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Down Syndrome Screening

Aneuploidy/Down syndrome screening and diagnosis

Aneuploidy refers to a missing or extra chromosome. It is a type of chromosome abnormality and is a common cause of genetic disorders such as; trisomy 21, Down syndrome; trisomy 18, Edward's syndrome; and trisomy 13, Patau syndrome. These three chromosome disorders make up the most common aneuploidic conditions in the United States today.

In human beings, each cell has 23 pairs of chromosomes for a total of 46. One copy of each pair is inherited from the mother and one from the father. The first 22 pairs are called autosomes. Scientists arrange one to 22 from largest to smallest in a karyotype (a visual diagram of the chromosome pairs). The 23rd pair is the sex chromosomes. Normal males have one X chromosome and one Y chromosome. Normal females have two X chromosomes. During cell division, each cell should receive one copy of each chromosome from the mother and father. When cells fail to divide properly, an extra or missing chromosome may result. Aneuploidy is classified as full (every cell affected) or mosaic (some but not all cells affected).

Most embryos cannot survive with an extra or missing autosome and are spontaneously aborted. Trisomy 16 is the most frequent aneuploidy; fetuses with the full version of this (every cell affected) rarely survive to term.

Aneuploidy is further broken into:

  • Monosomy – refers to the lack of one chromosome of the normal pair
  • Disomy – the presence of two copies of a chromosome (for humans this is the normal condition)
  • Trisomy – the presence of three copies, instead of the normal two
  • Tetrasomy/pentasomy – four or five copies – seen mostly in sex chromosomes

Aneuploidy is never caused by something the mother or father does or doesn't do. The risk of aneuploidy occurring in a fetus increases with a mother's age, but it is not the only factor involved. The majority of the time the cause is unknown. Parents with genetic disorders may have increased risk for passing a genetic disorder to their baby. Sanford's genetic counselors are here to help parents learn more about chromosome disorders, the risks and how it affects family planning.

What are the Risk Factors for Down Syndrome/Chromosome Disorders?

  • Maternal age over 35. It's important to note though that 80% of children born with Down syndrome are born to women under the age of 35.
  • Previous fetus/child with autosomal trisomy. The recurrence rate is 1.6 – 8.2 times the maternal age risk, depending on the type of trisomy, whether the previous pregnancy was spontaneously aborted, the maternal age at the initial occurrence and the mother's age at subsequent prenatal diagnosis.
  • Structural abnormalities identified by ultrasound.
  • Previous fetus/child with sex chromosome abnormality – not all sex abnormalities have a maternal origin and not all have risk of recurrence. When they do, the recurrence rate is 1.6 – 1.5 times the maternal age risk.
  • Parental carrier of chromo translocation –a type of genetic abnormality.
  • Parental carrier of chromosome inversion – a type of genetic abnormality.
  • Parental aneuploidy or mosaicism. Women with Trisomy 21, Down syndrome, have a 50 percent risk of having trisomic children.


Screening for Down syndrome/aneuploidy is an important part of prenatal health care. Screening is recommended for all pregnant women, but it is done based on the mother's personal decision. Often, genetic testing may become necessary if a routine ultrasound reveals structural abnormalities in the fetus.

Sanford offers a variety of screening and diagnostic tests to calculate risk and diagnosis chromosome disorders and abnormalities. We also offer high-risk pregnancy management and help parents navigate the decisions involved when an unborn baby is diagnosed with a chromosomal disorder. We support parents in their journey and provide advanced fetal medicine care. Happily, 95 percent of the time we can assure parents their child is healthy and there are no chromosomal abnormalities.

Screening vs. diagnosing Down Syndrome/chromosome disorders

It's important to know the difference between screening for Down syndrome/aneuploidy and diagnosing them. Non-invasive blood tests from the mother, along with a women's age can help determine the risk of your baby having a chromosomal abnormality, but it does not tell whether or not your baby has a chromosomal disorder. These genetic tests are useful to help reassure you that your baby is developing normally if your results fall in the normal range. When results cause alarm, they may indicate the need for diagnostic testing to rule out or confirm your baby has a chromosome disorder.

Screening Tests:
First Trimester Screening

A blood test can screen for a higher risk of having a baby with Down syndrome, trisomy 18, congenital heart defects and other birth defects. This test is performed between weeks 11 and 13 in pregnancy. If test results are abnormal, your provider will discuss further testing options to diagnosis the presence of these specific birth defects.

Second trimester screening (quad screen)

A second trimester screen is a blood test available to all women between weeks 15 and 20 of their pregnancy. The purpose of this test is to measure the chance your baby has a chromosome disorder such as Down syndrome, trisomy 18, Smith-Lemli-Opitz syndrome or a neural tube defect such as spina bifada. If results are abnormal, your provider will recommend further testing to obtain a diagnosis.

Risk of Miscarriage

None. These are blood tests drawn from the mother.

Diagnostic Tests

When a chromosomal disorder is suspected, your physician may recommend one or more of the following tests to diagnose or rule it out. These tests are considered invasive as they involve taking blood or tissue samples from the placenta or fetus. Because of this, there is a slight chance of miscarriage or complications. Your doctor will discuss the risks and benefits of each test and help you to make an informed decision you feel comfortable with.

Chorionic Villus Sampling

This is a prenatal test in which a sample of chorionic villi (tissue that has fetal genetic material) is removed from the placenta for testing for chromosomal abnormalities. This test is performed as early as nine weeks and can be done via the mother's cervix or abdomen. It can provide the earliest confirmation or ruling out of a chromosome disorder. This test is used to diagnose genetic disorders such as Down syndrome, cystic fibrosis and Tay-Sachs disease. It does not test for spina bifada or neural defects.

Risk of Miscarriage

Because this test is done when a woman is at highest risk for miscarriage, it is considered to have a higher risk for loss. Overall, the risk is one in 100 for miscarriage. That risk is slightly higher when the test is done via the cervix. If performed prior to nine weeks gestation (not recommended), there is also a risk for limb deformities.

Other Risks include:

  • Cramping
  • Vaginal bleeding
  • Rh sensitization (if you are Rh negative, you may be given Rh immunoglobulin after the test to prevent you from producing antibodies against your baby's blood cells)
  • Uterine infection

Genetic Amniocentesis

By testing the fluid surrounding your baby, our specialists can study your baby's chromosomes and identify genetic abnormalities. Amniocentesis is 99 percent accurate in detecting chromosomal abnormalities and 90 percent for neural tube defects. This test is done by inserting a needle through the mother's anesthetized abdomen and, with careful guidance by ultrasound, removing a sample of the amniotic fluid. The fluid is then tested at a laboratory with results usually back in two to three weeks. Amniocentesis is offered at 15 to 20 weeks gestation and occasionally as early as 11 weeks.

Risk of Miscarriage

Amniocentesis has a low risk for fetal loss at a rate of one in 300 to 500. Studies indicate the risk is lower at maternal fetal medicine centers that have expertise and experience in performing this test.

Other Risks include:

  • Injury to baby or mother
  • Preterm labor
  • Infection

PUBS (cordocentesis)

This test is a common procedure for twins in which an umbilical blood sampling is obtained to diagnose any abnormalities. It is used less now than in the past, due to advances in technology and other testing procedures. PUBS is used after a chromosome diagnosis and often when a fetus is critically ill. The results can be returned within 24 to 48 hours. This test involves removing blood from the umbilical cord and is offered at 18 weeks gestation or later.

Risk of Miscarriage

It is slightly higher at two in 100. It is not known if this increased risk is due to the procedure or to the critically ill nature of the fetus.

Other Risks include:

  • Bleeding from the needle entry site
  • Temporary slowing of the baby's heart rate
  • Uterine Infection

What does it mean for my baby?

If your baby is diagnosed with a chromosome disorder, your physician will discuss the options of termination or continuing the pregnancy. We support our patients during this time of extreme difficulty and provide genetic counseling and resources to help you and your family through your journey. We stress that every baby is unique and while statistics offer guidance, there is no way to predict with 100 percent accuracy the outcome of the pregnancy or survival rates. Without giving false hope, we are here to say miracles do happen. We encourage you to seek support from your maternal fetal health team, your family, spiritual guide and genetic disorder support groups during this time.