Throughout pregnancy, women are given routine checks by doctors and midwives at ante-natal clinics, surgeries and health centres. These test are carried out to ensure that both mother and baby are in good health. This information sheet outlines the main tests available which enable a more accurate diagnosis to be made.
This is a simple blood test carried out between the 16th and 18th weeks of pregnancy. A small amount of blood is taken from the woman's arm and tested to find out the level of alpha feto protein. A high level indicates the possible presence of a neural tube defect, eg spina bifida. A low level indicates the possible presence of Down's Syndrome.
However, there may be several other reasons for these results. it is important to stress that, when a high or low AFP level is detected, it does not necessarily mean that there s something wrong with the baby. The AFP blood test is a screening procedure and other tests are required to confirm the presence of a neural tube defect. These tests are a detailed ultrasound scan and/or amniocentesis.
The AFP blood test is not performed routinely at all ante-natal clinics. Where it is the policy of the hospital to screen for AFP levels, all mothers are tested unless they specifically request not to be included in the programme. When the test is not routine, parents can request that it is carried out.
The AFP blood test carries no risk to either mother or baby. The results are normally available within a few days to a week.
Ultrasound scanning uses sound waves to form a picture of the baby in the uterus. It is a completely painless procedure that can be carried out at any stage of pregnancy. For early scans (up to about 14 weeks), the mother will usually be requested to have a full bladder at the time of the procedure to help give a clearer image.
Occasionally, in very early pregnancy, ie 6-9 weeks of pregnancy, it may be necessary to carry out a transvaginal scan of the baby. A specially shaped transducer is inserted into the vagina and very good pictures of the baby can be obtained. The scan is not painful and is becoming standard practice in many hospitals.
Ultrasound is often used between the 12th and 16th week of pregnancy to confirm the age of the baby and to determine the presence of twins. however, for accurate results, ultrasound scans to detect serious abnormalities need to be performed at approximately 18-20 weeks.
Most centres offer a scan at this time to check that the baby is developing as expected. This ultrasound should pick up most cases of neural tube defects (such as spina bifida) and hydrocephalus.
An ultrasound scan is a non-invasive technique and there is no known risk to the health of either mother or baby.
Most centres use ultrasound to detect spina bifida and it would generally be the exception to use amniocentesis for this reason. The main use of this test would be to detect chromosonal disorders (eg Down's Syndrome).
Amniocentesis is usually carried out between the 16th and 20th weeks of pregnancy. Ultrasound examination is often used at the same time to ascertain the position of the baby and the placenta.
A local anaesthetic is usually given to numb the abdominal area. A fine needle is then inserted into the mother's abdomen and a small amount of the amniotic fluid surrounding the baby is withdrawn. The fluid is then sent to the laboratory for testing.
Amniocentesis does not usually require admission to hospital. Some women may experience soreness or a tightening feeling in the abdomen for a short time after the procedure. Mothers are recommended to rest for a while before leaving the hospital and then to take things easy for a couple of days, particularly avoiding any heavy lifting or strenuous exercise during this time.
If amniocentesis is performed, the results for detection of neural tube defects and other structural abnormalities are normally available within a week. Results for other conditions (for which amniocentesis is more likely to be used) will take longer. For example in: Down's Syndrome; Turner's Syndrome; inherited metabolic disorders (eg Tay-Sachs disease); and certain sex-linked conditions, results of screening will take about three or four weeks as these tests involve growing the cells from the fluid, in the laboratory. The sex of the baby can also be determined after amniocentesis.
Unfortunately, there is a slight chance that a miscarriage may occur after amniocenetesis. This risk is now less that one in 100 pregnancies. It is for this reason that the test is not performed routinely.
Chrionic Villus Sampling is a technique used to diagnose conditions which could otherwise not be detectable until about 16 weeks of pregnancy by amniocentesis. CVS cannot detect the presence of spina bifida or other neural tube defects.
Like amniocentesis, this test is usually only carried out if a woman is considered to have a greater risk of carrying a baby with an abnormality and is carried out by specialist centres. The CVS test is usually carried out between the 10th and 14th weeks of pregnancy (or possibly at 18-20 weeks, following the results of a blood test or detection of an abnormality on a scan).
An ultrasound scan is used to show the position of the developing placenta (part of which is the chorionic tissue) in the uterus. A tiny fragment from the edge of the chorionic tissue is removed for tests.
Most women do not find this test painful, although it can be a little uncomfortable. A general anaesthetic is not necessary. Mothers are recommended to rest for a while before leaving the hospital and then to take things easy for a couple of days, particularly avoiding any heavy lifting or strenuous exercise during this time. Results are available fairly rapidly - from 2-12 days, depending on what the test is screening for.
One of the greatest advantages of the CVS test is the ability to diagnose some congenital abnormalities early in pregnancy. Results are available much earlier than with amniocentesis.
An important aspect is the miscarriage rate after this test. The natural miscarriage rate is higher in early pregnancy, around the time the early CVS test is performed. It is therefore more difficult to know exactly what the miscarriage rate is for the CVS test, although it is felt to be slightly higher than for amniocentesis. However, if CVS is performed around the 18 week stage, then the risk of miscarriage is probably in the same order as that for amniocentesis. When considering CVS, it is important that this aspect of the test is discussed with the doctor and/or genetic counsellor before pregnancy begins.
You can obtain further detailed information and advice from your midwife, obstetrician or general practitioner.
You may feel that there is insufficient time to discuss these matters during an ante-natal clinic appointment. If so, talk to your midwife and arrange a further appointment. It is better to have a clear understanding of what a particular test involves before it is carried out. In most cases, these tests are quite routine to hospital staff but a new experience for expectant parents. As a result, you may feel unsure when or what to ask for, for fear of wasting staff time. Doctors and midwives will explain things in greater detail but do not always realise what areas cause uncertainty. So it is up to you to ask.
As already stated, not all hospitals routinely provide all the screening tests available. If they are not available locally, you can be referred to another centre where they are performed. Ask if you want to know about the test your hospital can provide.
