Screening is defined as the process of identifying apparently healthy people who may be at an increased risk of a disease or condition.
Prevalence, sensitivity & specificity
Prevalence is the number of individuals in a target population who have the condition.
Sensitivity is the measure of a screens ability to refer all those people who do have the target condition
Specifcity is the measure of a screens ability to not refer those people who do not have the target condition.
At the booking appointment the midwife should discuss why booking bloods are offered to all pregnant women and what we are screening for.
Rhesus Blood Group
In order for full informed consent to be gained, the woman must know and understand all the information about the screening.
Screening for Downs Syndrome
Before the booking appointment, women should have been given the appropriate information regarding screening for Downs Syndrome. Again this should be discussed with them to ensure they understand what the screening test is looking for, what it involves & when and how they will receive the results.
If they give their full informed consent and whether they are in the correct time frame, they will be offered the combined screening.
This is done between 11+2 and 14+1 weeks gestation.
involves combining the results of a nuchal translucency scan (check for fluid on the back of baby's neck through USS) with a blood test.
This then gives the woman a risk factor of having a baby with Downs Syndrome
This is a blood test taken between 14+2 -20 weeks gestation for women who have presented for antenatal care late and missed the opportunity for the combined screening.
This blood test measures the levels of:
Maternal Serum Alpha-Feto Protein levels (AFP)
Human Chorionic Gonadotrophin levels (hCG)
unconjugated oestriol (uE3)
These markers are all affected by maternal age, smoking status, ethnicity and family history and should be noted on the blood form.
Low Risk - a risk of less than 1 in 150 e.g. 1 in 220
High Risk - a risk equal to or greater than 1 in 150 e.g. 1 in 100
women with a high risk factor will be offered counselling and support as well as further diagnostic testing by either an Amniocentesis or Chorionic Villus Sampling.
The Diagnostic testing for Downs Syndrome involves undergoing an invasive procedure that extracts cells from either the amniotic fluid or the placenta.
Amniocentesis - This involves a fine needle being inserted through the mothers abdomen in order to extract some amniotic fluid surrounding the baby.
Chorionic Villus Sampling - This involves a fine needle also being inserted through the mothers abdomen to extract some of the chorionic villi of the placenta.
Both of the extracted matter gives an exact DNA match to that of the baby, allowing for examination for the extra chromosome 21.
Fetal Anomaly Scan
The Fetal Anomaly Scan is performed by Ultrasound at 18 - 20+4 weeks gestation.
It is performed so that by a qualified sonographer who looks in detail at the fetus from head to toe to check its growth and development.
The scan can also identify any serious abnormalities or defects that could increase the risk of mortality and morbidity.
Remember that all screening is OPTIONAL
Women should know all the information regarding her screening choices in order to make a full informed decision.