A CUB test (double test) is a combination of a fetal diagnostic ultrasound examination during week 11–14, often known as an "early ultrasound", and the taking of a blood sample from the mother. The CUB test does not lead to a diagnosis, but enables the likelihood of the fetus having certain specific chromosomal abnormalities to be determined.
Between weeks 11 and 14 during a pregnancy, all fetuses have a slight swelling under the skin of the neck (nuchal translucency/nuchal fold). An ultrasound examination involving the measurement of nuchal translucency is performed between week 11 and 14 of pregnancy.
If the swelling of the neck is larger than normal, there is a greater probability of chromosomal abnormalities in the fetus. Trisomy is an example of a chromosomal abnormality, where the fetus has an extra copy of chromosome 13, 18 or 21, also known as
The most common of these is trisomy 21 - Down's syndrome.
The probability of trisomy 13, 18 and 21 is calculated by combining the measurement of neck translucency with the results of blood samples taken from the mother (double test), other information about the mother and other information obtained during the ultrasound examination.
Increased swelling of the neck can also be observed in other conditions, such as heart defects. The fetus may be completely healthy even if the examination shows an increase in swelling under the skin of the neck.
What happens in practice?
In order to have a fetal diagnostic ultrasound examination, you must be referred by your doctor, your midwife or a department of fetal medicine or medical genetics. The referral must state the date of the last menstruation and the reason why you are being referred.
If you are offered a CUB test, you will be given more information by the department of fetal diagnostics/medical genetics concerning the procedure and when you will have a blood sample taken.
You will be summoned to an ultrasound examination during pregnancy week 11–14, as determined from the date of your last menstruation or information from previous ultrasound examinations.
The procedure is usually performed as an ultrasound examination of the abdomen (abdominal), but an ultrasound examination via the vagina (vaginal) provides a clearer overview. The procedure does not normally hurt and it is not dangerous for the fetus.
How long does it take to get the results?
You will normally be told the results of a CUB test immediately after the ultrasound examination. If the results of the blood test are not ready by the time of the ultrasound examination, you will receive the final results of the CUB test by telephone or letter. This can take up to two weeks.
The CUB test will not lead to a confirmed diagnosis, as it only calculates the probability that a condition exists. Only a placenta or amniotic fluid test can provide a confirmed diagnosis. If the examination shows that the probability is above a specific threshold (1:250), your doctor will discuss with you your options regarding NIPT, placenta and amniotic fluid tests.