Sanford Clinic Women's Health

First Trimester Screening

Approximately 4% of all babies are born with a birth defect or genetic condition. Often this happens without any warning and families and doctors are unprepared. A test known as a First Trimester Screen, however, can help identify women with the highest risk to have a baby with certain kinds of birth defects or to develop a problem later in pregnancy.

What is a First Trimester Screen?

The First Trimester Screen is a prenatal risk assessment available to all women between weeks 11 and 13 of their pregnancy. It identifies unborn babies at increased risk of having certain birth defects. A blood test is combined with an ultrasound measurement to estimate the specific risk for Down syndrome, Trisomy 18 and congenital heart defects.

Blood is drawn from the mother and is analyzed for two proteins called free Beta-hCG and PAPP-A which are found in a mother's blood during pregnancy. The levels of these substances may go up or down when a baby has a certain kind of birth defect. In addition to the blood draw, an ultrasound is used to measure the amount of fluid accumulation behind the neck of the baby, called the nuchal translucency (NT).

When the blood test is combined with the NT measurement, the screen will detect approximately 85% of Down syndrome, 85% of Trisomy 18, and 30-40% of heart defects. For 95% of women, the results of the First Trimester Screen will be normal and no further tests are needed. Although there is still a chance the baby could have a problem, the risk is significantly reduced. If the screen is abnormal, it does not necessarily mean that a baby has a birth defect; just that further investigation is needed.

What specific birth defects does a First Trimester Screen look for?

First Trimester Screening will estimate the risk of having a baby with Down syndrome, Trisomy 18, congenital heart defects and other birth defects.

What is Down syndrome?

Down syndrome is a genetic condition caused by the presence of an extra chromosome number 21. The extra chromosome is present from the time of conception. Adults with Down syndrome can be mildly or severely retarded, but most are in the moderate range. Individuals with Down syndrome may have other physical problems such as heart defects.

What is Trisomy 18?

Trisomy 18 is a serious genetic condition that is caused by having an extra chromosome number 18 in all the cells of the body. Problems with growth and development are present before birth and physical problems such heart defects are usually present.

Who should be offered screening?

Although the risk is higher for women over the age of 35, most babies with Down syndrome and Trisomy 18 are born to women less than 35. Therefore, First Trimester Screening is offered to all women who will be less than 35 at the time of delivery, and select women who will be age 35 and older. It is important to note that The American College of Obstetricians and Gynecologists (ACOG) (1994) and the American College of Medical Genetics (ACMG) (1993) recommend that all women who will be 35 or older at delivery consider having amniocentesis or CVS for prenatal chromosomal testing to diagnose 99.9% of all chromosomal abnormalities. First Trimester Screening is not a substitute for amniocentesis because:

• Screening does not give a diagnosis
• Screening estimates a risk for only 2 chromosomal abnormalities
• Screening will miss many of the hundreds of other types of chromosomal abnormalities,
• Screening may falsely reassure some patients who still have an increased chromosomal abnormality risk due to age. Despite this, the screen is available to those women over the age of 35 who have a higher risk for miscarriage or other complications, or who would like a better idea of their risk estimates before undergoing a prenatal diagnostic test, such as amniocentesis or chorionic villus sampling (CVS).

What if my screen result does not show an increased risk?

This is good news. First Trimester Screening cannot eliminate the risk for Down syndrome or Trisomy 18, but it can indicate a reduction in the risk.

Will First Trimester Screening detect all birth defects?

No. Most birth defects are not detected by this or any other test, but the screen will help detect some of the more common birth defects.

Besides First Trimester Screening, what other test should I have?

In order to screen for neural tube defects, such as spina bifida, either an ultrasound to examine the baby's spine or an alpha-fetoprotein (AFP) blood test done on the mother (between 13-22 weeks of pregnancy) is recommended. These birth defects usually occur without a family history.

What does a “screen positive” or “abnormal” First Trimester Screen result mean?

A screen positive result does not mean that there is a problem; just that the chance of Down syndrome or Trisomy 18 is higher and there is a need to look more closely at the pregnancy. In most cases, the baby will not have Down syndrome or Trisomy 18.

The next step is to visit with a perinatologist who may suggest further testing to help determine whether or not there is actually a problem. A perinatologist will offer you an amniocentesis or chorionic villus sampling (CVS) as a way to test the baby's chromosomes.

In an amniocentesis, the doctor inserts a thin needle through the abdominal wall and into the uterus to withdraw a few teaspoons of amniotic fluid (the fluid surrounding the baby). The baby's cells in this fluid are tested for Down syndrome and other chromosome abnormalities. Amniocentesis is more than 99% accurate in diagnosing, or more likely, ruling out a chromosome problem. Another option is chorionic villus sampling (CVS), which is done by removing a small sample of the placenta and examining the chromosomes. The baby and the placenta typically have the same chromosomes making this an effective way to check the baby's chromosomes. It is also extremely accurate. The main difference between CVS and amniocentesis is that CVS is done earlier in pregnancy (11-13 weeks) than amniocentesis (15-20 weeks). The risk of miscarriage for these procedures is much less than once thought with estimates being from 1 in 400 for CVS to 1 in 1600 for Amniocentesis.

Where can I go to have First Trimester Screening performed?

Sanford Clinic MFM is certified by the Fetal Medicine Foundation to perform the First Trimester Screen. An appointment consists of a consultation with a board certified genetic counselor, a blood draw, an ultrasound and a visit with a board certified perinatologist. You can make an appointment by calling (605) 328-7700.

The First Trimester Screen

• Identifies up to 91% of pregnancies with Down syndrome
• Identifies 97% of pregnancies with Trisomy 18
• Identifies 40% of pregnancies at risk for congenital heart defects
• Consists of an ultrasound exam and a maternal blood test
• Done between weeks 11 and 13 of pregnancy
• Results available in one week

The Second Trimester Screen (Also known as the Quad Screen)

• Identifies 69-76% of pregnancies with Down syndrome
• Identifies 60% of pregnancies with Trisomy 18
• Identifies 80-90% of pregnancies with neural tube defects such as spina bifida
• Consists of a maternal blood test
• Done between weeks 15 and 20 of pregnancy
• Results available in one week