WHAT IS SCREENING
Screening categories women into high and low risk. A screening test can never confirm a fetus as having Down syndrome.
It aims to identify women with a higher risk when the diagnostic test needs to be performed.
WHAT IS DOWN SYNDROME
Down syndrome refers to intellectually challenged individuals with unique physical characteristics due to trisomy 21 (presence of extra copy of chromosome no 21 ). It occurs in 1 in every 800 pregnancies.
Risk of having a baby with Down syndrome is related to mother’s age
25 year woman – 1 in 1200
35 year woman – 1 in 350
40 year woman – 1 in 100
WHY SCREEN FOR DOWN SYNDROME
Down syndrome is the commonest cause of intellectual disability among live born babies which can be detected during pregnancy.
30% of down syndrome pregnancy die during early pregnancy .
There is no associated major structural defect detectable on routine prenatal ultra sound.
Individuals have long life often reaching adulthood hence big social and financial burden in caring for such individuals.
HOW TO SCREEN FOR DOWN SYNDROME:-
1st trimester ultrasound
Includes evaluation of gestational age, nuchal translucency ( fluid behind the neck of the baby), presence of nasal bone .
Flow through ductus venoses, fetal heart rate and tricuspid regurgitation (blood through the abdomen and the right side of the heart) are additional features about fetal well being.
1 . Double marker – (1st trimester < 13 weeks ) blood test of mother to find levels of hormones beta hcg and papp-a. Their values are combined with ultrasound parameters and mother age and a probability of down syndrome is calculated.
2. TRIPLE TEST / QUADRUPLE MARKER – (2ND TRIMESTER 15 – 19 WEEKS ) triple screening measures AFP ( alpha fetoprotein), HCG ( human chorionic gonadotropin )and uE3 (estriol) When hormone inhibin A is also added it is called quadruple marker.
The amount of these substances helps your doctor find out the CHANCE that your baby has birth defects like Down syndrome, spin bifida or anencephaly. These tests can not show for sure that your baby has a birth defect. You would need a diagnostic test to confirm it.
FALSE POSITIVE – 5%
The odds of the fetus being affected when screen is positive is between 1 in 30 and 1 in 20 .
In India , Down syndrome screening in not a prioriIty for the state health program and therefore screening is offered mostly in private sector and research institute etc.
Prenatel screening for Down syndrome is still an informed choice and the couple has the right to decide.whether they would like to go through screening or not.
NIPT it is a non invasive test uses mothers blood but it is a very costly test .it is 97 to 99 % accurate. NIPT screens for –
Trisomy 21 (Down Syndrome)
Trisomy 18 (Edwards Syndrome)
Trisomy 13 (Ptau Syndrome)
CVS (Chorionic villus sampling) done between 10 -13 weeks or amniocentesis done between 16 –18 weeks. These genetic test analyzes baby’s own genetic material, collected from the amniotic fluid or placenta.