Testings During Pregnancy. FAQ
Testings During Pregnancy. FAQ
Testing, testing
investigations in pregnancy
What is the difference between diagnostic and screening tests?
Screening tests identify your baby’s ”risk factor” for a particular condition, but do not confirm that your baby definitely has a condition. For example, a screening test for Down’s syndrome may give your baby a risk factor of 1:200 This means that your baby has a 1 in 200 chance of being affected by Down’s syndrome. Another way to view this result could be that the baby is most likely to be healthy. If your baby has a high risk factor, you may then decide to have a diagnostic test, such as amniocentesis or chorionic villus sampling (see pp 122-123), which gives a definite yes or no as to whether or not a condition is present. These tests are more invasive, as they require a sample of amniotic fluid or blood from the fetus or placenta, and they carry a slight risk of miscarriage.
Certain screening tests, such as first- or second-trimester screening for Down’s syndrome, are offered routinely to all women regardless of any factor other than they are pregnant. These tests, in the form of scans or blood tests, identify who would benefit from further diagnostic tests. This avoids subjecting all pregnant women to diagnostic tests, which carry some risks (see p. 125). Any benefit from a test should outweigh the potential risk.
What do these tests look for?
Screening and diagnostic tests aim to identify abnormalities in the unborn baby, which may be congenital, genetic, or chromosomal. Congenital abnormalities are often detected in the 18-22 week scan (see p.121) and these include conditions such as heart abnormalities or extra digits These abnormalities can sometimes be treated after, or sometimes even before, the birth and are not inherited. Some conditions, such as spina bifida, are thought to be due to a combination of genetic and
environmental factors; a dietary deficiency of folic acid may also contribute to this condition. Other congenital abnormalities may be caused by infections caught in pregnancy.
Diagnostic tests are usually carried out to identify genetic or chromosomal abnormalities, such as Down’s syndrome, cystic fibrosis, sickle-cell anaemia, and muscular dystrophy. (Cystic fibrosis and muscular dystrophy are screened for if there is a family history) These conditions occur either because there is a problem with the inherited genetic material, for example a gene has mutated, or because there is a chromosomal problem, for example there may be an incorrect number of chromosomes, as in Down’s.
Down’s syndrome, or ”trisomy 21”, is a chromosomal abnormality in which there is an extra copy of the chromosome 21. It is the most common ”trisomy” disorder. Babies born with this condition have physical anomalies, such as slanting eyes and
a protruding tongue, and there is a high incidence of heart, intestinal, hearing, and sight problems. Down’s is the biggest single cause of learning difficulties. The majority of Down’s syndrome conceptions are lost through spontaneous miscarriage early on in pregnancy, although over 600 babies are born with Down’s syndrome in the UK each year.
I’m 38 - will I have more tests because I’m older?
Although the risk of Down’s syndrome increases as you get older (see p 118) currently many women regardless of age are offered one of two types of screening test for Down’s. This is either a first trimester screening that involves a blood test and a scan to measure nuchal translucency (see p.118), or second trimester screening which is a blood test only, called the triple, or Bart’s. test (see below) Both tests give the result as a risk or a percentage risk. If the test indicates there is a high risk of Down’s, then all women are offered a diagnostic test such as amniocentesis (see p.123). However, if you are over 35. amniocentesis is offered routinely in the UK.
Your midwife should discuss with you in detail all the tests that are available and give you written information about them. Ideally, you should have this information several weeks before you are asked to decide if you wish to go ahead with any screening or diagnostic tests so that you have plenty of time to consider the possible outcomes and whether these tests are something -you wish to undergo
Depending on your past medical history and other factors, such as your blood pressure during your pregnancy or problems you had in previous pregnancies, you may be offered additional scans to check your baby’s growth after 26-28 weeks.
We don’t want invasive tests as we will love the baby whatever. Can we refuse diagnostic tests?
Whether or not you have a diagnostic test is your choice and you can refuse at any time to have any test offered As well as the question of whether you are prepared to have a Down’s baby, there is also the risk of miscarriage to consider (see p.125) On the other hand, you might decide you want a definite diagnosis to be able to prepare for your child.
What blood tests will I be having, and when?
There are various blood tests offered during pregnancy As well as routine blood tests taken during antenatal checks to assess your health, there are also blood tests to screen for problems with the baby. Within the first 12 weeks you will be given a routine blood test to check your levels of haemoglobin, the oxygen-carrying part of blood. Although these fall slightly in pregnancy as the blood becomes more diluted, a significantly low haemoglobin level indicates iron-deficiency anaemia (see p 8 1). You will also have tests to identify your blood group, Rhesus factor, and rubella immunity (see p 15). and to screen for infectious diseases including syphilis, HIV, and hepatitis B. You may also be tested for sickle cell and thalassaemia, inherited blood conditions more commonly found in people of African, Caribbean, Indian, or southern Mediterranean origin.
Other blood tests may be offered to screen for congenital abnormalities in the baby Between 10 and 14 weeks, a blood test that measures the levels of the substance known as pregnancy associated plasma protein (PAPP-A) may be offered that is combined with the nuchal translucency scan (see opposite) to calculate a risk of Down’s syndrome.
If first trimester Down’s syndrome screening isn’t available, then second trimester blood screening tests are offered, which also include screening for neural tube defects, such as spina bifida These tests, carried out between 16 and 18 weeks, include the triple, or Bart’s, test, which measures the levels of the hormones AFR hCG, and oestriol; and the quadruple test, which, in addition to the other three hormones, measures inhibin A and PAPP-A.
Will I have a test for HIV?
Ali screening and diagnostic tests recommended in pregnancy are optional, so it is up to you and your partner to decide whether to have them. One of these is a blood test to check if you have the human immuno-deficiency virus, or HIV, and, indeed, some women only find out about their HIV status in pregnancy It is worthwhile to test for HIV in pregnancy as, if the result is positive, anti-retroviral medication, careful monitoring of maternal blood levels, and careful, safe delivery of the baby can reduce the chance of transferring the infection to the baby from 40 per cent to 2 per cent.
For pregnant women with HrV, a blood test is taken around the time of delivery to measure the levels of the virus. Depending on the results of the blood test, the obstetrician will either recommend a planned Caesarean section or decide that the levels are low enough to have a normal delivery.
After the delivery, HIV-positive mothers are advised to bottlefeed, again to reduce the risk of transferring the virus to the baby
How do ultrasound scans work?
Ultrasound scans use high-frequency sound waves — so high we can’t hear them — that bounce off solid objects and create a picture, visible on a
computer screen, of your baby, the placenta, and your organs in the surrounding area.
How many scans will I have and when?
All women should routinely be offered two scans, a dating scan between 10 and 14 weeks and an anomaly scan between 18 and 22 weeks. Some units routinely offer a screening scan for Down’s syndrome between 11 and 14 weeks, known as the nuchal scan (see opposite), although this isn’t available nationwide. You can also arrange to have private scans that may be 3D/4D (see p, 124) and which you have to pay for
I’m quite scared about my first scan. What happens during the scan and what does it feel like?
Although not painful, early scans can cause discomfort as you need a full bladder (see p, 124). Ultrasound scans can be carried out by a doctor,
a midwife, or a sonographer. You will lie on a couch and need to wear something that makes it easy to expose your tummy. The person doing the scan puts cold gel on the lower part of your tummy, which improves contact with the skin, making it easier to view the baby. You will feel a little pressure as a transducer is pressed against your skin and moved around to look at the baby from different angles and to take measurements The image produced by the scan is viewed on a screen similar to a computer monitor. The person carrying out the scan may spend some time first studying the image and taking measurements before talking to you about what they can see Although this can be unnerving, it does not mean that anything is wrong
Some units offer a transvaginal scan in early pregnancy, which can give an improved image at this stage. This internal scan is done using a probe that is covered by a condom and gently inserted into your vagina. The image is viewed on the screen in the same way as an abdominal scan. This may be offered before 10 weeks if there is bleeding or pain.
Many units offer to print an image from the scan for you to take home Although ultrasound scans
Should I have a scan? is ultrasound safe ill pregnancy’?
Ultrasound scans in pregnancy, first introduced 40 years ago, have become a routine part of antenatal care.
* Most research indicates that they are a safe way to view the baby even when extra scans are needed for medical reasons * Suggested links between additional scans and growth problems and dyslexia are tentative as babies scanned more often are more likely to have problems linked to other factors.
* Recommendations are that scans are carried out only for clinical reasons and the number done is kept to a minimum.
How long do scans last?
The length of time an ultrasound scan takes varies depending on the reason for the scan and the experience of the ultrasonographer.
During the dating scan, performed at around 10-14 weeks, the sonographer takes some basic measurements. This includes the measurement from the top of the head to the end of the bottom, known as the ”crown-rump” measurement, used to calculate how many weeks old your baby is and
therefore your due date. This scan can take around 20-30 minutes. The nuchal fold scan (see p 118), during which the sonographer measures the fluid at the back of the baby’s neck, takes around 20 minutes. Anomaly scans, performed between 18 and 22 weeks, are detailed scans that take approximately 40 minutes (see opposite) At this scan, the sonographer measures the baby and looks at physical and structural development. The size and position of the placenta are examined and the amniotic fluid around the baby is measured
If, during your pregnancy, your midwife has any concerns about your baby’s growth or wellbeing, she may refer you to an obstetrician who may recommend another scan. As this will be to identify a specific problem, such as whether there is a concern about your baby’s growth, it may take a bit longer. This may be in the form of a Doppler scan, which measures the blood flow in the uterus, placenta, and umbilical cord and can help to identify growth problems in the baby This procedure usually takes around 30 minutes.
Do I have to have scans in pregnancy?
Official guidelines are that all women should be offered two routine scans during their pregnancy, but the choice to have one is yours. As scans are screening tests to look for anything out of the ordinary some women choose not to have any as they prefer not to know about any problems until the baby is born, or are confident that they will continue with the pregnancy regardless. You need to decide whether you fall into this category
Can my partner come along for the scans?
There is no reason why your partner should be excluded from attending these appointments if you want him to be there and, indeed, it’s very common for partners to attend ultrasound scans For many couples, the scan is a special moment as it’s the first time they get to see their baby and begin to think of themselves as parents.
primarily area clinical screening tool to determine if your baby is growing and developing as expected, they are also an opportunity to see your baby for the first time and often see your baby moving even before you feel the first flutters inside your uterus.
So scans become part of the developing relationship between you and your partner and the baby. In recognition of this, most units offer the facility of providing photos of the scan for a small charge to cover printing costs. Ask your community midwife whether the maternity unit where you are having your scan has this facility.
Do you have to drink pints of water before a scan? I’m scared I’ll have an accident.
For the 10-14 week dating scan it’s important to have a full bladder to make it possible to view the baby. This is because until 12 weeks the uterus stays in the pelvis and the bowel obscures the view. a full bladder raises the uterus and pushes the bowel out of the way. You may need a full bladder for a nuchal scan, between 11 and 14 weeks, depending on when it is done. Some units do transvaginal. scans (a small ultrasound probe placed inside your vagina) before 10 weeks if the image from an abdominal scan is poor In this case, you won’t need a full bladder and research indicates that transvaginal scans are more comfortable in early pregnancy compared to abdominal scans. You don’t need a full bladder for the 18-20-week scan, as the position of the uterus has changed
I’m pregnant through IVE Will I have more scans than normal?
It’s usual to have one extra scan in an IVF pregnancy usually carried out by the centre where you had the
3D and 4D ultrasounds
Many companies now offer special scans that reveal your baby in three dimensions or moving on film or video. These 26-32 week scans can be quite expensive and are carried out for curiosity value and not for medical reasons The quality of the pictures is usually amazing and parents are sometimes able to spot genetic similarities between themselves and
procedure This scan is usually done around two weeks after the embryo has been transferred to confirm the pregnancy and make sure that the pregnancy is within the uterus rather than in a Fallopian tube (see ectopic pregnancy, p.25) Although the main purpose is to reassure you that all is well, the centre also has to inform the Human Fertilisation and Embryology Authority (HYEA) of the outcome of the IVF treatment. Once your pregnancy is confirmed, you will continue with routine antenatal care like any other pregnancy
Can they really tell the sex
of the baby early on? I’m 18
weeks and not sure if I want to know.
It is possible to identify the sex of a baby on routine ultrasound scans from around 20 weeks, but this is dependent on a number of factors, including the expertise of the person performing the ultrasound, the quality of the equipment being used, the position the baby is lying in, and the position of his or her legs. Even if all of these factors are favourable and the genitalia can be seen, there is an error factor, so
their baby However, the scan is often lengthy which means the baby is exposed to ultrasound for longer than is normal Also, if the baby is in the wrong position, it may be difficult to get a clear picture The position of the placenta, the amount of amniotic fluid, and the size of the mother can also affect the quality of the pictures obtained the information given about gender from a scan is never seen as 100 per cent accurate Some research has been carried out to try and determine gender at an earlier stage, but this was even less accurate. Sometimes when you are watching the scan you may be able to see the genitalia yourself and may decide you know the sex of your baby without being told. But remember you may be wrong. If you have an amniocentesis the sex of the baby can be definitely identified during the procedure.
Most units have a written policy only to reveal the baby’s sex if this information is requested Some units have a policy of not telling anyone the sex of the baby from scans alone, partly because they cannot be 100 per cent accurate and also because, in a small number of cases, the information about gender may lead to a request for termination. If you want to know the policy in your area, ask your community midwife.
I’ve seen lots of companies advertising scans and videos of scans - are these safe?
Many companies offer 3D scans (still pictures) and 4D scans (moving pictures copied onto video or DVD) (see opposite), and the detail in these can be very good. If you have a private scan, you should check the expertise of the person carrying out the scan, and check if the company has a referral policy to an appropriate consultant obstetrician if anything untoward is discovered, as not all companies employ the services of obstetricians or midwives.
There are twins in our family. When will they be able to check whether I’m having twins?
Most women find out that they are having twins
at their ultrasound dating scan between 10 and 14 weeks Very occasionally, one twin is hidden on the first scan and is seen at the second ultrasound scan, but nowadays this is less likely due to advances in scanning Family history also gives a clue to the possibility of twins, but only if they are fraternal, or non-identical (see p.129).
Is everyone offered amniocentesis?
Amniocentesis is a diagnostic test (see p 123) that is routinely offered if you are over 35 and so have a higher risk of having a baby with Down’s syndrome Alternatively, you may be offered the test if your family history suggests there may be a risk of your baby having muscular dystrophy, haemophilia, cystic fibrosis, or another genetic disorder. Also, if you have had a screening test that suggests your baby has a high risk for a congenital condition, you will be offered a diagnostic test to confirm or rule it out For example, if the nuchal scan (see p.118) showed a high risk of Down’s, amniocentesis may be offered.
I’ve heard that amniocentesis carries a risk. Is this true?
Amniocentesis does carry a small risk of miscarriage. It is thought that the risk of miscarriage is increased above the normal risk by 1 per cent immediately following an amniocentesis, but after two days the risk returns to normal. You need to balance the risk against the value of the test to you and also be aware that a normal test result is not a guarantee that there will not be any other problems, but is nonetheless reassuring
Can chorionic villus sampling cause miscarriage?
Chorionic villus sampling (CVS) is another diagnostic test used to establish whether a baby has Down’s syndrome (see p.122). Unfortunately, as with other invasive tests, this carries a risk of miscarriage, of around 1.5-2 per cent, with the risk reducing each day, Larger hospitals carrying out more than 100 CVS tests a year may have lower miscarriage rates due to the opportunity for the doctors to fine tune their ability to carry out the procedure.
When is cordocentesis used?
Cordocentesis is a diagnostic test used to diagnose Down’s syndrome and other problems
in a baby It can also detect infection from diseases such as toxoplasmosis (see p.45). Additionally,
cordocentesis is used to detect rubella infection (see p.15), as well as to perform a blood count on a baby that is suspected of having anaemia From 18 weeks, the baby’s blood is examined using a sample of
blood carefully extracted from the umbilical cord The test is carried out in a similar way to that of amniocentesis, though results are available within 72 hours. The risk of miscarriage is 1-2 per cent.
Will I get weighed at my antenatal appointments?
In 1941, routine weighing of all pregnant women at each antenatal appointment began Although it was thought that there was a connection between a mother’s weight gain and a baby’s birth weight,
it was decided more recently that this is not a good indicator of when a baby is not growing, and so over the last 10 years routine weighing at each appointment has been abandoned. Furthermore, weight gain can vary from woman to woman in normal healthy pregnancies as widely as 3-18 kg (7-401b).
Nowadays, all women are weighed once at the beginning of pregnancy and then, together with a height measurement, their BMI (body mass index) is calculated (see p.18), which helps to predict certain risk factors, for example in women who have a very high or very low BMI The only time that you might be weighed on successive visits is if there is a medical reason to do so, for example if you had significant weight gain in a short space of time that could indicate excessive fluid retention (oedema), a sign of pre-eclampsia (see p 89).
My friend is 27 and has had a Down’s baby - is that unusual?
Although the risk or chance of having a baby with Down’s syndrome increases with age, particularly over 35, the majority of Down’s babies are born to younger mothers. This is probably due to the fact that more women have their babies younger, and also because women over 35 are likely to have more tests The risk of having a baby with Down’s at the age of 20 years is 1 in 1,700. This risk increases to 1 in 1,400 by the age of 25 and by the time the mother reaches 35, the risk has increased to about 1 in 400
My partner wants to hire a Doppler so we can listen to the baby’s heartbeat. Is this a good idea?
During pregnancy your midwife listens to the baby , s heartbeat with an instrument called a Doppler sonicaid or pinard (ear trumpet). Most midwives use a sonicaid so the parents can hear the heartbeat too This passes sound waves through the abdomen. which pick up movement and bounce it back to the machine, where it is converted into sound.
Being able to hear your baby’s heartbeat during pregnancy is reassuring, especially when the earlier symptoms wear off but the baby’s movements have yet to be felt. However, your baby’s heart beats at a rate approximately double the rate of your heart. If the closest moving thing to the ”beam” is your blood pulsating through your aorta, the sonicaid will pick this up, and if you pick up your heart rate, this might cause you anxiety Also, depending on your gestation and the position of your baby the heartbeat will be found in different areas on the abdomen. If you can’t pick up a heartbeat, you may be unduly worried.
Midwives undergo specialist training to find the heartbeat and many won’t try to find the heartbeat until the baby is around 16 weeks, and even then may have difficulty Occasionally, due to the baby’s position, they may need to call another midwife or doctor to help them locate the heartbeat
It is up to you and your partner if you decide to hire a sonicaid, but it would be wise to be aware of the anxieties that may accompany this decision.
The 18-22 week anomaly scan
Your baby’s physical examination
Also known as the fetal anomaly or anatomy scan, this detailed scan is offered to all women between the 18th and 22nd week of pregnancy. At this stage of gestation, your baby has well-developed limbs and facial features and all its major organs and
body systems are in place and can be checked.
How is it done? The scan involves transmitting high-frequency sound waves through the uterus that bounce off the baby and the returning sounds are converted into an image (see p.119), The biggest echoes are from hard tissues, such as bones, which appear white in the image on the screen, while soft tissues are grey-flecked. Fluid-filled spaces, such as the stomach, bladder, blood vessels, and amniotic fluid surrounding the baby, do not return sound waves so appear black. It is
the difference between echoes and colours that enables the ultrasonographer to interpret images
What will be checked? The ultrasonographer starts by checking the fetal heartbeat and then counts the babies - rarely, twins are not revealed until 20 weeks! She will measure the head circumference and diameter (biparietal diameter), and the abdominal circumference and the femur (thigh bone) to date the pregnancy and ensure your baby is growing well She will check for abnormalities in the brain, face and lips, spine, abdomen, heart, stomach, kidneys, bladder, and hands and feet. Lastly, the placenta, umbilical cord, and amniotic fluid are examined You may be able to find out the sex of your baby, although you can ask not to be given this information (see p.124).
Diagnostic tests
Identifying fetal abnormalities
Diagnostic tests give a definitive answer as to whether or not your baby has an abnormality such as Down’s syndrome These tests are not carried out routinely and you will be offered one only if a screening test indicated that your baby had a higher risk for Down’s syndrome, if you are over 35, or you have a family history that puts you at a higher risk of having a baby with an abnormality, All diagnostic tests also carry a small risk of miscarriage and you will need to weigh up the pros and cons before deciding to go ahead with one
Chorionic villus sampling (CVS) This is a
diagnostic test that involves taking a tissue sample from the placenta to identify for certain whether your baby has Down’s syndrome or a genetic
Chorionic villus sampling
needle and
syringe ultrasound transducer
abnormality This can be done as the placenta contains the same genetic information as the baby The test is carried out between 11 and 13 weeks, The advantage of this test is that it can be performed earlier in pregnancy than amniocentesis, so if an abnormality is found and you decide to terminate, it is early enough to have a suction termination.
How is it done? There are two procedures for CVS: one method extracts a sample of the placenta via the abdomen, and the other method carries out the procedure vaginally With the abdominal method, a fine needle is inserted through your abdomen and, using an ultrasound scan for guidance, the doctor removes a very small sample of tissue from the placenta You have to wait about
ultrasound transducer
catheter 10 days for the results, which means that if your baby has an abnormality and you want to terminate your pregnancy, you can do so well before you start to feel your baby kicking.
To carry out CVS vaginally the doctor inserts a small tube through your vagina and the cervix, which then passes through the uterine wall. As with the abdominal method, the doctor then takes a small sample of tissue from the placenta, using ultrasound for guidance. The sample is sent to a laboratory where it is grown in a culture for around seven days. The sample is then studied under a microscope to check for chromosomal abnormalities or other defects.
Amniocentesis Amniocentesis is a diagnostic test used mainly to identify a chromosomal abnormality and it is the most commonly used test for identifying Down’s syndrome. During the test, a sample of amniotic fluid containing cells from the baby’s system is taken from the uterus. It is a relatively quick and painless
Amniocentesis test
ultrasound — needle and
transducer syringe
placenta uterus — amniotic
fluid
cervix
procedure and may be offered at around 16-19 weeks of pregnancy It is offered later than CVS because there may be insufficient fetal cells in the amniotic fluid before this stage of pregnancy The results from this procedure are usually very accurate and, although there is a slight risk of miscarriage, this is lower than the risk of miscarriage with CVS, especially in units where a large number of the tests are carried out and the doctors are particularly practised at conducting the test Apart from the slight risk of miscarriage, the main disadvantage of amniocentesis is that it has to be carried out later in pregnancy, so if the result comes back as positive, then you will be half way or even further into your pregnancy should you decide to terminate and would need to be induced to undergo a vaginal delivery
How is it done? Using an ultrasound scan to guide the procedure, a long, thin needle is inserted through the mother’s abdomen into the amniotic sac and a small sample of amniotic fluid is extracted. This contains fetal cells, which are then grown in a culture in a laboratory to be analysed. As there is a small risk of miscarriage, you may be advised to rest for a day or two afterwards to minimize this risk. Depending on the maternity unit and the laboratories used, there may be a chance that the result could be back before the end of one week, but the majority of units still have to wait two or even three weeks. It usually takes 2-3 weeks for the fetal cells to grow, Very occasionally this does not happen and you may need to have another amniocentesis test
Cordocentesis This diagnostic test is also known as ”fetal blood sampling” or “umbilical vein sampling”. In this test, blood is taken from the baby’s umbilical cord to diagnose Down’s syndrome when earlier screening tests have shown a possible problem. Since this is an extremely specialized procedure, it can only be carried out at a regional specialist fetal medicine centre in certain parts of the country.
Nuchal fold and dating scans
Ultrasound examinations
A dating scan at 10-14 weeks measures fetal growth so that a gestational age can be given The nuchal fold scan, or nuchal translucency scan, is offered between weeks 11 and 14 and assesses the risk of Down’s syndrome. Only some hospitals offer this scan at the moment The risk of Down’s syndrome rises with age. At 20 the risk is 1:1527; at 25 it is 1:1352; at 30 it is 1:1895; at 35 it is 1: 356; and at 40 it is 1,97,
What does the dating scan look for? The distance is measured from the top of the baby’s head to its bottom (crown-rump measurement), and the diameter of the head is recorded, known as the biparietal diameter - the distance between the parietal bones either side of the head
How is the nuchal fold scan done? The sonographer will measure the width of the fold of
skin behind your baby’s neck to see if any excess fluid has collected there This measurement is calculated into a risk ratio based on your age The ratio is considered high if it is above 1:300. You will be given the results of the scan immediately If your baby has a high risk, you will be offered further tests and, depending on the results and after counselling, the choice of continuing your pregnancy with support or having a termination.
Is it reliable? The nuchal fold scan is considered to be 80 per cent accurate, which means there is a 20 per cent (1:5) chance of it being inaccurate If your hospital offers you a blood test (PAPP-A, see p.119) with the scan, it becomes 85 to 90 per cent accurate. When the nasal bone is also measured, the accuracy rises to 95 per cent. Your local maternity unit should be able to provide you with information as to how accurate their scans are.