Posted on Tuesday 10th May 2011
An antenatal ultrasound scan
Pregnant women in north Cumbria are now being offered improved antenatal screening for Down’s syndrome.
The new test, known as the Combined Test, offers a better detection rate and therefore reduces the number of women subsequently offered unnecessary invasive diagnostic procedures which carry the risk of miscarriage.
As well as being more reliable, the Combined Test is done earlier in pregnancy, giving parents more time to decide how to proceed once they have their test results. The two-part test is carried out at around 12 weeks of pregnancy, when the expectant mother goes to the hospital for her early pregnancy (‘dating’) scan.
Screening for Down’s syndrome is optional. If a woman accepts screening, the fluid under the skin at the nape of the baby’s neck - known as Nuchal Translucency (NT) - is measured using ultrasound, usually during her early pregnancy scan. Following the scan a blood sample is taken from the mother, to measure the amount of two hormones from the placenta, found naturally in the mother’s blood in early pregnancy. The results of both parts of the test, together with other factors, are combined to estimate the chance of the unborn baby having Down’s syndrome.
Locally, the Combined Test has a detection rate for Down’s syndrome of 85% to 90% and a ‘screen positive’ rate of between 2% and 2.5%. The screen positive rate is the number of women identified as being at ‘higher risk’ (or chance) of their baby being affected by Down’s syndrome; this group of women will be offered further counselling and diagnostic testing. Women in the ‘higher risk’ group are offered Chorionic Villus Sampling (CVS) or Amniocentesis – both diagnostic tests, which may lead to miscarriage in around 1% of women undergoing the procedures.
The alternative screening test for Down’s syndrome, later in pregnancy, is the Quadruple (or Quad) Test; a blood test that measures the levels of four hormones in the mother’s blood. This test is most accurate at 16-17 weeks of pregnancy and is offered to women who access maternity care later in pregnancy or for whom it is not possible to complete the Combined Test in early pregnancy.
Compared to the Combined Test, the Quad Test has a lower detection rate of 79% to 82% and a significantly higher screen positive rate of around 3.5% ¬- meaning more women are identified in the ‘higher risk’ group and offered diagnostic tests, with the potential risks to their unborn child.
Nationally, the move from the Quadruple Test to the new Combined Test means there will be potentially 23,000 fewer subsequent diagnostic tests carried out each year, cutting the number of related miscarriages by around 230.
Nuchal Translucency measurements above the expected level in early pregnancy may be associated with a higher likelihood of the unborn baby having Down’s syndrome. However, an increased NT may also be linked with other health problems (physical or genetic) in babies.
Georgina Quigley, Antenatal Screening Co-ordinator for North Cumbria University Hospitals NHS Trust, said: “We are constantly working to improve our screening programmes, and also to provide women with information about the screening tests available to help them to make an informed choice. The Maternity Department is pleased to be able to offer combined screening at both West Cumberland Hospital and the Cumberland Infirmary.
“The Combined Test is the nationally recommended method of screening for Down’s syndrome in pregnancy because women benefit from a more accurate screening test available earlier in pregnancy; and an earlier diagnostic test, should that be required.
“The improved accuracy means 40-50% fewer women each year will need to be offered diagnostic tests, therefore fewer pregnancies will be lost as a result. This really is a significant improvement in the screening programme.”
Pregnant women are encouraged to access maternity care early in pregnancy to ensure the best possible antenatal care for them and choice regarding screening in pregnancy. Women have direct access to a midwife and are advised to contact maternity care services as soon as their pregnancy is confirmed.
For further information visit http://fetalanomaly.screening.nhs.uk/.