The Second Trimester Screening test is carried out at 14 to 21 weeks of pregnancy.

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 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).

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).