First trimester screening includes an ultrasound exam to measure the size of the clear space in the tissue at the back of a baby's neck (nuchal translucency). In Down syndrome, the nuchal translucency measurement is abnormally large — as shown on the left in the ultrasound image of an 11-week fetus. For comparison, the ultrasound image on the right shows an 11-week fetus with a normal nuchal translucency measurement. Show First trimester screening is a prenatal test that offers early information about a baby's risk of certain chromosomal conditions, specifically, Down syndrome (trisomy 21) and extra sequences of chromosome 18 (trisomy 18). First trimester screening, also called the first trimester combined test, has two steps:
Typically, first trimester screening is done between weeks 11 and 14 of pregnancy. Using your age and the results of the blood test and the ultrasound, your health care provider can gauge your risk of carrying a baby with Down syndrome or trisomy 18. If results show that your risk level is moderate or high, you might choose to follow first trimester screening with another test that's more definitive. Why it's doneFirst trimester screening is done to evaluate your risk of carrying a baby with Down syndrome. The test also provides information about the risk of trisomy 18. Down syndrome causes lifelong impairments in mental and social development, as well as various physical concerns. Trisomy 18 causes more severe delays and is often fatal by age 1. First trimester screening doesn't evaluate the risk of neural tube defects, such as spina bifida. Because first trimester screening can be done earlier than most other prenatal screening tests, you'll have the results early in your pregnancy. This will give you more time to make decisions about further diagnostic tests, the course of the pregnancy, medical treatment and management during and after delivery. If your baby has a higher risk of Down syndrome, you'll also have more time to prepare for the possibility of caring for a child who has special needs. Other screening tests can be done later in pregnancy. An example is the quad screen, a blood test that's typically done between weeks 15 and 20 of pregnancy. The quad screen can evaluate your risk of carrying a baby with Down syndrome or trisomy 18, as well as neural tube defects, such as spina bifida. Some health care providers choose to combine the results of first trimester screening with the quad screen. This is called integrated screening. This can improve the detection rate of Down syndrome. First trimester screening is optional. Test results indicate only whether you have an increased risk of carrying a baby with Down syndrome or trisomy 18, not whether your baby actually has one of these conditions. Before the screening, think about what the results will mean to you. Consider whether the screening will be worth any anxiety it might cause, or whether you'll manage your pregnancy differently depending on the results. You might also consider what level of risk would be enough for you to choose a more invasive follow-up test. Request an Appointment at Mayo Clinic RisksFirst trimester screening is a routine prenatal screening test. The screening poses no risk of miscarriage or other pregnancy complications. How you prepareYou don't need to do anything special to prepare for first trimester screening. You can eat and drink normally before both the blood test and the ultrasound exam. What you can expectFirst trimester screening includes a blood draw and an ultrasound exam. During the blood test, a member of your health care team takes a sample of blood by inserting a needle into a vein in your arm. The blood sample is sent to a lab for analysis. You can return to your usual activities immediately. For the ultrasound exam, you'll lie on your back on an exam table. Your health care provider or an ultrasound technician will place a transducer — a small plastic device that sends and receives sound waves — over your abdomen. The reflected sound waves will be digitally converted into images on a monitor. Your health care provider or the technician will use these images to measure the size of the clear space in the tissue at the back of your baby's neck. The ultrasound doesn't hurt, and you can return to your usual activities immediately. ResultsYour health care provider will use your age and the results of the blood test and ultrasound exam to gauge your risk of carrying a baby with Down syndrome or trisomy 18. Other factors — such as a prior Down syndrome pregnancy — also might affect your risk. First trimester screening results are given as positive or negative and also as a probability, such as a 1 in 250 risk of carrying a baby with Down syndrome. First trimester screening correctly identifies about 85 percent of women who are carrying a baby with Down syndrome. About 5 percent of women have a false-positive result, meaning that the test result is positive but the baby doesn't actually have Down syndrome. When you consider your test results, remember that first trimester screening indicates only your overall risk of carrying a baby with Down syndrome or trisomy 18. A low-risk result doesn't guarantee that your baby won't have one of these conditions. Likewise, a high-risk result doesn't guarantee that your baby will be born with one of these conditions. If you have a positive test result, your health care provider and a genetics professional will discuss your options, including additional testing. For example:
Your health care provider or a genetic counselor will help you understand your test results and what the results mean for your pregnancy. By Mayo Clinic Staff Request an Appointment at Mayo Clinic Nov. 17, 2020 Share on: FacebookTwitter Show references
First trimester screening
AdvertisementMayo Clinic does not endorse companies or products. Advertising revenue supports our not-for-profit mission. What happens if Down syndrome test is positive during pregnancy?A positive screening test result means that your baby's chance of having Down syndrome is higher than average. Your test results may include a number that describes how high the risk is. But a high risk doesn't mean your baby will have Down syndrome.
What is high risk for Down syndrome?● Screen positive (high risk) – A patient with a result of 1 in 50 would have a "high" risk. The "1" in 50 means that, among 50 patients with this same risk, one of them would have a developing baby with Down syndrome.
What stage of pregnancy does Down syndrome occur?It occurs by accident at conception (when the egg is fertilised by the sperm), not because of anything the parents have done. Although the chance of having a child with Down syndrome increases with the age of the mother, anyone can have a baby with Down syndrome. It affects about one in every 700-900 babies.
Can Down syndrome be cured during pregnancy?Currently, if the screening detects a third No. 21 chromosome in the cells of a baby, there is no prenatal therapy for Down syndrome.
|