The vast majority of babies are normal. However all women, whatever their age, have a small risk of delivering a baby with a physical or a mental problem. In some cases this may be due to a chromosomal problem such as Down Syndrome, or a problem with the baby's spine such as spina bifida.
The only way to know for sure whether or not your baby may have a chromosomal problem is by having an invasive test, such as an amniocentesis (amnio). However, this test has a small chance of causing a miscarriage of about 0.1% or less (about 1 chance in 1,000 or less).
This STS test helps you and your partner decide whether the chances that your baby may have a chromosomal problem is high enough to warrant having an invasive test, such as an amnio.
The Second Trimester Screening test is carried out at 14 to 21 weeks of pregnancy.
This test measures the level of four proteins normally found in your blood during pregnancy. These are AFP, hCG, oestriol, and Inhibin-A. Based on the level of these four proteins, your age and the results of a detailed ultrasound scan (the Fetal Anomaly Scan), we can calculate the chances that your baby may have Down syndrome or other chromosomal problem such as Trisomy-18, or spina bifida.
As the STS blood tests are analysed in a special laboratory in the United Kingdom, the results of this test will take up to 10 days before they will be available. This STS test can identify about 80% of cases of Down syndrome, about 95% of cases of Trisomy-18, and about 95% of babies with spina bifida.
It is important to remember that a screening test does not guarantee a normal baby. Therefore you should carefully compare the advantages and disadvantages of a screening test (like STS) compared with a diagnostic test (like amnio).
Only you can decide if you wish to have an invasive diagnostic test, like amnio, and you will always have the right to choose this definitive test if you prefer. It is also recommended that you have a further scan at 20 weeks to check the baby so that we can reassure you about problems that the amnio cannot detect.