Screening for Down syndrome is available to about 53.5% of mothers on a maternal age basis, and the remaining 46.5% of health boards provide serum screening for all ages.
There are several methods used in prenatal screening, these are usually used separately, and a number of factors are taken into account to determine which method should be used. Amniocentesis has been around for 20 years and is probably the most well known screening method. It involves testing a sample of the amniotic fluid surrounding the foetus, ultrasound is used to guide a needle through the abdomen, into the womb and a small amount of amniotic fluid (20ml) is removed. The procedure is usually carried out at 14-16 weeks. Amniocentesis tests for chromosome disorders, and is 99.8% reliable for chromosome number, there is however a risk of miscarriage (usually 1/250 or less) after the procedure. This is one of the reasons why amniocentesis has only been offered to over 35’s (since they have a much higher risk of having a Down syndrome child)(Webb 1990).
Previous studies on amniocentesis concentrated on problems that might arise during pregnancy or immediately after, these studies found that children whose mothers had amniocentesis are more likely to have breathing problems in the first few days after birth. A study performed by Jo-Anne Finegan in Toronto followed 88 women who had, had amniocentesis, there was an increased incidence of ear infection in this group. Finegan tested the stiffness of the eardrum and found children in the amniocentesis group were more than three times as likely to have abnormal readings. It is thought that there is a disruption of the delicate balance of pressure across the eardrum when the amniotic fluid is removed, which could cause the problems(Webb 1990).
Chorionic villus sampling is another form of sampling, it involves taking a small piece of placenta and genetic testing is carried out on it, there is a slightly higher chance of foetal loss with this procedure (Dick 1996).
A more recent form of prenatal testing involves serum markers. Blood is taken from the pregnant women and the maternal blood is tested for three hormones, this test is called the ‘triple screen’ test. The three hormones tested are alpha foeto-protein (AFP), human chorionic gonadotrophin (HCG) and oestradiol (E3). AFP is based on the fact that Down syndrome foetuses tend to be smaller on average, have smaller placentas and thus secrete less AFP. All three hormones can be tested individually, but are not so reliable this way. AFP results detect 20%-25% if carried out alone (Cuckle 1984), tests combining more than one measure detected 48%-91%. A study using AFP and HCG detected 90% of cases in women over 35 and 43% in women under 30, it also found that detection rates were better when the test was performed before week 17 (Gouldie et al 1995).
Prenatal sonography looks at the foetus using ultrasound, by measuring the iliac angle in the pelvis the risk of Down syndrome can be measured. A study performed shows that the mean iliac angle is 60o for normal foetuses and 75o in foetuses with Down syndrome, by measuring the iliac angle in foetuses the liklihood of Down syndrome can be worked out, for example if the iliac angle is 50o the liklihood is 1/588 of the foetus having trisomy 21, if however the angle is 80o then the chance is 1/50. This type of prenatal testing although not as reliable as amniocentesis is much safer.
Saridogan et al 1996 pointed out a number of reasons why Down syndrome may not be detected, first of all women may decline the test, this may be due to ignorance of the test or to cultural/religous reasons. Another reason may be due to the late presentation of the woman, as stated above testing before week 17 gives the best results.
The triple test is not 100% reliable, there are incidences when there is a negative test, and the child is born with Down syndrome the reason for this is not always known.
Prenatal Screening Procedures
In an uncomplicated pregnancy, expect about a dozen doctor visits
First VisitBlood tests: To check the woman’s blood group and sometimes, to check for presence of hepatitis B virus, which might be transmitted to the baby.
Cervical smear test: To test for an early cancer of the cervix (if a test has not been performed recently). Also called a Pap smear.
First Visit and Throughout the PregnancyBlood tests: To check for anemia in the woman, and in women with Rh-negative blood groups, to look for the presence of Rhesus antibodies.
Urine test: To check for proteinuria, which could indicate a urinary tract infection or preeclampsia.
Blood and urine test: To check for diabetes mellitus.
Blood pressure check: To screen for hypertension, which interferes with blood supply to the placenta and is a sign of preeclampsia.
First Visit and After ANY InfectionBlood tests: To screen for rubella, which can cause defects in the baby, and for syphilis and HIV (the AIDS virus) which can also be passed on.
First 12 WeeksChorionic villus sampling: May be performed if there is a risk of certain genetic (inherited) disorders being passed on.
16 to 18 WeeksUltrasound scanning: Is carried out to date the pregnancy accurately and to detect any abnormalities present in the fetus.
Amniocentesis: Carried out on older women and those with spina bifida or Down’s syndrome to detect possible abnormalities in the fetus.
Blood test: In some cases, the amount of alpha-fetoprotein in the blood is tested to determine whether the baby has spina bifida.
Fetoscopy and fetal blood sampling: In some cases, these are carried out if there is doubt about the normality of the baby.
High-risk or overdue pregnancies Blood and urine tests: To assess placental function and fetus health.
Electronic fetal monitoring: To check on the fetal heart beat.
Ultrasound scanning: Extra scans may be recommended to assess fetal growth and development, location of placenta, amount of amniotic fluid.
SOURCE: Encyclopedia of Medicine, American Medical Association, Random House, 1989.