Guest article by Dr Lakshmi Kiran, who is a fetal medicine expert in Hyderabad. You can contact her at : drlkiran@yahoo.com
Chromosomal abnormalities are one of the major causes for perinatal death and mental handicap in children. The only definitive way to diagnose is by invasive testing (ie, CVS/Amniocentesis). These tests are associated with
a risk of miscarriage of about 1% and therefore these tests are carried out only in pregnancies considered to be at high-risk for chromosomal defects.
So, there are various methods designed to identify the high risk population. These tests are the screening tests which, even though do not give a definitive diagnosis, will give the patient specific risk score, which determines whether the woman is screen positive or screen negative.
Who should be screened?
Every woman has a risk that her fetus/baby has a chromosomal defect. The risk for many of the chromosomal defects increases with maternal age. So, every pregnant woman is entitled to screening.
Pre-screening probability
The risk of Down's syndrome varies with maternal age:
- 1:1,500 at 20 years
- 1:800 at 30 years
- 1:270 at 35 years
- 1:100 at 40 years
- >1:50 at 45 years and over1
The risk also increases after a previously affected pregnancy:
- With regular trisomy 21, the recurrence risk is 0.75% more than the maternal age related risk.
- Following trisomy due to a translocation, the recurrence risk is dependent on the type of translocation and which partner carries the translocation.
The challenge of an antenatal screening programme is to identify women in whom a risk of Down's syndrome is sufficiently high to justify such an invasive test and to minimise the risk of miscarrying a healthy baby.
So, various methods are designed to increase the detection rate with as less false positive rate as possible.
Screening Tests :
There are two methods of screening : Serum screening and ultrasound screening. The methods also vary based on the age of pregnancy.
First trimester: The serum screen measures free beta-hCG (human chorionic gonadotrophin) and Pregnancy-associated plasma protein A (PAPP-A) between 10 to 13+6 weeks.
The NT (Nuchal translucency) scan between 11 – 13+6weeks.
In this scan, on ultrasound, mainly the thickness of the nape of fetal neck, presence of nasal bone are assessed.Various other parameters like the fetal heart rate, blood flow across tricuspid valve and ductus venosus are also checked.
Both together is called the “combined screening” ,which has 90% detection rate.
Second Trimester: Quadruple test: If a woman books later in pregnancy (when NT scan is not possible) the quadruple test can be taken between 15 to 20 + 0 weeks of gestation. This measures free beta-hCG, alpha fetoprotein (AFP), inhibin-A and unconjugated estriol (uE3) . It is less accurate than the combined test.
Second Trimester Ultrasound(Anomaly scan +Genetic Sonogram):
In this scan we not only look for major structural defects in the baby, which is mandatory in every pregnancy, but also look for certain soft markers which suggest the increased likelihood of chromosomal abnormalities.
The first and second trimester tests could all be done and interpreted together and that is called “Integrated test”
The risk assessment should be done individually for every woman after taking into consideration, the various factors like maternal weight, ethnic group, assisted conception, whether previous pregnancy was affected, bleeding in pregnancy, Insulin dependent diabetes, smoking, gestational age of the pregnancy, singleton or multiple pregnancy. The patient specific risk is arrived at by considering all above parameters and the screening tests.
- The integrated test offers the most effective and safe method of screening for women who attend in the first trimester.
- The quadruple test is the best test for women who first present in the second trimester.
The parents are counselled in detail about this in detail based on which they can make a decision whether they would want to opt for a diagnostic test or not.
Women found to be carrying a baby with Down's syndrome will be offered expert counselling and support, they may be offered a termination of pregnancy or they may choose to continue with the affected pregnancy with support.
Screening for Down's syndrome in multiple pregnancy:
Around 2% of pregnancies affected by Down's syndrome are twins. The screening is mainly by NT scan. Combined screening may be beneficial. The risk assessment and interpretation and management depends on whether the twins are dichorionic or monochorionic.
For further information, i suggest visiting the websites:
www.fetalmedicine.com and www.patient.co.uk.
