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 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 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 Diagnostic tests),
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 or fetal injury.
tests, such as first- or second-trimester screening for Down 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 the question
Is it true that amniocentesis carries a risk?). 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 may be detected in the 18–22 week scan
and these include conditions such as heart abnormalities or extra
digits. These abnormalities can sometimes be treated after 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
tests are usually performed to identify genetic or chromosomal
abnormalities, such as Down syndrome, cystic fibrosis, sickle-cell
anemia, 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.
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. Disabilities
resulting from Down syndrome fall into a wide range from minimal to
severe. The majority of Down syndrome conceptions are lost through
spontaneous miscarriage early on in pregnancy. The incidence of Down
syndrome births in the US is about 1 in 800.
I'm 38—will I have more tests because I'm older?
Although the risk of chromosomal defects increases as you get
older, currently many women regardless of age should be offered two
types of screening tests. This is either a first trimester screening
that involves a blood test and a scan to measure nuchal translucency , or second trimester screening, which is a blood test only, called the quad screen.
These tests give the result as a risk or a percentage risk. If the test
indicates there is a high risk of a problem, then all women are offered
a diagnostic test such as amniocentesis. However, if you are over 35, amniocentesis is offered routinely in the US.
Your doctor or
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 want to go ahead with any screening or diagnostic tests so
that you have plenty of time to consider possible outcomes and whether
these tests are something you want to undergo.
Depending on your
medical history and your pregnancy blood pressure or problems in
previous pregnancies, you may be offered additional ultrasounds to check
your baby's growth after 26–28 weeks.
We don't want invasive tests since we will love the baby no matter what. Can we refuse diagnostic tests?
Genetic screening and diagnostic testing is always optional and
you can decline any or all of them. Some parents believe such testing
would cause undue worry even though the rate of false positives is
approximately five percent. Many couples feel that they would like to
know if their baby may have a problem, so they can prepare.
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 prenatal checkups 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 hemoglobin, the oxygen-carrying part of blood.
Although these fall slightly in pregnancy as the blood becomes more
diluted, a significantly low hemoglobin level indicates iron-deficiency
anemia . You will also have tests to identify your blood group, Rhesus factor, and rubella immunity,
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 anomalies in the baby. Along with the
nuchal translucency first trimester screen, between 11 and 14 weeks, a
blood test can be done to measure the level of PAPP-A. This hormone test
helps determine risk for Down syndrome.
trimester nuchal translucency (with PAPP-A) is not done, a second
trimester quad screen is offered between 15 and 22 weeks. Together with
your age, race, weight, diabetic status and these four markers, a risk
of carrying a baby with Down syndrome, Trisomy 18, and neural tube
defects can be determined. The most common reason for a positive quad
screen is incorrect dating or multiple pregnancy.
Will I have a test for HIV?
All 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
immunodeficiency 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 since, if the result is positive, antiretroviral 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 percent to 2 percent.
For pregnant women
with HIV, 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 cesarean section or decide
that the levels are low enough to have a normal delivery.
After the delivery,
HIV-positive mothers are advised to bottle-feed, 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
How many scans will I have and when?
Frequency of ultrasounds performed in pregnancy varies by
practice. Some practices offer scans at each visit but most offer at
least one scan at 18–20 weeks. This is used for assessing fetal organs.
The heart's chambers, brain, kidney, bladder, diaphragm, and placental
site can be seen at this time. Measurements of the baby's head,
abdominal circumference, and thigh bone length complete the scan. A due
date is given based on measures.
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 since you need a full bladder (see drink a quart of water before a scan).
Ultrasound scans can be performed by a doctor, a midwife, or a
sonographer. You will lie on an exam table and need to wear something
that makes it easy to expose your belly. The person doing the scan puts
warm gel on the lower part of your belly, 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 doing 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
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 doctors offer to
print an image from the scan for you to take home. Although ultrasound
scans primarily are a 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. You may be able to take home a photo from the scan. Many
facilities encourage siblings to accompany mom and dad to see the new
How long do scans last?
The length of time an ultrasound scan takes differs depending on
the reason for the scan and the experience of the ultrasonographer. If
an early scan is performed at around 6–12 weeks, the technician takes
some basic measurements. This includes the measurement from the top of
the head to the tail bone, known as the “crown-rump” measurement, used
to calculate gestational age and due date. This scan takes around 15
The nuchal translucency scan,
during which the technician measures the fluid at the back of the
baby's neck, takes about 20 minutes. Anomaly scans, performed between 18
and 22 weeks, are detailed scans that take approximately 20–30 minutes.
During this procedure, the sonographer measures the baby and looks at
physical and structural development. The position of the placenta is
also examined and the amniotic fluid around the baby is measured.
Cervical length may also be assessed at this scan.
Another scan, the
doppler, measures blood flow within the umbilical vein and arteries and
as it enters and exits the fetal heart. If baby seems small for dates
and is not receiving adequate oxygen, doppler flow studies may give an
indication of the problem. This scan takes about 10–20 minutes.
A level ii ultrasound
can take 30–45 minutes and is indicated if genetic screening tests or
earlier ultrasounds are abnormal, if an anomaly is suspected, if baby is
small for dates, or if there is too much or too little fluid.
If your doctor has any
concerns about your baby's growth or well-being, additional scans may
be recommended to re-evaluate growth, fetal presentation, or placental
Do I have to have scans in pregnancy?
Most women are offered at least one ultrasound during their
pregnancy, but the choice to have one is yours. Most health-care
providers feel they can provide the safest care if they perform a scan
although performance of routine ultrasounds may not change pregnancy or
fetal outcomes. There are indications for some scans and your midwife or
doctor should explain the reasons for each.
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.
Do you have to drink a quart 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. Some
practices do transvaginal scans (a small ultrasound probe placed inside
your vagina) in the first trimester 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 than abdominal scans. You don't need a full bladder for the
18–20-week scan, since the position of the uterus has changed.
I'm pregnant through IVF. Will I have more scans than normal?
It's usual to have one extra scan in an IVF pregnancy, usually
done by the center where you had the 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 .
Although the main purpose is to reassure you that all is well, each
center keeps data on outcomes and successful in vitro attempts. Once
your pregnancy is confirmed, you will continue with routine prenatal
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, the position the baby is in,
and the position of his or her legs. Even if all of these factors are
favorable and the genitalia can be seen, there is an error factor, so
the information given about gender from a scan is never 100 percent
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 you may be wrong. If you have an amniocentesis,
the sex of the baby can be definitely identified. Most ultrasound
facilities have guidelines about revealing the gender of a baby. If
parents want to know and if gender can be reliably assessed, the results
are given. If there is some degree of uncertainty, the technician often
gives a percentage of probability. As gestational age advances,
accuracy improves. Usually, if a couple does not want to know, the
gender is not documented so the care provider does not know either.
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 3D and 4D ultrasounds),
and the detail in these can be very good. If you have a private scan,
you should check the expertise of the person performing the scan, and
check if the company has a referral policy to an appropriate medical
facility if anything abnormal is suspected, since 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 gives a clue to the possibility of twins, but only if
they are fraternal, or nonidentical.
Is everyone offered amniocentesis?
is a diagnostic test
that is routinely offered if you are over 35 and so have a higher
risk of having a baby with a chromosomal defect. Alternatively, you may
be offered the test if your family history suggests there may be a risk
of your baby having a 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 translucency showed a high risk of Down syndrome, 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 one percent 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
Can chorionic villus sampling cause miscarriage?
Chorionic villus sampling (CVS)
is another diagnostic test used to establish whether a baby has a
. Unfortunately, as with other invasive tests, this carries a risk
of miscarriage but it is thought to be less than one percent. Larger
centers performing many 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 syndrome
and other problems in a baby. It can also detect infection from
diseases such as toxoplasmosis. Additionally, cordocentesis is used to detect rubella infection,
as well as to perform a blood count on a baby that is suspected of
having anemia. From 18 weeks, the baby's blood is examined using a
sample of blood carefully extracted from the umbilical cord. The test is
performed in a similar way to that of amniocentesis, though results are
available within 72 hours. The risk of miscarriage is 1–2 percent.
Will I get weighed at my prenatal appointments?
Yes, this is still a routine assessment performed at each visit
though you can decline any aspect of testing or care during your
prenatal visit. If weight gain is an issue for you, you can ask that
this not be revealed to you.
Your BMI, a ratio
derived from your height and weight, has important implications for your
health and therefore for your baby. Lower weight gains may be
recommended for obese women whereas higher ones may be more suitable for
women with a low BMI. It is generally believed that weight gain in
excess of 40 lbs increases the risk of excessive fetal weight and its
attendant risks in labor and birth. Such weight is often more difficult
to lose in the postpartum period.
Rapid weight gain in the
later part of pregnancy may be an indication of edema (water retention)
and may help assess the need for further testing or monitoring.
Your weight gain
during the pregnancy is not a very good indication of your nutrition
levels, so ask your midwife or doctor for additional information and
advice on eating a balanced, healthy diet. A referral to a registered
dietician or nutritionist may be available too.
My friend is 27 and has had a baby with Down syndrome—is that unusual?
Although the risk or chance of having a baby with Down syndrome
increases with age, particularly over 35, the majority of babies with
Down syndrome are born to younger mothers. This is probably due to the
fact that more women have their babies younger. The risk of giving birth
to a baby with Down syndrome at the age of 20 years is 1 in 1,400. This
risk increases to 1 in 1,200 by the age of 25 and by the time the
mother reaches 35, the risk has increased to about 1 in 350.
My partner wants to rent a Doppler so we can listen to the baby's heartbeat. Is this a good idea?
In pregnancy, your health-care provider listens to the baby's
heartbeat with an instrument called a Doppler. Most providers use this
device 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 Doppler 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 special
training to find the heartbeat and many won't try to find the heartbeat
until the baby is around 10 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 rent a doppler, but it would be wise to
be aware of the anxieties that may accompany this decision.