First trimester screening for Down syndrome, also known as nuchal translucency screening, is atest offered to women in their first trimester of pregnancy to determine the chance that they willhave a baby with Down syndrome.
Down syndrome is a genetic condition caused by having an extra copy of chromosome 21. It isalso known as trisomy 21. Children with Down syndrome experience developmental delay andsome degree of mental retardation. They usually have characteristic facial features and may haveother health conditions including an increased chance for congenital heart defects, thyroidproblems, infections, respiratory problems, vision and hearing problems. However, each childwith Down syndrome is unique.
More information about Down syndrome can be found through local support groups or at:
First trimester screening is performed early in the pregnancy – usually between 10 ½ and 13 ½weeks of pregnancy. The screening combines measurements from both a blood test and anultrasound examination.
A small sample of the mother’s blood is required for the screening. This sample may be takeneither by finger stick or by blood draw from the arm. The sample is then analyzed for twopregnancy hormones (usually free beta or intact hCG and PAPP-A). Both substances arenormally produced during pregnancy.
The ultrasound examination involves measuring the amount of fluid accumulated under the skinat the back of the baby’s neck. This normal accumulation of fluid is known as the nuchaltranslucency (or NT) measurement and it is often increased when a developing baby has Downsyndrome. It is critical that this measurement be accurate, therefore it should always beperformed by a physician or sonographer that has proper certification like that provided by theNuchal Translucency Quality Review Program of the Maternal-Fetal Medicine Foundation.
The ultrasound and blood results are then combined with maternal factors such as age and weightto calculate the chance for Down syndrome in the current pregnancy.
You can usually expect your test results to come back in approximately one week or less fromthe time the blood sample is obtained.
You will receive an individual assessment of the chance for Down syndrome in your pregnancy(for example 1 in 800). While each patient will interpret their personal risk somewhat differently,in general, laboratories will choose a cut-off and any risk that is higher than the cut-off will bereported as “screen positive”. All values lower than the cut-off are reported as “screennegative”.
Your individual risk based on the ultrasound examination and blood test is a very accuratereflection of the chance for your developing baby to have Down syndrome.
Ninety percent (90%) of pregnancies with Down syndrome will have results in the screenpositive range. This means that roughly 9 out of 10 of pregnancies with Down syndrome willcome back as ‘screen positive’. However a ‘screen positive’ result does not mean that the babyhas Down syndrome, it simply means that the risk is high enough that you should considerfurther evaluation of the pregnancy. The only way to know for certain whether or not adeveloping baby has Down syndrome is by performing a diagnostic test such as chorionic villussampling (CVS) or amniocentesis (these tests are described below).
First trimester screening can also determine whether the pregnancy has an increased chance fortrisomy 18. Trisomy 18 is a chromosome change that results in severe mental retardation andmultiple birth defects. First trimester screening detects approximately 90% of pregnancies withtrisomy 18.
When a laboratory calculates the chance for a developing baby to have Down syndrome theresults can range from 1 in 5 (20%) to 1 in 10,000 or less (0.01%). Each laboratory establishes acut-off risk and any risk that is over that cut-off is considered to be screen positive. Being toldyour result is screen positive does not mean that the developing baby definitely has Downsyndrome; it simply means that the chance is high enough for you to be offered further testing. Infact the majority of women with ‘screen positive’ results do not go on to have a baby with Downsyndrome.
Screening tests are not diagnostic and do not provide definitive answers. Screening tests identifya high-risk group.
Chorionic villus sampling and amniocentesis are procedures that can determine definitivelywhether or not a developing baby has a chromosome difference like Down syndrome or trisomy18 with greater than 99.9% accuracy.
Chorionic villi sampling (CVS) is typically done between 10 and 13 weeks of pregnancy. In thisprocedure the doctor obtains a small sample of tissue from the edge of the placenta using either athin needle inserted through the abdomen or a small catheter inserted through the cervix. Themethod used depends on the location of the baby and of the placenta.
Amniocentesis is a procedure in which a thin needle is inserted through the woman’s abdomeninto the amniotic sac to withdraw a small sample of fluid from around the developing baby.Amniocentesis procedures are safest when done after 15 weeks of pregnancy.
The cells collected from either procedure can be used for chromosome analysis. This testingallows the laboratory to determine if a baby has an extra chromosome (as with Down syndrome)or if the baby has exactly the number of chromosomes that we would expect.
Both procedures are associated with a small chance for miscarriage. The decision whether or notto pursue a CVS or an amniocentesis is a personal one. Speaking to a genetic counselor or yourobstetrician can help you make the decision that is best for you.
A genetic counselor is a health care professional that can explain your test results and reviewwith you details about further testing options. Your doctor can refer you to a genetic counselor oryou can find one in your area by selecting the “Find a Counselor” option on the National Societyof Genetic Counselor’s website at www.nsgc.org.
A ‘screen negative’ result means that the chance for you to have a baby with Down syndrome ortrisomy 18 is low. While this is good news, you should remember that the test is not a diagnostictest and some children with Down syndrome will be born to women in this low risk group. Firsttrimester screening does detect the majority of pregnancies with Down syndrome and trisomy 18,however about 10% of affected pregnancies have a negative first trimester screening result eventhough the baby actually has one of these conditions. Your individual result is the best estimateof the likelihood of this occurring in your pregnancy.
First trimester nuchal translucency measurements greater than 3.5 mm have been associated withan increased chance for some structural birth defects, especially heart defects. Your doctor mayrecommend a fetal echocardiogram at 18-20 weeks to rule out major congenital heart defects.
You should also consider a high-resolution ultrasound examination when you are between 18and 20 weeks along. However, most pregnancies that are screen positive will result in thedelivery of a healthy baby.
It is also important to remember that this test has been designed to detect pregnancies atincreased risk for Down syndrome and trisomy 18. It cannot detect all birth defects or geneticconditions. Your obstetrician will likely recommend some other tests during the pregnancyincluding:
If you are concerned that you may have an increased chance to have a baby with a problem dueto your family or personal history we encourage you to speak to your obstetrician in more detailor to contact a genetic counselor in your area. You can find a genetic counselor in your area byselecting the “Find a Counselor” option on the National Society of Genetic Counselor’s websiteat www.nsgc.org.
First trimester screening is available for twin pregnancies, but the results are a little moredifficult to interpret when there are two developing babies instead of one. Therefore the detectionrate is lower and the chances for trisomy 18 cannot always be provided. Different laboratoriesapproach the interpretation of first trimester screening in twin pregnancies differently. Some willprovide a separate assessment for each twin while others evaluate the pregnancy as a whole. It isrecommended that you talk with your doctor for more details about first trimester screening in atwin pregnancy.
Anyone who desires more information about her baby’s development should consider screening.More than 95% of babies with Down syndrome and trisomy 18 are born to families with nohistory of these conditions. In some cases, the diagnosis of a problem before birth may lead todifferent options during the pregnancy or special management of the delivery to improve theoutlook for the baby.
No. Women who are 35 or older at the time of delivery do have an increased risk of having ababy with Down syndrome or trisomy 18. However, babies with these conditions can be born towomen of any age. In fact, more than 75% of babies with Down syndrome are born to womenwho are under 35 because women under age 35 are having more babies than women over 35.
Some women 35 and over choose to have an invasive test without having any screening.However, screening techniques have become so good that women of all ages should considerscreening to better quantify their risk before deciding whether to have an invasive procedure.
Information provided by:Maternal Fetal Medicine FoundationNuchal Translucency Quality Review Program