Diagnostic testing tells you for sure if your baby has any of these three conditions. NIPT is another screening test but one that is highly accurate in detecting Down syndrome and trisomy 18.

As part of your screen result, you have been given a number that estimates the chance your baby has one of these conditions. For example, the chance of Down syndrome is 1:100. This is the same as saying:

• There is a 1% chance your baby will have Down syndrome; or
• There is a 99% chance your baby will not have Down syndrome; or
• If 100 women had the same result, one of them would have a baby with Down syndrome and the other 99 would not.

What happens now?

What is an amniocentesis?

What is non-invasive prenatal testing (NIPT)?

My prenatal screen result is screen positive for Down syndrome or trisomy 18? Should I have extra testing?

What if I am having trouble deciding whether having an amniocentesis is right for me?

What if the result of the amniocentesis show that the baby has one of these conditions?

How is diagnostic testing different from screening?

Diagnostic testing tells you for sure whether or not your baby has one of the conditions being screened for. Screening tells you the chance that your baby has one of the conditions to help you decide whether or not to have diagnostic testing. Screening is non-invasive, and therefore considered a good, safe first step.

What diagnostic tests are available?

Amniocentesis

A very thin needle is put into the woman’s belly into the womb. A few teaspoons of the fluid surrounding the baby (amniotic fluid) is taken out through the needle. The needle is guided by ultrasound, so that it does not touch the baby. 

The amniotic fluid contains skin cells from the baby. Each cell has a complete set of the baby’s genetic information. Testing can be done on these cells to accurately determine if the baby has Down syndrome, trisomy 18, trisomy 13, or an abnormal number of the sex chromosomes. 

Amniocentesis can tell you for sure whether or not the baby has Down syndrome or trisomy 18. However, amniocentesis is associated with a one in 200 risk of miscarriage. In other words, if 200 women have an amniocentesis, one would lose the pregnancy as a result of the procedure.

Amniocentesis is usually performed after 15 weeks of pregnancy. If a rapid lab test is done that looks only at chromosomes 13, 18, 21 and the sex chromosomes, results are available in three days. If all chromosomes are analyzed, results are available in two weeks.

Chorionic Villus Sampling (CVS)

Chorionic villus sampling is a diagnostic test in which a needle is put either into the woman’s belly or the vagina. The needle is used to collect a small piece of the placenta, known as the chorionic villus. The needle is guided by ultrasound so that it does not touch the baby. 

The sample contains cells from the baby. Each cell has a complete set of the baby’s genetic information. As with amniocentesis, CVS can tell you for sure whether or not the baby has Down syndrome or trisomy 18. However, there is a one in 100 chance of miscarriage from the CVS procedure. 

CVS is done between 11-13 weeks of pregnancy. If a rapid lab test is done that looks only at chromosomes 13, 18, 21, and the sex chromosomes, results are available in three days. If all chromosomes are analyzed, results are available in two weeks. There are times when the rapid lab test does not give conclusive results. In such cases, the full analysis is required with results available around two weeks after the procedure.

Detailed Ultrasound

In a detailed ultrasound examination, a specialist looks carefully at every part of the baby to ensure the baby is growing and developing as expected. The ultrasound can tell whether the baby has an open neural tube defect. It can also confirm how far along you are in the pregnancy, and whether twins are present.

Unlike amniocentesis or CVS, detailed ultrasound has no risk of miscarriage. It is usually done between 18 and 20 weeks of pregnancy.

An ultrasound uses harmless sound waves to make pictures of how your baby is growing and developing. In an ultrasound exam the health care provider moves a handheld device, called a transducer, along your abdomen. To get a clearer picture, your abdomen is covered with a thin layer of gel. The transducer sends sound waves into your womb. A computer gathers the information from these sound waves to make a live picture of the baby seen on a monitor. 

The ultrasound exam is painless. Ultrasound is available only when requested by your health care provider.

When are ultrasounds done, and what can be seen on an ultrasound?

Some women are booked for an ultrasound early (before 14 weeks) in pregnancy to check the baby’s heartbeat, see if there is more than one baby, and check how far along the pregnancy is by measuring the size of the baby. Most detailed ultrasounds are booked between 18-20 weeks of pregnancy.

What is a nuchal translucency (NT) ultrasound?

A nuchal translucency (NT) ultrasound measures a fluid-filled space at the back of your baby’s neck. The NT scan is done between 11 and just under 14 weeks of pregnancy.

NT ultrasound is one part of an Integrated Prenatal Screening (IPS), involving two blood tests, which can be done in women carrying a single or twin pregnancy. The combination of the NT ultrasound and the two blood tests tell you the chance of your baby having Down syndrome, trisomy 18 or an open neural tube defect. Your health care provider will have your screen result within ten days after the second blood test.

NT ultrasound alone may be done in women carrying triplets or more.

How is an NT ultrasound done?

The main part of an NT ultrasound involves an ultrasound scan. To accurately date your pregnancy, the sonographer will measure your baby from the top of its head to the bottom of the spine (called a Crown Rump Length [CRL] measurement). The sonographer will then measure the width of the fluid-filled space at the back of the baby's neck. The skin will appear as a white line, and the fluid under the skin will look black.

Regardless of whether a woman has an NT ultrasound in her first trimester, a detailed ultrasound in the 2nd trimester looking at the anatomy of the whole fetus is recommended for all pregnancies. The detailed ultrasound occurs at 18-20 weeks and can be arranged by your health care provider.

Genetic Counsellors

Genetic counsellors are health professionals with a master’s degree in medical genetics and counselling. Genetic counsellors work as members of a health care team, usually working with a medical geneticist (doctors that specialize in the area of genetic conditions), providing individuals and families with information on the nature, inheritance, and implications of genetic disorders to help them make informed medical decisions.

When you go to see a genetic counsellor, you can expect the appointment to be about an hour long. Sometimes women are booked for an ultrasound just prior to seeing a genetic counsellor. For example, women with an above screen-cut off prenatal screening result showing an increased chance for an open neural tube defect will first have and ultrasound and then meet their genetic counsellor. The ultrasound report will then be available to discuss during your appointment.

During your session, the counsellor will explain in detail all of the information that has been gathered about your pregnancy including the prenatal screening result, any ultrasounds you may have had, and any family or personal health history that may be important. They will answer all of your questions and do their best to address all of your concerns. They will then explain all the options you have and help you make decisions that are right for you. Their goal is to make sure you are well informed when making these decisions and arrange for testing or consultations with other health care providers as needed depending on your decisions.

Maternal Fetal Medicine Doctors

Maternal-Fetal Medicine doctors, also called Perinatologists, specialize in the diagnosis, treatment, and ongoing care of pregnant mothers if they or their babies have an increased chance of a health condition. A woman’s health care provider will refer a woman to a maternal fetal medicine doctor when her medical history, or screening, or diagnostic testing during pregnancy increases the chance or diagnoses a health condition in either mother or baby.

An example of why a woman is referred to a maternal fetal medicine doctor is if a woman has a medical condition needing extra care during pregnancy, like lupus, which is a chronic autoimmune condition.

When you see a maternal fetal medicine doctor, different tests or screens may be offered depending on why you were referred to see this specialist. These diagnostic tests or screens are to monitor the health of mother and baby, and perhaps diagnose a health condition.