17 Mar 2011
An early-pregnancy, non-invasive screening for Down syndrome is being offered to all mums-to-be, reducing the need for unnecessary invasive tests which carry the risk of miscarriages
A first trimester screening for Down syndrome is proving to be successful in picking up the risk of abnormality in foetuses. It could spare older mums-to-be unnecessary procedures which could carry the risk of miscarriage.
Doctors applaud the change in medical practice, which picks up more than 90 per cent of Down syndrome pregnancies, while making it unnecessary for a larger pool of women to undergo invasive tests.
This also means the chance of mothers miscarrying healthy babies is greatly reduced.
Although figures are not available, doctors generally agree that fewer pregnant women go through the invasive tests now.
Next Monday marks World Down Syndrome Day, which promotes awareness and understanding of people with Down syndrome.
The date – March 21 – represents the three copies of chromosome 21, which is unique to people with Down syndrome. Such people have 47 chromosomes instead of the usual 46 or 23 pairs. The extra chromosome normally appears with the 21st pair.
In a study between 2005 and 2008, KK Women's and Children's Hospital offered the first trimester screening to all pregnant women to pick up tell-tale signs of Down syndrome at the early stage of their pregnancy. Of these women, 12,585 opted for the screening.
One in every 20 women were deemed high risk. Of those who were "high risk", one in 22 women were later confirmed by invasive tests such as amniocentesis or chronic villus sampling (CVS) to be carrying a Down syndrome foetus.
Through the series of screening and tests, a total of 30 Down syndrome pregnancies were picked up. All the pregnancies were terminated.
In only one case did a mother, considered to be low risk, go on to give birth to a baby with Down syndrome.
Today, the first trimester screening is offered to all pregnant women in public hospitals and an increasing number of private clinics.
Without the initial screening, more than 20 per cent of mothers-to-be aged 35 years and older would have been offered an amniocentesis or CVS. An amniocentesis is usually performed between the 16th and 20th weeks of pregnancy and carries a risk of miscarriage of one in 300. A CVS can be done between the 11th and 15th weeks but has a risk of miscarriage of one in 100, said Dr Chee Jing Jye, medical director of The Obstetrics & Gynaecology Centre, a subsidiary of the Singapore Medical Group.
Dr Chee added that in these procedures, a needle is inserted in the woman's uterus to draw out either amniotic fluid (amniocentesis) or tissue from the placenta (CVS). The fluids contain cells from the baby which are examined for the chromosomal abnormality.
Until the first trimester screening test became available less than a decade ago, a woman over the age of 35 would have routinely been offered a CVS in her first trimester or an amniocentesis in her second trimester.
This is because older mothers are at increased risk of bearing a child with Down syndrome. The frequency rises from one in 1,400 babies for a woman between 20 and 24 years old, to one in 25 for a woman over 45 years of age.
People with Down syndrome have mild to moderate intellectual disability and other health problems. They generally have flat faces with small noses, small ears, protruding tongues and upward slanting eyes.
Worldwide, one in every 700 live births is to a Down syndrome baby.
Offering pregnant women the screening test has become routine in the last five or six years.
Dr Tan Wei Ching, consultant at Singapore General Hospital's (SGH) department of obstetrics and gynaecology, said: "We ask every single pregnant patient if she'd like to proceed with the screening, as opposed to it being offered on an ad hoc basis."
From 2006 till last year, 3,000 women were screened at SGH, of whom 3.6 per cent were found to be at high risk. Some 70 per cent of those chose to proceed with diagnostic tests.
The first trimester screening examines features unique to foetuses with Down syndrome between 11 and 14 weeks of pregnancy, said Dr Chee.
In an ultrasound scan which sends high frequency sound waves through the uterus to monitor the progress of the pregnancy, doctors check the nasal bone and nuchal (skin at the back of the neck) translucency of the foetus to assess the risk of the foetus having Down syndrome.
They look out for the absence of the nasal bone and a thicker-than-normal neck skin, which increases the risk of the foetus having the abnormality.
The mother's blood is also tested for a combination of markers – pregnancy-associated plasma protein A, which tends to be low in Down syndrome pregnancies; and beta human chorionic gonadotropin, which is raised in such pregnancies.
Other factors, such as the mother's age, are keyed into a computer program known as the Fetal Medicine Foundation risk calculation software to assess the risk.
If it is one in 250 or higher, doctors would typically recommend an amniocentesis, said Associate Professor Mahesh Choolani, a senior consultant at the National University Hospital's obstetrics and gynaecology department.
Ultimately, the decision whether to have an amniocentesis lies with the couple, he said.
Ms Tham Yim Leng, a nurse clinician at SGH, said: "A low risk score on the screening doesn't mean you have no risk at all, while a high risk score does not necessarily mean that your baby would be born with Down syndrome."
Mr Moses Lee, executive director of the Down Syndrome Association of Singapore, estimated that 30 babies with the chromosomal abnormality are born each year.
It is not known how often women choose not to proceed with the screening and tests because they do not want to abort due to personal and religious reasons.
Only an amniocentesis or CVS provides a definitive diagnosis of the condition. Both are outpatient procedures and take only a few minutes, with results in about two weeks.
With women putting off marriage and having children later, the occurrence of Down syndrome because of ageing mothers has increased.
The screening test gives them an early indication of where their pregnancy might lead.
Adjunct Professor George Yeo, chief of obstetrics at KKH, said: "All pregnant women should be made aware of the screening and be offered the screening and other tests."
There is a 1 in 25 chance of a woman over the age of 45 of bearing a baby with Down syndrome
First trimester screening test
1 Ultrasound scan:
Doctors check the nasal bone and nuchal (skin at the back of the neck) translucency of the foetus to assess the risk of the foetus having Down syndrome. They look out for the absence of nasal bone and a thicker-than-normal neck skin which increases the risk of the foetus having the abnormality.
2 Blood test:
The mother's blood is tested for pregnancy-associated plasma protein A, which tends to be low in Down syndrome pregnancies; and beta human chorionic gonadotropin, which is raised in such pregnancies.
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Dept of Obstetrics and Gynaecology
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