1st Trimester Prenatal Screening

The health of her unborn child is often a pregnant woman’s greatest concern.

Prenatal (before birth) screening can help deal with this concern. Prenatal screening helps find out if your baby is at greater risk of certain birth defects. This information will help you understand the first trimester prenatal screening test and what the next steps might be if your results show your baby is at increased risk.

Who should have prenatal screening?

The American College of Obstetricians and Gynecologists recommends that prenatal screening for Down syndrome, trisomy 18, and neural tube defects be offered to all pregnant women, regardless of their age.

Do I have to have prenatal screening?

You should discuss your options with your doctor; however, the decision about whether or not to have prenatal screening is up to you. The following information will help you decide.

What is Down syndrome?

Down syndrome (trisomy 21) is a condition caused by an extra copy of chromosome number 21. Babies with Down syndrome have a distinct physical appearance and some degree of mental retardation. They will often have other birth defects. About 1 in every 800 babies is born with Down syndrome. The chance of having a child with Down syndrome increases as a woman gets older; however, a woman of any age can have a baby with Down syndrome.

What is trisomy 18?

Trisomy 18 is a condition caused by an extra copy of chromosome number 18. Babies with trisomy 18 have severe mental retardation and multiple birth defects. Ninety percent of babies born with trisomy 18 die within their first year of life. Trisomy 18 occurs in only 1 out of 6,000 births.

What is first trimester screening?

The first trimester screen is a test that provides information early in pregnancy about your baby’s chance of having Down syndrome or trisomy 18.

The first trimester screen includes a blood test and usually a special ultrasound measurement. The blood test alone can detect about 68% of the babies with Down syndrome and 60% of those with trisomy 18. When the ultrasound measurement is included, the first trimester screen can detect 84% of the babies with Down syndrome and 75% of those with trisomy 18.

In addition to a first trimester screen, your doctor may order a separate screening test in the second trimester to provide information on the baby’s neural tube. The neural tube develops into the spine and brain of the baby very early in pregnancy. If the neural tube doesn’t develop properly, the result can be a defect in the baby’s spine and/or brain. Neural tube defects occur in 1 to 2 out of 1,000 births and are not associated with the mother’s age.

How is first trimester screening done?

Sometime during your first trimester of pregnancy, you will be asked to give a small sample of blood from your arm. Using this sample, the level of two substances will be measured. The level of these substances may be abnormal when a fetus is at increased risk of Down syndrome or trisomy 18.

Additionally, an ultrasound will be performed between your 10th and 13th week of pregnancy. This ultrasound is done to take a measurement at the back of the baby’s neck. This measurement is called nuchal translucency (NT).

The results of the blood test are combined with the NT measurement to determine the risk of Down syndrome and trisomy 18. Your age, weight, due date, ethnic background, and medical and family history will be used to help your physician interpret the results.

What if my result is “screen negative”?

A “screen negative” result means that it is unlikely your baby has either Down syndrome or trisomy 18. But, a negative screen does not guarantee the birth of a healthy baby.

What if my result is “screen positive”?

If your result is “screen positive”, it means that your baby has an increased risk of having Down syndrome or trisomy 18. It does not mean that your baby definitely has one of these disorders. In fact, the baby does not have these conditions in the majority of cases. Your doctor can help you understand the risk and explain the additional tests that may be recommended.

What are these additional tests?

If your test result is screen positive, your physician may order an additional procedure like chorionic villus sampling (CVS) or amniocentesis. During CVS, a small amount of cells are removed from the developing placenta. During amniocentesis, a needle is inserted into the mother’s abdomen and some of the fluid surrounding the baby is taken out. This fluid contains cells from the baby.

Using cells collected during CVS or amniocentesis, a chromosome analysis can be performed to determine if the baby has Down syndrome or trisomy 18.

Since there is a risk of miscarriage associated with CVS and amniocentesis, you should discuss the risks and benefits of each procedure with your doctor.

What if the additional tests are negative?

If the additional tests are negative, your baby is likely to be healthy; however, negative test results do not guarantee that your baby will not have some form of birth defect.

What if the additional tests are positive?

If an additional test is positive, genetic counseling will help you learn more about your baby’s condition. This will help you make decisions in the best interest of you, your family, and your baby.

Summary

First trimester screening:

• Helps determine what your chances are of having a baby with Down syndrome or trisomy 18
  
  
• Includes a blood test and, usually, a special ultrasound
  
  
• Detects up to 84% of babies with Down syndrome and about 75% of babies with trisomy 18
  
  
• Does not guarantee a healthy baby if the result is “screen negative”
  
  
• Is not diagnostic: A “screen positive” result does not necessarily mean your baby has a problem. Additional testing is needed.
  
  
• Does not provide information about neural tube defects

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