At a glance
Also known as
Triple screen; Quadruple test; Maternal Serum Screening (MSS); AFP maternal; Non-Invasive Prenatal Testing (NIPT)
Why get tested?
Prenatal screening provides pregnant women with the option to refine the probability of having a fetus with a common fetal chromosomal anomaly such as Down syndrome or a structural defect such as spina bifida.
When to get tested?
First trimester screening for Down syndrome is usually available 9th week of pregnancy (9 weeks to 13 weeks 6 days). Second trimester screening for Down syndrome and neural tube defects (spina bifida) are usually available from the 14th week to 20 weeks.
Non-invasive prenatal testing (NIPT) of cell-free DNA is available from the 10th week of pregnancy. Most laboratories offering NIPT will test for trisomy 21, trisomy 18, trisomy 13, fetal sex, Monosomy X, sex chromosome aneuploidies, and DiGeorge syndrome (22q11.2 deletion syndrome).
For first and second trimester serum screening, a clotted blood sample is drawn from a vein in the mother's arm.
- First trimester screening- the laboratory results of maternal serum free beta component of human chorionic gonadotrophin (free ß-hCG) and pregnancy-associated plasma protein A (PAPP-A) are combined with the ultrasound nuchal translucency measurement to calculate the probability of the fetus having Down syndrome ( trisomy 21), trisomy 13 or trisomy 18.
- Second trimester screening- maternal blood is analysed for alpha-fetoprotein (AFP), free ß-hCG and unconjugated oestriol (uE3) to determine the probability of the fetus having Down syndrome or a neural tube defect.
The key information required for accurate risk assessment:
- Maternal age
- Gestational age ( preferably by ultrasound)
- Sample collection date
- Maternal weight
3. NIPT- Blood is collected into special tubes to protect the DNA. A first trimester ultrasound should be done prior to NIPT. This allows for confirmation of viability, accurate gestational dating, identification of multifetal pregnancies and placental and other abnormalities. There are several laboratories offering NIPT in Australia.
Test preparation needed?
You may be instructed to have a full bladder when having the nuchal translucency ultrasound test performed.
What is being tested?
The maternal serum screening tests involve the measurement of different pregnancy-associated hormones, which are found in all pregnancies.
Several different biochemical substances are measured in the blood. Which substances are measured depends on the time during pregnancy that the sample was taken. The combinations of tests may be known by different names depending usually on how many tests are measured. For example, the first trimester screening uses the concentrations of pregnancy-associated plasma protein A (PAPP-A), the free ß component of human chorionic gonadotrophin (free ß-hCG) combined with ultrasound measured nucal translucency to calculate the probability of Down syndrome. The triple test combines the results of three different substances (AFP, hCG and unconjugated oestriol (UE3); the inclusion of inhibin A in a four-test nael is called the "Quadruple test".
NIPT measures the amount of cell-free DNA circulating in the mother's blood.
First trimester screen:
- pregnancy associated plasma protein A (PAPP-A) is one of several proteins produced by the placenta. The PAPP-A concentration increases throughout pregnancy in the mother's blood until term and low levels of maternal serumm PAPP-A are associated with fetal Trisomy 21.
- Free beta human chorionic gonadotrophin (free ß-hCG) is a hormone produced by the trophoblast tissue of the placenta. Concentrations of the free ß-hCG increase to a peak at around 8-12 weeks of gestation and then decline to 10% to 15% of the peak concentrations and plateau from 20 weeks until term. Free ß-hCG provides a more sensitive marker of Down syndrome than total hCG.
- Ultrasound measurment of fetal nuchal translucency (NT) or the thickness of the fluid accumulated at the back of the baby's neck. An increased NT is strongly associated with a higher probability of Trisomy 21.
The ultrasound part of this test can only be performed between 10 weeks and 13 weeks 6 days pregnancy. The biochemistry part of the test can be performed from 9 weeks until 13 weeks 6 days of pregnancy.
Second trimester screen, MSS or ‘triple test’:
Alpha fetoprotein (AFP
) is the major blood protein produced in early fetal life and it is excreted into the amniotic fluid by the immature fetal kidneys. AFP passively diffuses into the maternal blood.
- AFP concentration peaks in fetal blood between 12 and 14 weeks and inmaternal serum between 28 and 32 weeks of gestation. An abnormal increase in maternal serum AFP is the basis of the serum screening test for NTDs at about 15-16 weeks of pregnancy.
- Human chorionic gonadotrophin (hCG) is a hormone produced by the placenta. Concentrations of free ß-hCG provide a more sensitive marker of Down syndrome than total hCG.
- Unconjugated oestriol (uE3) is a hormone produced by placenta from precursor hormones produced by the baby, so it requires both placenta and baby to be healthy and developing normally.