Pre and Post-test Counselling
- Counselling of patients for first trimester screening is usually performed by trained general practitioners, radiologists, obstetricians or clinical geneticists.
- The objective of counselling is to inform patients about the benefits, risks and implications of screening to allow them to consent to the tests, understand the results and be suitably knowledgeable to make further decisions based on the results.
- First trimester screening is a three fold process involving taking into account the patient's age and past obstetric history, measurement serum b-HCG and PAPP-A levels at 9 -14 weeks gestation as well as a fetal ultrasound scan at 11 - 14 weeks.
- The primary aim of screening is to identify women at increased risk of having babies with Trisomy 21 (Down's Syndrome), Trisomy 13 (Patau's Syndrome) and Trisomy 18 (Edward's Syndrome) and who may therefore benefit from further testing. However, the process of scanning the fetus may, in some circumstances, reveal other morphological abnormalities such as a cystic hygroma (suggestive of Turner's Syndrome), anencephaly, anterior abdominal wall and limb defects and other abnormalities.
- As a screening test, first trimester screening has a high sensitivity of approximately 90% for the detection of aneuploidies and a false positive rate of 5%.14
- Calculation of the adjusted risk considers the results of the serum bHCG and PAPP-A, the nuchal translucency thickness from the ultrasound scan as well as several sociodemographic factors of the mother, most importantly her age.
- Currently, an adjusted risk of greater than 1 in 300 is considered high risk and these women are further counselled on additional testing with chorionic villus sampling (CVS) or amniocentesis. Whilst CVS can be performed earlier, the risk of a miscarriage is almost twice as high as for amniocentesis.
- Assessment of neural tube defects (eg, spina bifida) is usually performed during the 19 week fetal morphology ultrasound scan and this cannot be easily detected in the first trimester screen.
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