Pre-natal screening tests
by Dr. Derrick Thompson
Tests for chromosomal abnormalities and neural tube defects.
The accuracy and reliability of these tests depend on knowing exactly how long you have been pregnant, so I recommend an early dating ultrasound scan which can be arranged or performed by your doctor at your first visit, preferably 6-8 weeks after the first day of your last period. This should give you an accurate EDD. You can undergo the following tests to check whether your baby is normal. The main objective is to diagnose if your baby may have a chromosomal abnormality, for example, Down syndrome or a neural tube defect.
Combined first trimester screening for Down syndrome (or early combines screening test, ECST)
A blood test during the tenth week of your pregnanct. this measures PAPP-A (pregnancy associated plasma protein A) and free beta HCG. In Down syndrome-affected babies, the PAPP-A is low and the free beta HCG is high.
An ultrasound scan during the twelfth week of pregnancy. A measurement is made of nuchal translucency. Nuchal translucency is a measurement of the thickness of the fluid-filled tissue at the back of the fetal neck. Nuchal translucency usually measures less than 2-3 mm in thickness but this does vary depending on the age of the mother. The thicker the nuchal translucency, the greater the risk of Down syndrome. In other conditions, for example, Trisomy 18 and Turner’s syndrome, there is also an increased amount of subcutaneous fluid beneath the skin at the back of the neck.
Into a computerized database, your doctor will enter the results of the blood tests and the nuchal translucency, together with your age, weight, gestation of your pregnancy at the time of your blood test and whether you have had a chromosomal abnormal baby previously. If you have conceived following in-vitro fertilisation with donor eggs, the age of the donor is also included.
If your risk factor is greater than one chance in 250 (>1:250), a chorionic villus sampling (CVS) or amniocentesis should be performed. A risk factor of <1:250 means that there is only a very small chance that your baby will be affected by Down syndrome (Trisomy 21) or Edwards syndrome (Trisomy 18). It is important to realise that about one in 20 women will be classed “at risk”. Yet only one in 50 of these women will be carrying a Down syndrome baby.
Chorionic Vilus sampling (CVS)
Your doctor may recommend this test if:
- your risk factor is > 1 in 250
- you will be 35 or more years of age when you baby is due to be born
- you have had a previously affected baby.
A CVS is performed by inserting a fine needle through your lower abdomen or vagina under ultrasound control, and obtaining a tissue sample from the placenta. A chromosome analysis is performed on the cells in this tissue. Every cell in the body contains 46 chromosomes. Normally each cell contains 22 pairs of autosomal chromosomes and a pair of sex chromosome: XX (female) and XY (male). However in Down syndrome, each cell contains an extra chromosome of the 21st pair, hence the name Trisomy 21. Thus each cell contains 47 chromosomes instead of 46 chromosomes.
A CVS will also identify other chromosomal abnormalities, for example:
- an extra number 18 chromosome, Trisomy 18 (Edward’s syndrome)
- an exmtra nuber 13 chromosome, Trisomy 13
- an absent X chromosome in a female fetus, XO instead of XX (Turner’s syndrome)
- and extra X chromosome in a male fetus, XXY instead of XY (Klinefeleter’s syndrome)
Some couples, especially if there has been a history of infertility, choose to have an amniocentesis instead of a CVS, where the risk of subsequent miscarriage is only about one chance in 200 (0.5%).
An amniocentesis is performed at 16 weeks gestation where, under ultrasound control, a fine needle is inserted into the fluid sac surrounding the fetus and a sample of amniotic fluid withdrawn. This fluid contains exfoliated skin cells form your baby which are cultured and then examined for chromosomal abnormalities. As with CVS, you can get a preliminary result in 24 hours, but the final result will not be available for 10-14 days.
CVS vs. Amniocentesis
The benefit of CVS is that a diagnosis can be made by 12-13 weeks gestation. If a woman decides to terminate her pregnancy, her doctor canb perform a suction curettage. Undergoing a termination of pregnancy at 18-20 weeks gestation, on the other hand, would invilve either a medical termination with misoprotosol and mifepristone, if available, or a surgical procedure known as dilatation and evacuation (D&E). This is a difficult procedure and should only be performed by a doctor skille din this technique.
Following a CVS or amniocentesis, you might experience some mild lower abdominal discomfort and cramping. You may notice some vaginal blood spotting. If your blood loss is heavier, like the start of a period, you need to contact your doctor. Similarly, you need to contact your doctor if you experience a loss of amniotic fluid after an amniocentesis. Therefore I suggest that following a CVS or amniocentesis, you relax at home with your feet elevated. Also avoid intercourse for the next 48 hours.
Maternal serum screening test (MSST)
If your doctor has not performed the early combined screening test (ECST), you can have a maternal serum screening test (MSST) between 15-18 weeks gestation. This blood test is low-cost and convenient, and is used if there are limited ultrasound facilities.
The ECST is a more sensitive test and therefore preferred over the MSST for a diagnosis of chromosomal abnormality. The ECST does not give a risk factor for neural tube defects, but the ultrasound at 12 weeks gestation and 18-20 weeks gestation will diagnose most neural tube defects. I do not recommend that you have an ECST as well as a MSST.
The MSST measures 4 hormones and is also known as the “quadruple test”.
The hormones measured are:
- free beta HCG
- aplha fetoproteion (AFP)
- unconjugated oestriol (UE3)
- dimeric inhibin (A)
In Down syndrome, AFP and UE3 are reduced, and B-HCG is increased. In neural tube defects, AFP is increased.
This test will help to identify 9 out of 10 pregnancies with anencephaly. 4 out of 5 neural tube defects, and 2 out of 3 cases of Down syndrome. It is very important to realise that this test is only for Down syndrome and neural tube defects, and that a low-risk result does not guarantee that your baby will be free of other birth defects. The test results are usually available within a week.
For Down syndrome, approximately one in 20 women screened will have an increased risk of one in 250 or greater. However, only one in 50 will have an affected fetus. For neural tube defects, the level of AFP alone is used to determine the risk of having a fetus with an open neural tube defect. About one in 50 of all women screened will be found to be at increased risk, but only one in 20 will have an affected fetus.
Fetal anomaly scan (18-20 weeks)
The second part of prenatal screening involves fetal anomaly scan at 18-20 weeks gestation. This is an exciting ultrasound scan as it is the first time you can see your fully developed baby and find out the sex, if you so desire. More importantly, your doctor can perform a detailed examination of your baby. Let me say, however, even though a wide range of abnormalities can be detected on ultrasound, the 18-week ultrasound scan cannot guarantee the complete normality of a baby.
This scan also confirms:
- your due date
- your baby is growing normally
- if you have multiple pregnancy, wither with twins or triplets
- the position of the placenta
- the amount of amniotic fluid around your baby, and whether it may be excessive (polyhydramnios) or diminished (oligohydraminos)
- if you have an ovarian cyst
- if you have uterine fibroids, which may cause pain if they swell, or cause your baby to lie in an abnormal position; this may prevent a normal delivery.
- your cervix is closed and not opening prematurely.
This ultrasound scan also screens for Down syndrome by measuring nuchal fold thickness.
2D ultrasound is normally used for the fetal anomaly scan. This scan takes photographic slices through tissues, and a picture is built up from the cross-section of the tissue. As a result of improving technology, we now have 3D and 4D scans. 3D ultrasound shows lifelike images of your baby, especially surface features which allow you to examine the face and cheek of the fetus for a hare lip and/or cleft palate.
4D ultrasound is a moving 3D image. However 2D ultrasound is still the best method for diagnosing fetal abnormalities at this point in time.
The main aim of the 18-20 week ultrasound is to assess the normality of your baby. However the second important function is to check the position of the placenta to see if it is low lying. At 18 weeks, the placenta occupies about one third of the wall of the uterus, but by the time of birth it occupies only one sixth of this space. Therefore, the placenta may appear to be low lying at 18 weeks, At 28-30 weeks gestation, the lower part of the uterus thins out – called the development of the lower segment – and the placenta appeasrs to migrate upwards and away form the cervix.