Ante-Natal Screening
Introduction
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.
The AFP Blood Test - Maternal Serum Alpha Feto Protein
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
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.
Amniocentesis
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.
