The series of tests that you are given throughout your pregnancy helps to ascertain whether you or your baby have any problems. Some of these tests are given routinely to every pregnant woman. Others may only be offered to women whose babies are considered to be in 'at-risk groups'of having or developing a particular condition that might require special care or intervention.
Several factors are taken into account when determining whether you fall into an at-risk group:
- Medical history, for example, previous pregnancies.
- Family history, for example, if there are inherited conditions such as cystic fibrosis among members of your family or your partner's family.
- Ethnic group (and that of your partner) and where your family comes from. (Higher rates of thalassaemia are found in people of Mediterranean origin; sickle-ceil anaemia is mainly found among people of African descent, and cystic fibrosis occurs in white people of western European origin.)
The purpose of screening tests
Screening tests are designed to ascertain the level of risk and include blood tests, urine tests and ultrasound scans. If a screening test indicates an increased risk for a condition, it will usually be followed by a diagnostic test, which will determine whether or not the condition is present.
Some hospitals offer certain screening tests routinely to all women, while others offer them only to women in at-risk groups. For example, some hospitals offer all pregnant women blood tests to check for the possibility of Down's syndrome, but others offer them only to older women.
At different stages during pregnancy, you will be given ultrasound scans to check on your baby's development.
This may form part of the routine ultrasound scan at 11 -14 weeks. The fold of skin at the back of your baby's neck (the nuchal fold) is checked to see whether excess fluid is present, which is a possible sign of Down's syndrome.
Given at 18-20 weeks, this scan provides a check of the baby's growth and development and an opportunity to check for signs of spina bifida, changes that might be associated with Down's syndrome, and conditions that might require antenatal monitoring and neonatal care.
As well as the blood test given to all women at their first appointment, you may be offered blood tests (also referred to as serum screening) to check for specific conditions if you are at risk.
Serum screening at around 10 weeks
If you are at risk of having a baby with Down's syndrome, you will have a blood test to establish the levels of the fetal hormones plasma protein A and human chorionic gonadotrophin (HCG) in your blood.
Serum screening at around 16 weeks
You will be given further tests to check the levels of fetal hormones in your blood at about 16 weeks. Depending on the hospital, this will be an AFP test (to check for alphafetoprotein, or AFP), a triple test (which also checks for HCG and oestriol), or a triple-plus test, or quadruple test (which checks for inhibin A, too). Abnormal levels of these hormones may be an indication of spina bifida or Down's syndrome.
Your blood can also be tested for inherited conditions, for example, sickle-cell anaemia, haemophilia, thalassaemia and cystic fibrosis, according to your ethnic, family or geographical background. For conditions such as sickle-cell, if you test positive, your partner will also be screened. It is important to remember that these tests only indicate the possibility of a baby inheriting a condition. If the results are positive, you will be offered diagnostic tests in order to investigate further.
To test or not to test?
There are a number of things to consider when deciding whether or not to have any screening test. It can be very reassuring to find that the results indicate a low risk, but what if they indicate a high risk? Even if you are sure you will love your baby whatever, and would never consider a termination, you may wish to consider screening. A low-risk assessment will be reassuring, while a higher-risk assessment will give you the opportunity to decide whether to have follow-up diagnostic tests, so you can be fully prepared for your baby's arrival.
Many women find the stress of screening very disturbing and they prefer not to have it done. Your midwife and doctor have your best interests at heart and should talk through the options of further tests with you. However, the decision rests with you. At this stage, it may be wise to start thinking about what you will do if the result is positive.
False positive or false negative?
You should remember that, although the accuracy of detection rates has improved, no screening method is perfect. For example, raised levels of alphafetoprotein (AFP) in your blood may have other causes, such as a twin pregnancy.
On average, only 1 in 20 women is assessed as being at high risk of carrying a baby with Down's syndrome and offered diagnostic testing. However, of these assessments, only 1 in 60 will be correct. Similarly, screening misses about 30 per cent of all cases of Down's syndrome.