You have a very low chance of your baby being deformed and all parents expect a ‘normal’ baby. Usually about 1-2 babies out of 100 babies born have some fetal abnormality. It is well recognized that some women are at increased risk of fetal abnormalities ( Ex- Above 35 years ). However over 90 % of these abnormalities occur in ‘low risk’ women as high risk women are a minority of pregnant women.
What are the types of abnormalities?
Basically there are two types of abnormalities. 1. Chromosomal or genetic abnormalities – These are the abnormalities of genes or chromosomes of the baby resulting in various types of defects which may or may not be compatible with life.( Ex- Down’s Syndrome ) 2. Structural abnormalities – Defects or abnormalities of the structure of the baby (Ex- Hole in the heart, split spine- Spina Bifida) ABNORMALITIES OF GENES OR CHROMOSOMES – To have a basic understanding about these abnormalities it is essential to be familiar with what genes and chromosomes are.
What are Genes and Chromosomes?
The genetic blue print ( plan or design of what you are ) we inherit from our parents is called genes. All our characteristics such as colour of hair, shape of the nose and complexion etc are written in our genes. These genes are arranged and packed in a structure called ‘chromosomes’. Humans have approximately 23,000 genes arranged on 23 pairs of chromosomes. Similar to bricks of a house , we are composed of billions of ‘cells’ and all these cells have genes, arranged in chromosomes. Each cell has 23 pairs of chromosomes ( 46 single chromosomes) derived from parents. We have special cells required for reproduction which are called sperms and ova(eggs) and these cells have half of the genetic material of a normal cell. Therefore a sperm or an egg has 23 chromosomes each , and when they meet , the resulting ‘new life’ will acquire 46 ( 23 pairs). These chromosome pairs are numbered from ‘one’ to ‘twenty three’. Twenty third pair contains either ‘an X and a Y’ ( male ) or ‘two Xs’( female ) and ‘X’ and ‘Y’ called sex chromosomes.
How do we get genetic or chromosomal abnormalities?
According to above facts ,we know that eggs and sperms have half of the genes and chromosomes of a normal cells. Hence there has to be a special process of division for this to happen in the testis ( where sperms are produced) and the ovary( where eggs are produced). During this process of special division of cells, some eggs or sperms can end up in having an excess or a reduction or loss of genes or chromosomes simply by CHANCE. Therefore 60- 70 out of 100 of these abnormalities are just due to ‘chance’. Fortunately most of these pregnancies end up in miscarriages before 12 weeks and very few go up to full term. Age also increases the risk of having these abnormalities. The older you are the higher the chance of these abnormalities . Marriages among blood relatives (consanguineous marriages ) is another cause which makes them vulnerable for genetic abnormalities as variation of genetic material is minimal among relatives. The effect of two genes, one each from father and mother is necessary for some of our characteristics . Some of these genes are strong( Dominant ) and some are weak(recessive) . The effect of a weak gene from one parent is usually covered or suppressed by the presence of a strong gene from other parent. As there is minimal variation of genetic material among blood relatives, the chances of occurrence of two weak genes is twenty five times more in them . Therefore this will give rise to rare genetic diseases to be more common among children of blood relatives. Avoidance of consanguineous marriages is the only way to prevent this genetic abnormalities as most of these diseases are incurable . Some genetic diseases run in families and it is important to know if there are family members affected by the same kind of disease at present or in the past. Bleeding disorders like Haemophilia and blood disorders like thalassaemia are examples of those. A minority of these disorders are due to environmental factors like radiation and medications like anti-cancer drugs etc. Rarely, more than one of the above factors can give rise to an abnormality in the same individual.
How common are these disorders?
Fortunately ,most pregnancies are healthy and normal .Approximately about 1-2 pregnancies out of 100 pregnancies can have these problems. These can be rare genetic disorders which can be seen at birth or sometimes they can manifest in adulthood( Ex- Adult onset polycystic kidney disease ). If there is an additional chromosome in one of the 23 pairs of chromosomes or if one chromosome is missing in one of the 23 pairs, it will give rise to a chromosomal abnormality. Additional chromosome in 21st pair is called Downs syndrome, additional chromosome in 13th pair is called Patau syndrome, additional chromosome in 18th pair is called Edward’s syndrome and if one of the sex chromosome is missing in the presence of only an ‘X’ chromosome in 23rd pair it is called Turner’s Syndrome.
What is the commonest chromosomal abnormality?
As most of the chromosomal abnormalities end up in miscarriages, still births or early infant deaths , these are not common. The commonest one which is compatible with life called ‘Down’s syndrome’. One out of 600 pregnant women end up in having a baby with Down’s syndrome. The older you are, the higher the risk of having a baby with Down’s Syndrome. One out of about 900 women at the age of 30 years give birth to a baby with Down’s syndrome, whereas one out of 350 women give birth to a baby with Down’s syndrome at 35 years of age. At 40 years one out of each 100 mothers give birth to a baby with Down’s syndrome. As most of the mothers who fall pregnant are below 35 years this syndrome is more frequently seen in low risk groups and in younger mothers.
What is DOWN’S SYNDROME?
Down's syndrome is a life-long condition and majority of these children are not independent. This condition cannot be cured .They have learning difficulties and delays in development . As majority of these children are not independent, always there has to be someone to look after these children . This can have an effect towards the other family members . This can also have a financial impact as these children have special needs like special schools etc. Having a child with Down’s Syndrome can give rise to social , family and financial problems , especially in this part of the world where there are no special facilities etc . As the awareness of conditions like these is minimal among people , the stigma associated with having a baby with this condition too can lead to lot of problems, compared to developed countries where people are more aware of these conditions . All Down’s syndrome babies have same kind of faces with flat nose and face and upward slanting eyes. Most of these babies are ‘floppy’ (Hypotonic) and have very flexible joints which improve as they grow older. Many babies with Down's syndrome have a single crease which runs right across the palm of the hand ( single palmer crease). Doctors often look for this characteristic crease as a sign that the baby may have Down's syndrome.. They may have a larger than usual gap between the big toe and the second toe (sandal gap). By looking at the above features the doctor can usually arrive at a diagnosos of Down’s syndrome. The confirmatory test involves a blood test to confirm extra chromosome in the 21st pair and is called karyotyping or chromosome analysis test. These babies are more prone to chest infections and pneumonia etc. About fifty percent of these babies can have obstruction to the bowel at birth( Duodenal Atresia), congenital heart diseases, blood cancers (leukaemia) etc . There lifespan is also shorter than a normal.
Why does Down’s Syndrome happen?
This happens because of the presence of an extra chromosome in the 21st pair in baby’s cells. This condition is also called ‘Trisomy 21’. ( Tri= three ; somy = bodies – which means there are three chromosomes in 21st pair, instead of two ). In nearly 95 out of 100 babies it happens due to ‘chance’ during the process of cell division. These babies are called ‘non-dysjunction Down’s’ . There is an age related rise of the chance of having a baby with this type of Down’s as you grow older. About 5 out of 100 Down’s Syndrome babies are due to the presence of some abnormality in one of the parent which is not obvious in parents. These babies are called ‘Translocation Down’s’ . Karyotyping or chromosome analysis test of the parents will be needed if the baby is found to be a translocation Down’s where there is a higher risk of the same occurring again.
Is it my fault or can it happen again?
It just happens it is never anyone’s fault. Therefore you do not have to feel guilty. It has never been linked to food you eat, things you do etc. it occurs in all races and religions. There is a slight risk of you having a baby with Down’s syndrome if you have delivered a baby with Down’s syndrome. Can I find if my baby is having Down’s syndrome before the baby is born, and about Down syndrome screening in pregnancy in next issue…. TESTING FOR DOWN’S SYNDROME IN PREGNANCY
Should I have the test for Down’s syndrome?
In developed countries all pregnant mothers are offered this tests irrespective of the age of mother. Some parents want to find out if their baby has Down’s syndrome and some parents do not .Only parents can decide about this .Information about what Down’s syndrome is , and about the tests available to detect will help you to make a decision.
What are the tests available to detect Down’s syndrome in pregnancy?
Basically there are two types of tests that are offered to expectant mothers. These tests are offered in two stages. First type of tests offered are called SCREENING TESTS. All mothers are offered these tests if they opt for screening, as these tests do not carry any risk to the baby or the mother. These tests help us to find out your actual RISK of having a baby with Down’s syndrome but do not give a definite answer. It is important to realize that these tests do tell us which mother has an increased risk of carrying a baby with Down’s syndrome. Second stage tests are called ‘DIAGNOSTIC TESTS’ and these give definite information as to whether your baby is Down’s syndrome or not. But as these tests increase the risk of miscarriage, these tests are offered only to mothers who are at high risk of having a baby with Down’s syndrome. Screening tests which are described in this article will tell the exact risk in numbers ( Ex- a risk of 1 in 100 has a higher chance than a risk of 1 n 500 ) so that you can decide if you are going to have the diagnostic test.
What are the diagnostic tests for Down’s syndrome?
Basically there are only two reliable tests to detect babies with Down’s syndrome before they are born. First test is called Chorionic villus sampling (CVS) and the other is called amniocentesis. These test can tell you if your baby is having Down’s syndrome or not for sure. Since these tests give a definite answer these tests are called ‘diagnostic tests’.
What is Chorionic villus Sampling (CVS)?
This is a test which involves insertion of a tiny needle through mother’s tummy into the womb to take a little sample of tissue from the placenta. An Ultrasound scan is used to guide the needle during this procedure. The sample is analyzed in the laboratopry and the chromosomes are counted to see if they are normal. This test can be done as early as eleventh week of the pregnancy
What is amniocentesis?
This is a widely used procedure which takes about ten minutes. This procedure is done under Ultrasound guidance . It involves taking little amount of water around the baby by inserting a tiny needle through mothers abdomen and through the womb in to the pregnancy sac which contains amniotic fluid(water around the baby is called ‘amniotic fluid’. This fluid contains cells from the baby which will be examined in the laboratory and chromosomes of the baby will be counted to see if they are normal.
Are there any risks of having these ‘diagnostic tests’?
Yes. One to two women out of every two hundred women who undergo these tests can end up in having a miscarriage. Therefore this test cannot be offered to all mothers universally as the risk of having a baby with Down’s syndrome is low in many women. Therefore these tests are offered only to those who are at high risk of having a baby with Down’s syndrome. Therefore Down’s syndrome testing in pregnancy is offered in two stages. The first kind of tests carry NO risk of miscarriage. These tests are called ‘SCREENING TESTS’ and these tests do not give a definite answer as to whether your baby is having Down’s syndrome or not. Instead these tests will tell us which expectant mothers have an increased risk . Only those mothers who are at increased risk are offered a ‘diagnostic test’. But in the past only screening test used was just based on woman’s age and diagnostic tests were offered to all women above a certain age.
What are the screening tests?
( tests to find out if you are at increased risk of having a baby with Down’s syndrome) In the previous issue we discussed what Down’s syndrome is, why does it occur and about problems of a child with Down’s syndrome. As you know the risk of having a baby with Down’s Syndrome increases with age. At the age of 25 years one out of every 1350 mothers carry a baby with Down’s Syndrome. At 35 years of age ,one out of every 350 mothers carry a baby with Down’s syndrome. It increases to one in 100 at 40 years of age. This is your age related risk or your ‘background risk’. This is the generalized risk calculated for all women for a given age. This is similar to average age of cessation of periods(menopause). If the average age of menopause is fifty years it does not mean that all women are having menopause exactly at the age of fifty. It depends on lot INDIVIDUAL factors which decide on age of menopause. Similarly , your individual risk of you having a baby with Down’s syndrome may be higher or may be lower than your background risk which is dependant on your age. Although there is a higher risk with advancing mother’s age , we see babies with Down’s syndrome born to younger mothers too. This is because when you consider the number of pregnancies , majority is less than 35 years of age. Therefore new screening tests have been developed which can provide women of all ages with information about the chance of their baby having Down’s syndrome. These tests are called ‘SCREENING TESTS’. It is very important to understand that these screening tests cannot tell you that your baby definitely does or does not have Down’s syndrome. It will act as a guidance to make a decision in analyzing one’s situation or the risk of having a baby with Down’s syndrome. It is vital to understand the concept of screening. In the past , age was used as a screening tool and all women above a certain age( usually above 35 years) were offered a diagnostic test. But very soon doctors realized that this is not a good method of screening as we can detect only 30 Down’s syndrome babies out of 100 Down syndrome babies if age of the mother is used as a screening tool. Also, this method was not helpful in diagnosis of Down’s syndrome babies born to mothers who are less than 35 years and this fact made introduction of other methods of screening. New screening tests were introduced after extensive research and observation of thousands of mothers carrying babies of Down’s syndrome. These new screening tests involve some blood tests ( called ‘serum screening’) and special Ultrasound scans.
BLOOD TESTS FOR SCREENING
All babies produce certain types of hormones ( a type of chemicals found in blood) when they are inside the womb and some of these hormones find their way to mother’s blood circulation through the umbilical cord and the placenta.( We know that tests to detect these hormones in mother’s urine or blood is the basis of the ‘pregnancy test’). It was observed that the levels of these substances found in mother’s blood vary if it is an abnormal baby. This is the basis of the blood tests to check for the risk of you having a baby with Down’s syndrome. The concentration of these hormones vary depending on the stage of the pregnancy. The level of this hormones at 11 weeks of pregnancy may be less than the level at 13 weeks. Therefore it is mandatory to know the exact stage of pregnancy ( the number of weeks) worked out from a ‘dating scan’ done between 11weeks to 13weeks and 6 days from your last menstrual period. If you have not had a dating scan you cannot undertake these tests as the results can give false information which can confuse you. A computer programme is used to work out the chance of the baby having Down’s syndrome and this programme uses the results of blood tests together with mother’s exact age, weight and the stage of pregnancy worked out only from a dating scan done between 11weeks to 13weeks and 6days. Timing of the test and details of the hormones or substances measured vary depending on the test you have. Basically there are two tests available. The ‘TRIPLE TEST’ is offered around sixteen weeks of pregnancy and checks for three kinds of hormones. This test alone can detect little less than 60 Down’s syndrome babies out of 100 Down’s syndrome babies when diagnostic tests described above are offered to high risk mothers found in this way. Therefore this is becoming less popular now as this is a late test and also since this is less reliable compared to other tests. ‘Double marker test’ which involves checking for two substances in mother’s blood from 11 weeks to 13 weeks and 6days of pregnancy is more reliable and become more popular now. This test helps detect about 70 Down’s syndrome babies out of 100 Down’s syndrome babies when diagnostic tests are offered subsequently. This is becoming more and more popular as early diagnosis is an attractive feature of the test. Important fact you should bear in your mind is that no screening test is valid if you have not had a ‘dating scan’ between 11weeks to 13 weeks and 6days since interpretation of these blood tests is based on the correct age of the baby. The most effective and most reliable method of screening is by having an Ultrasound scan between 11 weeks and 13 weeks and 6 days. This special Ultrasound scan scan is frequently known as ‘nuchal scan or NT test and done only by accredited doctors or sonographers ( for competent doctors in Sri Lanka please go to the following link and click on ‘accredited sonographers’ on the left side- http://www.fetalmedicine.com/f-downs.htm ) . Ultrasound screening has become the most popular method of screening world-wide and is offered to all mothers universally in most developed countries. Combination of results of blood tests together with findings of Ultrasound scans is the most reliable method of screening and this can detect about 90 Down’s syndrome babies out of 100 Down’s syndrome babies. Ultrasound scans in screening for Down’s syndrome in pregnancy in next issue….
SCREENING FOR DOWN’S SYNDROME IN PREGNANCY
THE ROLE OF ULTRASOUND SCANS
In the previous issues we discussed what Down’s syndrome is, ‘diagnostic tests’ and about ‘screening tests’ available to detect this condition during pregnancy. We know that these tests are offered in two stages as the second stage tests carry a small risk to the baby. Therefore ‘screening tests’ are offered first to see if one is having a higher risk of carrying a baby with Down’s syndrome. As screening tests do not have any risk to the mother or the fetus they are offered universally to all mothers who opt for screening for chromosomal abnormalities. Ultrasound scan has become the most popular, most reliable and a cost effective method out of all screening tests available. Its safety during pregnancy is another feature for this to become the most popular tool of screening.
What is Ultrasound?
The strength(frequency) or power of the sound that human ear can hear is measured in terms of Hertz .Human can hear sound frequencies between 20 to 20,000 Hertz. Any frequency beyond 20,000 Hertz is called Ultrasound as human cannot hear. Some animals can hear high frequency sounds (ex-Dogs and bats). In medical Ultrasound we use high frequency sound waves that we cannot hear which are measured in MegaHertz.
Is Ultrasound scan harmful during pregnancy?
NO it is not harmful. According to medical evidence the Ultrasound waves used in an Ultrasound scan during pregnancy do not cause any damage or any adverse effect to the baby or the mother what so ever. From the time of conception up to birth Ultrasound scans can be done with out any adverse effect to the baby. Very few mothers are aware that an Ultrasound scan is an essential investigation in pregnancy. This Ultrasound scan is called ‘THE DATING SCAN’ and it should be done at 12 weeks. What is Ultrasound screening? An Ultrasound scan is performed to check if the baby is having the risk indicators. Presences of some features during an Ultrasound scan indicate that the baby has a high risk of having a chromosomal abnormality like Down’s syndrome. Is this a special Ultrasound scan? Yes it is. This Ultrasound scan can only be done between 11 weeks and 13 weeks and 6days from your last menstrual period. This is the golden period for assessment of risk for chromosomal abnormalities during pregnancy. If you pass this period with out an Ultrasound scan, even the serum screening( blood tests to assess the risk) may not be reliable. In developed countries all mothers are offered this test irrespective of the age of the mother. This test is commonly called ‘Nuchal Transleucency scan’. What is NUCHAL TRANSLEUCENCY SCAN ? This is also called NT test or NT scan. This is a special scan that is done between 11 weeks and 13weeks and 6 days. All babies have some amount of fluid lying under the skin at the back of the neck. The thickness of this layer of fluid can be measured by performing an Ultrasound scan. This layer of fluid is called ‘Nuchal Transleucency’( NT). The thickness of this layer depends on the mother’s age and the length of the baby. Following surveillance of thousands of babies between 11 weeks and 13 weeks, researchers observed that there is a higher risk of a baby having chromosomal abnormalities when the thickness of this fluid behind the neck is more than normal. Utilizing this data scientists developed a computer software which uses the thickness of fluid behind the neck(Nuchal transleucency) together with length of the baby and mother’s risk to work out a risk for Down’s syndrome for that baby. By using this computer software you can find your individual risk for a particular pregnancy even if you are at high risk according to your age. What is a ‘positve’ NT test? There is nothing called a positive NT test .It was a term used in the past. But now parents are provided with accurate assessment of risk rather than creating arbitrary definitions of ‘HIGH’ and ‘LOW’ risk or ‘POSITIVE’ and ‘NEGATIVE’ tests . A computer software is used to calculate the accurate risk so that the parents are given the risk in numbers. Ex- Your age related risk is 1 in 100 at 40 years and the adjusted risk following the nuchal test is 1 in 158 ( this shows that her individual risk is lower than other women of her age) . This way of giving the risk in numbers is a sensible way, so that the parents can get an idea about the magnitude of their risk . It is vital to understand how big your individual risk is , compared to your age related risk. As perception of the risk depends on the expectation of the parents, this concept of numerical value of the risk is very important. What is the significance if my NT test reveals that I am ‘high risk’? It only means that you have a higher risk of your baby having a chromosomal abnormality like Down’s syndrome or a structural abnormalities of heart (Ex- a ‘hole in the heart’). It does not give a definite answer as to whether your baby is abnormal or not. If you and your husband think that this risk is high , then you have to go for the second type of test called ‘diagnostic tests(amniocentesis -described in the previous issue) to have a definite answer. What if my risk is low? Then you have a low chance of your baby having an abnormality. But it is not an assurance of having a completely normal baby. You can compare this to someone walking on the right side of the road with someone walking on the left side. The one who is walking on the right side of the road is less likely to meet with an accident where as the one walking on the left side is more likely, although both are at risk of meeting with an accident. What are the other Ultrasound markers? Ultrasound markers are indicators of high or low risk of a particular condition depending on their presence or absence. Nuchal transleucency (NT) which is discussed above is also an Ultrasound marker which increases the risk of carrying a baby with Down’s syndrome when the thickness is higher than normal. Similarly there are other markers that can be detected at the time of the nuchal Ultrasound scan. Nasal bone , reversed blood flow through the right side of the heart ( Called Tricuspid regurgitation), Blood flow through the continuation of umbilical vein ( called Ductus venosus flow), angle between roof of the mouth and face ( called Fronto-Maxillary facial angle ) to detect flat face of babies of Down’s syndrome are the other Ultrasound markers . What is the ‘Nasal Bone’? NASAL BONE or the nose bone is another Ultrasound marker for Down’s syndrome. About 60 to 70 out of 100 Down’s syndrome babies do not have a nose bone if an Ultrasound scan is done to look for t he nose bone between 11weeks and 13weeks and 6days.In contrast, only five out of hundred of normal babies do not have the nose bone at this stage. Therefore if your baby has a nose bone at this stage it is a reassuring feature. Absence indicates high risk and does not mean that the baby is having Down’s syndrome for certain. How reliable are these tests in detecting Down’s Syndrome babies?How reliable are these When nuchal transleucency ,nasal bone and other Ultrasound markers are used to assess the risk, about 90 out of 100 Down’s syndrome babies can be detected when diagnostic tests are offered to them when the risk is high. Ultrasound screening has become very popular because of its better rate of detection. When the diagnostic tests are offered on the basis of age alone we can detect only about 30 Down’s syndrome babies out of 100, and when serum screening is used it is only about 60 out of 100. A combination of serum screening and Ultrasound markers have the best detection rates at the moment. Computer software package is used to calculate the combined risk in this manner and it is mandatory to know the exact age of the baby by performing a dating scan between 11 weeks to 13 weeks . If you are pregnant and if you want to have this scan or screening, please log on to this website http://www.fetalmedicine.com/f-downs.htm and click on ‘accredited sonographers’ on the left side and find out accredited Sri Lankan doctors ) . Structural abnormalities of baby.. in the next issue
අපගේ සම්පත් දායකයකු වන කොළඹ "ලංකා හොස්පිටල්ස්" ආයතනයේ නාරී හා ප්රසව රෝග පිළිබඳ විශේෂන්ඥ වෛද්ය විජිත් විද්යාභුෂණ මහතා විසින් සකස් කල ලිපියක් ඇසුරිනි. Thanks to our kind resource person Dr Vijith Vidyabhushana, Consultant VOG at Lanka Hospitals, Colombo 05.
Dr Vijith Diyabhushana, MBBS (Colombo), MS (Colombo), MRCOG (UK), DFFP (UK), RCR/RCOG, Dip Advanced Obstetric Ultrasound (UK), Accredited Ultrasound Specialist
ඔබට ඇති වෛද්ය ගැටළු අසන්න මෙහි ක්ලික් කරන්න, Ask a Doctor | වෛද්යවරයාගෙන් අසන්න
මෙම ලිපිය ඔබට ප්රයෝජනවත් වුනානම් හෝ ඔබේ මිතුරියකට ප්රයෝජනවත් වේ යයි සිතනවානම්, කරුණාකර පහත ඇති "recommend" ක්ලික් කිරීමෙන් එය බෙදා හදා ගැනීමට අමතක කරන්න එපා. තවද, මෙම ලිපිය පිළිබඳව ඔබේ අදහස්, යෝජනා සහ අනෙකුත් මවුවරුන්/කාන්තාවන් හට ප්රයෝජනවත් වන ඔබේ අත්දැකීම් පහත ලියා තැබීමටද අප ආරාධනා කරමු. ස්තුතියි!
If you found this article interesting and helpful, please share with your friends too. Just click the "recommend" button below. Also, we welcome your comments, views and experiences to share with other moms and ladies. Thank you!