Blood Tests and Amniocentesis

During pregnancy you'll be offered a number of antenatal tests, to check the growth and development of your baby. Throughout pregnancy you'll be offered blood tests and when you're between 15 and 20 weeks pregnant, you'll be offered amniocentesis.

Blood Tests

Blood tests are a type of screening test and you'll be offered a variety of them throughout the various stages of your pregnancy, as they help ascertain the health of your baby. In addition, your blood can offer insight into your health too. For example, it will be tested to see if you're rhesus positive (you have rhesus factor), rubella, hepatitis B and sometimes HIV.

The alphafeto protein (AFP) test is the best way of screening for spina bifida. If the result shows the levels of AFP in your blood are high, then it may be a sign that your baby has spina bifida or anencephaly.

To screen for Down's syndrome, blood tests will measure the amount of AFP and other markers in the blood. The exact test used depends on the individual hospital, but most are double, triple or quadruple tests, which means they're looking for two, three or four markers within the blood.

The blood tests are more accurate if you know exactly how many weeks pregnant you are and will give you an idea of what your risk of having a baby with certain conditions is. They're available for most women, except those who are expecting twins or multiples. In the case of twins or multiples, the nuchal translucency test, which is performed early in pregnancy and amniocentesis, are the best options.

When it comes to the results, if your result comes back as screen negative, it means you have a less than one in 250 chance of having a baby with Down's syndrome. This doesn't conclusively mean that your baby definitely doesn't have Down's syndrome, but that it's very unlikely he does.

A screen positive result means that you have over a one in 250 risk of your baby being born with Down's syndrome. Again, the test can't conclusively say that your baby will have the condition, just that the chances are higher. However, the likelihood of your baby having Down's is still low, but a test such as amniocentesis - which is a diagnostic test, rather than a screening test - will be able to give you a more exact answer.

Amniocentesis

Amniocentesis is a type of diagnostic test, which means that it tells you for certain if your baby has a specific condition. It's usually carried out when you're between 15 and 20 weeks pregnant and involves having an ultrasound scan and a sample of amniotic fluid taken from your womb. The test is offered to all pregnant women, but it's particularly worthwhile if:

• You've had a previous test or screening result which has suggested a high risk of a birth defect or Down's syndrome.
• You're over 35 years old, as the risk of genetic abnormalities increases at this stage.
• There's a history of abnormalities or genetic abnormalities in your family.

Amniocentesis can help identify certain conditions, such as chromosome conditions, like Down's syndrome or Edward's syndrome; musculoskeletal disorders, such as Duchenne muscular dystrophy, cystic fibrosis or spina bifida; metabolic disorders and blood disorders, such as sickle cell anaemia, thalassaemia or haemophilia.

Before the test you'll be warned that there is a tiny risk - less than 1% - of miscarriage following the test. Due to this, it's advisable to rest for a few days following the procedure.

To take a sample of amniotic fluid, a thin needle is passed through the wall of your abdomen and womb and into the amniotic fluid, which is what your baby is floating in. Some hospitals may offer a local anaesthetic, to numb you as the needle goes in. A sample of the fluid is taken out using a syringe. The fluid contains cells and other substances shed from your growing baby and is a vital source of DNA and other information.

After the test, the sample is sent away for laboratory testing and you should receive the results within one to three weeks.