About the test

There are two screening options available for you if you choose to have antenatal screening for Down syndrome and other conditions.

The two options now available include first trimester combined screening and second trimester maternal serum screening. The screening tests are able to pick up most cases where there is an increased chance that you are carrying a baby with Down syndrome, Turner syndrome, Trisomy 13 and Trisomy 18.  Occasionally it can pick up that you are potentially at risk of a pregnancy-related health condition.

First trimester combined screening

  • This screening is available if you are less than 14 weeks pregnant.
  • It includes both a blood test and a scan.
  • Results from both the blood test and the scan are combined with other information like your age, weight and baby's gestational age (how many weeks pregnant you are) to find a screening result.
  • Screening results may show that there is either a low or an increased chance that you are carrying a baby with one or some of the conditions which can be found during screening. The actual result you will receive will say either, 'increased risk' or 'low risk'.
  • Most women who receive a result which shows that there is an increased chance that they are carrying a baby with a condition will actually not be carrying a baby with a condition. An increased risk screening result simply puts people into a group where the chances are higher for them than other women having screening.

If your screening test shows that there is an increased chance that your baby has a condition, you will be offered further testing to see if baby actually has that condition or not. This further testing is called diagnostic testing.

There are two parts to this screen. You need to have both a blood test and a scan before a result can be found.

1. The blood test - first trimester maternal serum screening

  • Is best taken at 10 weeks but can be taken between 9 weeks and 13 weeks and 6 days.
  • Tests for the levels of two analytes (or chemicals) in your blood.
  • The analytes include:
    • β-HCG (Beta Human Chorionic Gonadotrophin)
    • PAPP-A (Pregnancy Associated Plasma Protein A)

2. The scan - Nuchal translucency scan

  • Is best taken at around 12 weeks of pregnancy but can be taken between 11 weeks and 2 days, and 13 weeks and 6 days.
  • This is an ultrasound scan which is specifically designed to screen for conditions in the baby.
  • It measures the fluid-filled space at the base of the baby's neck.  This measurement tends to be larger when babies have some conditions.

Second trimester maternal serum screening

  • This screening is available if you are between 14 and 20 weeks pregnant.
  • It is a blood test only.
  • Results from the blood test are combined with your measurements like your age, weight and baby's gestational age (how many weeks pregnant you are) to find a screening result.
  • Screening results may show that there is either a low or an increased chance that you are carrying a baby with one or some of the conditions which can be found during screening.  The actual result you will receive will say either, 'increased risk' or 'low risk'.
  • Most women who receive a result which shows that there is an increased chance that they are carrying a baby with a condition will actually not be carrying a baby with a condition.  An increased risk screening result simply puts people into a group where the chances are higher for them than other women having screening.
  • If your screening test shows that there is an increased chance that your baby has a condition, you will be offered further testing to see if baby actually has that condition or not.  This further testing is called diagnostic testing.

There is only one part to this screen.

The blood test - second trimester maternal serum screening

  • Is best taken between 14 and 18 weeks but can be taken up until 20 weeks.
  • Tests for the levels of four analytes (chemicals) in your blood.
  • The analytes include:
    • β-HCG (Beta Human Chorionic Gonadotrophin)
    • µE3(Unconjugated oestriol)
    • AFP (Alpha fetoprotein)
    • Inhibin A.

Diagnostic testing

If screening shows that there is an increased risk for a genetic condition (like Down syndrome, Trisomy 13, Trisomy 18 or Turners syndrome), amniocentesis or chorionic villus sampling (CVS) are the options available for diagnostic testing.  Amniocentesis is usually the test you will be offered.

Some other conditions require diagnostic ultrasound rather than genetic testing. Like spina bifida, for example, although you may be offered both of these diagnostic tests; that is, both a diagnostic ultrasound scan and an amniocentesis, in some cases.

For more information about diagnostic testing follow some of the links in our useful links page.

Page last updated: 24 November 2014