First Trimester Screening

First Trimester Screening

First trimester screening is designed to identify whether you may be at increased risk of having a baby with Down syndrome or trisomy 18. These conditions are types of chromosomal abnormalities. The screening involves a maternal blood test and an ultrasound measurement called “nuchal translucency.”

First trimester screening can be done between 11 and 13 weeks gestation (sooner than other screening tests), and results take about one week. First trimester screening must be performed by an ultrasonographer or physician who is certified in nuchal translucency measurement.

It’s important to remember that first trimester screening is an indicator, not a definitive answer, about a baby’s health or condition. Even with an “abnormal” screen, most of the time the baby is healthy. Likewise, not every baby with Down syndrome or trisomy 18 will be identified through first trimester screening. Once a first trimester screening is performed, additional tests can give a definite answer about these and other conditions, if desired.

Screening for Down syndrome and Trisomy 18

When a pregnant woman sees her doctor in the first trimester of pregnancy she may be offered a screening test for Down syndrome and trisomy 18. This recent advancement in prenatal care is called first trimester screening. First trimester screening is designed to identify women who may be at increased risk of having a baby with Down syndrome or trisomy 18. It is important to remember that this is a screening test and that even when it is “screen positive”, most often the baby is healthy. It is also important to remember that not every baby with Down syndrome or trisomy 18 will be identified with this screen.

First trimester screening involves a specific ultrasound measurement called “nuchal translucency” and a maternal blood draw. First trimester screening assesses the chance of having a baby with Down syndrome and trisomy 18. It is typically drawn between 11 and 13 weeks 6 days gestation. Results take approximately 6-7 days.

Screening for High Risk Pregnancies: Noninvasive Prenatal Screening (cell free DNA testing)

For women that will be 35 years or older at delivery, have an abnormal fetal ultrasound, or an abnormal first trimester screening, there is a newer type of chromosome screening that is available. Noninvasive prenatal screening (cell free DNA testing) is a maternal blood test that detects common chromosome abnormalities in pregnancies at 10 or more weeks based on directed analysis of fetal DNA in maternal blood. They can also detect the presence of X and Y chromosomes. The detection rate for common trisomies such as 18, 21 and gender chromosomes is estimated to be greater than 99% and detection rate for trisomy 13 is ~ 90%.
If a high risk patient is interested in screening, we also recommend undergoing the first trimester ultrasound at the same appointment. The first trimester ultrasound is an early assessment of your baby; A normal scan would be reassuring, however, it is much too early in pregnancy to rule out all possible complications. An abnormal scan may be an early red flag that baby has an underlying birth defect or chromosome condition.

What are Chromosome Abnormalities?

Humans typically have 46 chromosomes in every cell. These chromosomes carry genes, which are the blueprints for human growth and development. If a child is born with more or less than 46 chromosomes in each cell, he or she will have a problem with mental and physical development. One example of a chromosome abnormality is Down syndrome (trisomy 21) and another is trisomy 18. These two chromosome conditions are screened for with first trimester screening.

Down Syndrome (Trisomy 21)

Down syndrome is a chromosome problem that occurs when a baby inherits an extra chromosome #21 at conception. This extra chromosome causes the features of Down syndrome, which can include heart defects, mental retardation and other physical disabilities. Down syndrome occurs in 1:800 pregnancies in the general population. The risk of having a baby with Down syndrome increases with increasing maternal age. However, 80% of babies with Down syndrome are born to women less than 35 years of age. Most couples who have a child with Down syndrome have no family history of the condition.

Trisomy 18

Trisomy 18 is a chromosome problem that occurs when a baby inherits an extra chromosome #18 at conception. It is medically more severe than Down syndrome and babies with trisomy 18 typically will be stillborn or will not live past 1 year of age. Babies that do survive will have severe mental retardation and can have a variety of other birth defects.

Frequently Asked Questions

What is First Trimester Screening?

How Accurate is First Trimester Screening?

What Does a Positive Result Mean?

My Result was Positive. Now What?

What Other Test Should I Have?

Should I Have the Triple Screen if I Have First Trimester Screening?

Learn More

fmlh-mcw-boxLearn more about the Maternal Fetal Care Center on the Froedtert & the Medical College of Wisconsin website.