Maternal screening

The results from all of the biochemical tests and ultrasound in first trimester screening are used together to estimate the possibility of a Down syndrome affected baby. The triple test is used as a primary screening test (without ultrasound) to assess the chance of the pregnancy being affected by Down syndrome. A raised level of AFP by itself might indicate the possibility of a neural tube defect.

Interpretation of the screening test results is as follows:

First trimester screening

• First trimester values of PAPP-A tend to be lower in Down syndrome affected pregnancies when measured between 10 weeks and 13 weeks 6 days. The difference in PAPP-A levels between Down syndrome and normal pregnancies is most pronounced around 9 weeks.
• Values of free beta-hCG tend to be higher in Down syndrome affected pregnancies.
• Nuchal translucency is increased in pregnancies affected by Down syndrome.

Second trimester screening

• The maternal AFP level tends to be lower than usual in pregnancies affected by Down syndrome. A baby with an open neural tube defect has an opening in the spine, head or abdominal wall that allows higher than usual amounts of AFP to pass into the mother’s blood. In this case maternal AFP levels will be higher than usual.
• The maternal hCG level tends to be higher than usual in pregnancies affected by Down syndrome.
• In pregnancies affected by Down syndrome the oestriol level tends to be lower than usual.
• The second trimester screen can be performed from 14 weeks for the diagnosis of Down syndrome, but the test can only be used in the diagnosis of spina bifida after 15 weeks gestation, so generally it is delayed until this time.

Calculation of the likelihood of a Down syndrome affected pregnancy

A detailed mathematical calculation is performed, using the results of the various biochemical screening tests, together with details of mother’s age and weight, to calculate the chance of the baby being affected by Down syndrome (trisomy 21), or another chromosomal abnormality such as Edward’s syndrome (trisomy 18). Other test results may be included in this calculation such as the nuchal translucency measurement from an ultrasound scan. Additional information about family origin and smoking history may be included in the calculation to improve its efficiency. If the chance is greater than a certain cut-off value, further tests may be offered.

The screening tests may be performed as early as 9 or 10 weeks of pregnancy in some centres or around 15 weeks in others. Screening can be performed up until about 22 weeks of pregnancy.

The interpretation of the screening test result should be provided by the midwife, a genetic counsellor or doctor. They will be able to explain the meaning of the result and offer the choices available to the mother. If the result appears to be in the ‘higher risk’ category, more definitive tests are needed to confirm a diagnosis. For possible chromosome abnormalities, an amniocentesis or chorionic villus sample will usually be needed. Other disorders may be detectable using a detailed ultrasound scan.

It is important to remember that the tests described here are screening tests and not all Down syndrome affected pregnancies will give results in the ‘higher risk’ category. The screening results are a guide to those women who might benefit from further more invasive tests. Only a small proportion of women who are in the ‘higher risk’ category for Down syndrome, or have a high AFP result, will have an affected baby.

The screening results are very dependant on an accurate age for the baby and this is why a dating ultrasound scan is recommended prior to screening. If the gestational age of the baby is not correct, the screening result may be falsely high or low. An early ultrasound scan will show the possibility of twins or triplets and these will result in high maternal AFP levels.

More definitive tests are needed to diagnose a chromosome abnormality or a neural tube defect. To diagnose Down syndrome, an amniocentesis or chorionic villus sampling procedure must be performed. A small amount of fluid surrounding the baby, or a small piece of the developing placenta, is taken using a very fine needle. It is then possible to use laboratory techniques on one of these samples to look at the chromosomes of the baby and detect possible problems. There is a small risk of infection or miscarriage associated with these invasive procedures and therefore careful consideration should be given before having them performed.

The likelihood of carrying a baby with Down syndrome increases with mother’s age. Over the age of 35, about 40% of women will be in the ‘higher risk’ group after screening, just because their age-related risk is so high. However, it is important to stress that most women in the ‘higher risk’ group will NOT have an affected baby.