Screening in Pregnancy

July 26, 2011

These pages are designed to help parents to be. They cover the most common tests available to parents and include those which appear `routine' in every pregnancy. Parents are reminded that they should make their own mind up about which tests they need if any.

 

These pages are designed to help parents to be. They cover the most common tests available to parents and include those which appear `routine' in every pregnancy. Parents are reminded that they should make their own mind up about which tests they need if any.

SCREENING FOR ABNORMALITY



Although the vast majority of babies are normal when born, all pregnant women worry about abnormality in their baby. Abnormalities such as Down syndrome become more likely the older the mother is, but there is a small risk at any age.

Not all abnormalities are detectable before birth, but many more are becoming detectable with combinations of tests such as ultrasound, blood serum screening and amniocentesis. Equally the tests are not 100% accurate and your doctor will discuss the accuracy of each test with you.

SCREENING TESTS

Screening tests do not give a definite yes or no answer. They assess your risk of a problem in each pregnancy by combining information about you and your pregnancy to decide if you are high or low risk for an abnormality. An analogy is when an insurance company uses information about you and your car to decide if you are low or high risk as a driver. Even when you are very high risk it doesn't mean you will actually have an accident!

 

Fetal viability scan (6-10 weeks)

This ultrasound scan is used to accurately date a pregnancy, check the number of babies, their heartbeats and check that the pregnancy is progressing normally.

Nuchal translucency (NT) scan (10-13 weeks)

Nuchal translucency is the name given to the appearance of a fluid space at the back of a baby's neck. Studies all over the world have shown that it can be seen in almost all babies. If there is more than the normal amount, the risk of abnormality is increased. By combining the mother's age and the size of the space, the chance that the baby has Down syndrome or other chromosome abnormality can be estimated. Not all abnormal babies can be found with this test but about 80-85% of Down syndrome babies are detected.

Nuchal translucency can be combined with blood screening in the 'Primark Test'. This is said to detect up to 90% of Down's pregnancies depending on maternal age.

Blood screening (10-19 weeks)

By measuring two hormone levels (alpha fetoprotein = AFP and human chorionic gonadotrophin = HCG) in a small sample of the mother's blood ("serum screening" or "triple test") between 15 and 19 weeks, the risk of Down syndrome can be estimated, taking into account the risk due to the mother's age. With serum screening alone about 50-60% of Down syndrome babies are detected.

Most often this test is taken at 15-19 weeks of pregnancy but it can also be checked earlier at 10-12 weeks using different hormones. The blood results at 10-12 weeks can also be added to a nuchal translucency measurement in the `Primark Test' to give a more accurate risk for Down syndrome. With blood testing alone about 50-60% of Down syndrome babies are detected, but when combined with the nuchal translucency up to 90% will be detected, depending on maternal age.

Some hospitals also comment on the risk of the baby having spina bifida based on a high level of AFP in the mother's blood between 16 and 18 weeks. This test has been replaced by ultrasound examination in the majority of hospitals because it correctly identifies the majority of spina bifida babies directly by looking at the spine and head.

Fetal normality scan (18-22 weeks)

At this stage of pregnancy the baby's anatomy can be looked at in detail. Particular attention is paid to the baby's head, brain, spine, face, heart, limbs and internal organs. Many physical abnormalities are detectable at this stage, some of which will be associated with chromosome abnormalities but others are not. Some physical abnormalities are visible as early as 14 weeks but it is far more dependent on baby's position and mother's size. Most hospitals offer a scan as routine, some as early as 16 weeks, but now mostly nearer 20 weeks.

If anything unusual is identified you will be seen by your hospital doctor and referred onto a specialist in Obstetric scanning / Fetal Medicine for a more detailed scan. This may be available in your own hospital or it may mean a trip to another hospital . The findings and their implications will be fully discussed with you at the end of the scan and you may be offered additional diagnostic tests.