Chorionic villus sampling and amniocentesis
Most children are born healthy. In 3 to 5% of all newborn babies, a congenital defect is found. A number of such abnormalities can be detected before birth (prenatally) using chorionic villus sampling (CVS) or amniocentesis. Women with an elevated risk of such abnormalities are offered the option of having this test performed.
This brochure provides information about CVS and amniocentesis.
When can a pregnant woman be considered eligible for prenatal diagnostic tests?
A number of reasons for considering pregnant women eligible for prenatal diagnostic tests are given below:
- the woman's age: 36 or older (see table)
- parents who have a child with a chromosomal abnormality
- one or both parents is a carrier of a chromosomal abnormality
- parents with an elevated risk of a child with a hereditary condition that can be established by means of a DNA test or a metabolic test
- an elevated risk revealed during a screening test
Table
The chance of Down's syndrome (trisomy 21 or "mongolism") in connection with the age of the mother
| age |
change of Down's syndrome |
overall chance of a chromosomal abnormality |
| 25 |
1 in 1250 (0,08%) |
1 in 476 (0,2%) |
| 30 |
1 in 965 (0,10%) |
1 in 385 (0,26%) |
| 36 |
1 in 300 (0,3%) |
1 in 170 (0,59%) |
| 37 |
1 in 234 (0,4%) |
1 in 127 (0,79%) |
| 38 |
1 in 182 (0,5%) |
1 in 102 (1,0%) |
| 39 |
1 in 141 (0,7%) |
1 in 83 (1,2%) |
| 40 |
1 in 110 (0,9%) |
1 in 66 (1,5%) |
| 41 |
1 in 86 (1,2%) |
1 in 53 (1,9%) |
| 42 |
1 in 66 (1,5%) |
1 in 42 (2,4%) |
| 43 |
1 in 52 (2,0%) |
1 in 33 (3,1%) |
| 44 |
1 in 40 (2,5%) |
1 in 26 (3,8%) |
| 45 |
1 in 31 (3,0%) |
1 in 21 (4,8%) |
(Palomaki/Haddow/Hook, 1987)
What tests are possible? Chromosomal test
The carriers of hereditary characteristics are known as chromosomes. Humans have 46 chromosomes in every single cell, divided into 23 pairs (see fig. 1).
Fig. 1 Chromosomes A number of defects in the chromosomes can be detected before birth by studying cells taken from the placenta or the amniotic fluid. The sex of the child can also be determined in this way. The majority of chromosomal abnormalities are associated with severe physical and mental defects. Many congenital abnormalities, however, are not associated with chromosomal defects and are therefore not revealed during this test.
Chromosomes from the unborn child are present in both the chorionic villi (= tissue from the developing placenta) and the amniotic fluid.
AFP Amniocentesis also allows the level of AFP protein (alpha foetoprotein) to be determined. In children with a defect in the spinal column (spina bifida) or the cranial vault (anencephaly), this protein is often present in the amniotic fluid in abnormally high quantities. The chance of spina bifida is not related to the age of the woman.
DNA DNA tests can be performed for some hereditary conditions that run in families (e.g. haemophilia or muscular dystrophy). DNA tests are generally performed on the chorionic villi. In such cases, investigations into the family background are always required before the prenatal DNA test. This family study should preferably be completed before the woman becomes pregnant.
'Intake' appointment
Before you come for any actual tests, an 'intake' interview will be held at the out-patient clinic. The following items will be discussed during this appointment:
- the reason(s) for the test(s)
- the pros and cons of the test(s) and the associated risk(s)
- an explanation of what will happen during the procedure
- what you can expect after the procedure
A ultrasound scan will be carried out to determine exactly how far the pregnancy has progressed if this has not been done already. The ultrasound scan can be used to determine the date for the test. When you come for your appointment, please bring the following documents:
- your health insurance company's policy document or proof of registration
- a letter from whoever referred you (GP/obstetrician/ gynaecologist)
- if you know your blood group, please bring details (e.g. pregnancy chart)
The chorionic villus sampling test
The chorionic villus sample or biopsy (CVS) is an outpatient procedure. An ultrasound examination will have been performed beforehand to determine the exact duration of the pregnancy. Your partner, a friend or a member of your family may be present during the procedure.
There are two ways of obtaining the samples, either via the vagina (transcervical CVS) or through the wall of the abdomen (transabdominal CVS). The choice as to which of the two methods will be used depends on the positioning of the uterus and the location of the placenta. The gynaecologist who performs the test will decide at the actual time of the test which of the techniques (via the abdomen or the vagina) is preferable. In the majority of cases, the CVS is performed via the vagina. The test is easier to perform if the bladder is half full.
Transcervical CVS
The transcervical CVS is performed when 11 to 13 weeks pregnant, counting from the first day of the last menstruation. For this test, the bladder needs to be half full. You will be positioned on a gynaecological examination chair. An ultrasound scan will be done to determine the position of the placenta. A speculum is then inserted into the vagina. The vagina is then disinfected using an iodine solution.
Fig. 2 Schematic diagram of the transcervical CVS procedure
Using the ultrasound scan as a guide, a little grasping device (biopsy forceps) (about 1.5 mm across) is introduced via the cervix to the location of the placenta (see fig. 2). This does not cause any pain. A few of the chorionic villi are extracted from the developing placenta. A check is done straight away to ensure that enough chorionic villi have been obtained. If insufficient material has been obtained, it may be necessary to repeat the procedure and remove some more tissue. The appointment lasts 10 to 20 minutes; the procedure itself takes no more than one minute.
Transabdominal CVS
The transabdominal CVS can be performed after 11 weeks' pregnancy, counting from the first day of the last menstruation. You do not need to have a half-full bladder for this test. When you are on the examination couch, an ultrasound scan will be used to determine the position of the placenta. The abdomen is then disinfected with an iodine solution. After the puncture site has been anaesthetised, a very thin needle (0.9mm thick) is placed into the placenta under direct ultrasound guidance (see fig. 3).
Fig. 3 Schematic diagram of the transabdominal CVS procedure
A few chorionic villi are extracted and a check is done straight away to ascertain whether sufficient material has been obtained. If insufficient material has been obtained, it may be necessary to repeat the procedure. Extracting the chorionic villi by this method can be rather uncomfortable. The appointment also lasts 10 to 20 minutes for this method; the CVS procedure itself takes 1 - 2 minutes.
Blood group
If your blood group is rhesus negative, you will be given an injection immediately after the test to prevent your body from making antibodies against the red blood cells of the foetus, in the event that its blood group is rhesus positive. Since we do not know whether or not this is the case, all women with a rhesus negative blood group are given this injection (also known as the "anti-D" injection). This prevents the foetus from becoming anaemic.
What happens afterwards?
After the transcervical CVS, you may experience a little blood loss ('spotting') for a few days. This is generally a lot less than during a menstruation. Blood loss should not occur after a CVS through the abdominal wall. We recommend that you take things easy on the day of the test (for either type of test) and do not over-exert yourself. You can resume your normal activities the next day.
The results
The results are generally known in 14 days. You will be informed when to expect the results at the time of the test. At the time of the CVS you can let us know whether you want to know the sex of your baby. When you are called with the results, you can ask what the gender is, irrespective of whether you told us before that you didn't want to know. The complete results will be sent in writing to your GP and/or the person who referred you.
An abnormal result will be discussed in detail with you (and your partner) at the hospital. If you do decide to terminate the pregnancy, this can be done surgically by vacuum aspiration or medically.
This is a procedure that does not damage the uterus and the likelihood of subsequent pregnancies is not affected.
There is a possibility that you may feel in need of psychological/ social counselling before, during or after any such a termination. You can discuss this with the physician treating you, who can then help you contact an expert in that field.
Reliability of the chorionic villus sample test
The report of a CVS test is 100 % reliable. However, in approximately 2% of all cases the results are unclear or the quality of the chromosomes is insufficient to be able to detect minor structural chromosomal abnormalities. An additional amniocentesis test may then be needed. Obviously this will be discussed with you. Depending on the results, it may also be necessary to do chromosomal tests on you and your partner.
What risks are associated with the CVS test?
Because the CVS test is carried out during a period when spontaneous miscarriages can occur, it is not sufficiently clear exactly what the additional risk of a miscarriage is as a consequence of the CVS test. In the light of current literature, we estimate that 1 in 200 women (0.5%) will miscarry after a CVS procedure. The problem is that we can never know for sure in such cases whether the pregnancy would have resulted in a miscarriage in any event, in the absence of the CVS test. A miscarriage as a consequence of the CVS test usually occurs within one week.
Amniocentesis
Amniocentesis is an outpatient procedure. An ultrasound examination will have been performed beforehand to determine the exact duration of the pregnancy. Amniocentesis is performed after 15 weeks, counting from the first day of the last menstruation. You do not need to have a full bladder for this test.
While you are on the examination couch, an ultrasound scan is used to determine the puncture site. This takes account of the positioning of the foetus and the location of the placenta. The skin of the abdomen is then disinfected with iodine. A thin needle (0.7mm thick) is introduced into the amniotic fluid through the skin of the abdomen and the abdominal wall, under ultrasound guidance. Approximately 16 ml amniotic fluid is withdrawn. This is less than one-tenth of the total amount of amniotic fluid.
Anaesthesia is not required; the majority of women do not experience any pain during the examination. Your partner, a friend or a member of your family may be present during the procedure.
The appointment lasts about 10 - 15 minutes, while the actual extraction of the amniotic fluid takes about 30 seconds.
Reliability
The report of an amniocentesis is 100 % reliable. However, in approximately 0,5% of all cases the results are unclear or the quality of the chromosomes is insufficient to be able to detect minor structural chromosomal abnormalities. Obviously this will be discussed with you.
Depending on the results, it may also be necessary to do chromosomal tests on you and your partner.
What happens afterwards?
Some women experience a cramp(menstruation)-like feeling in the lower abdomen for a little while. We recommend that you take things easy on the day of the test and do not over-exert yourself, and in particular do not lift things. You can resume your normal activities the next day.
The results
Chromosomal investigations require the cells from the amniotic fluid to be cultured. This process takes 3 weeks. The level of AFP in the amniotic fluid is also determined. Those results are known within 1 week.
As soon as all the results are known, you will be called.
You can indicate at the time of the amniocentesis whether you want to know the sex of your baby. When you are called with the results, you can ask what the gender is, irrespective of whether you told us before that you didn't want to know. The complete results of the test will be sent in writing to your GP and/or the person who referred you. An abnormal result will be discussed in detail with you (and your partner) in the hospital. If you do decide to terminate the pregnancy, this can be done in the hospital by inducing contractions prematurely. This does not damage the uterus and the likelihood of subsequent pregnancies is not affected.
There is a possibility that you may feel in need of psychological/ social counselling before, during or after any such termination. You can discuss this with the physician treating you, who can then help you contact an expert in that field.
What risks are associated with amniocentesis?
The chances of a miscarriage as a result of amniocentesis are about 1 in 300 (0.3%). A miscarriage as a result of amniocentesis mostly occurs within 3 to 4 weeks.
If your blood group is rhesus negative, you will be given an injection immediately after the test to prevent your body from making antibodies against the red blood cells of the foetus, in the event that its blood group is rhesus positive. Since we do not know whether or not this is the case, all women with a rhesus negative blood group are given this injection (also known as the "anti-D" injection). This prevents the foetus from becoming anaemic.
Follow-up form
After the test, we will give you a form with questions about the progress and outcome of the pregnancy. We urgently request you to fill in this form after the baby has been born and send it back to us in the enclosed reply envelope. Your personal data will be treated in confidence. Insurance If you are eligible for chorionic villus sampling or amniocentesis, it is advisable to inform your health insurers about this in advance and ask whether the examination is reimbursable.
Summary
| Type of test |
CVS |
amniocentesis |
| timing of test |
11-13 weeks |
16-17 weeks |
| results |
14 days |
3 weeks |
| reliability of test |
100 % |
100 % |
| method |
vaginal or abdominal |
abdominal |
| risk of miscarriage |
5 in 1000 |
3 in 1000 |
| method of pregnancy termination |
vacuum aspiration/medical |
induced premature labour |
Location
The gynaecology outpatient clinic is in corridor A, second floor.
Finally If you have any further questions after reading this information, please contact the Gynaecology secretarial office, tel. +31(0)70 - 312 46 69.