Pregnancy
& Ultrasound
Article
Introduction
Ultrasound is an ubiquitous technology
in all areas of medicine but many people are either ignorant of
the nature of the technology or have a misguided notion of it. It
is not unusual to find people putting it in the same bracket as
x-ray. This misguided impression can, quite understandably evoke
significant anxiety especially where pregnancy is concerned. Let’s
start by stating a simple but important act. Ultrasound does
not involve ionising radiation.
Ultrasound
is a form of sound waves generated at very high frequency. This
will be in the range of 3.5 to 7 milion cycles per second (Megahertz).
Normal sound waves audible to the ear are at a much lower frequency
of less than 20 thousand cycles per second.
How
Ultrasound Works
The
sound waves are produced by a crystal built in the probe used for
the test. The waves are then reflected from the various structures
they encounter as they travel through the body. The data produced
by the reflected waves is then processed by the computer in the
scanning machine to produce an image on the screen representative
of the reflecting structures. That is how the detailed image of
the pregnant uterus and baby within, complete with movements, is
produced and seen in real time.
The
conventional position for an obstetric ultrasound is on your back,
usually propped up comfortably to allow you a clear view of the
monitor screen. A clear, water-based conducting gel is applied to
the abdomen. The gel helps transmit the sound waves. A hand-held
probe is then moved over the area in a systematic way.
Alternatively,
the scan is performed with the probe placed in the vagina (transvaginal
ultrasound scanning). This technique often complements conventional
ultrasound techniques by providing better detail especially in the
very early phase of a pregnancy. A woman will normally be advised
accordingly if this is deemed to be the more appropriate route or
sometimes as an additional measure if the abdominal route has not
provided all the required information.
A
full bladder is necessary to get a good picture. Therefore, the
person due to be scanned will be asked to drink a few glasses of
water (or similar) an hour or so before the test and to refrain
from urinating before the procedure. A full bladder is, however,
not necessary for the transvaginal Ultrasound.
There
may be some slight discomfort from pressure on the full bladder
otherwise the procedure is perfectly painless. The conducting gel
may feel slightly cold and wet. There is no perception of the sound
waves and of-course you cannot hear them.
When
to have a Scan
Scans
may be performed in the First Trimester
to:
- Confirm
a normal pregnancy
-
Confirm viability
- Assess
the baby's age (gestation)
- Assess
potential problems such as threatened miscarriage, ectopic pregnancy
etc.
- Assess
the baby's heartbeat
- Confirm
number of embryos/fetuses
- Identify
any abnormalities of the placenta, uterus, and other pelvic structures
In
the Second Trimester (after 13 weeks)
- Assess
the baby's age, growth, position, and sometimes gender.
- A
detailed look at the baby’s vital organs and limbs to identify
or rule out any possible congenital problems.
- Identify
any developmental (growth) problems
- Rule
out multiple pregnancies
- Evaluate
the placenta, amniotic fluid, and other pelvic organs.
In
the Third Trimester (after 26 weeks):
Scans not normally required at this stage except when monitoring
pregnancy development and fetal wellbeing in special situations:
these may include:
-
Multiple pregnancy
-
Suspected fetal growth problems
-
Chronic medical conditions such as diabetes, hypertension, epilepsy
etc.
-
Pre-eclampsia
Other situations where a scan may be called for are when there
is a need to:
-
Detect cervical changes that might predict preterm labour.
- Verify
breech presentation or other uncommon fetal or cord position before
delivery.
- Get
valuable information leading to treatment of any conditions arising
in the course of a pregnancy in a bid to improve a woman’s
chances of having a healthy baby.
Scan
Aided Tests and Procedures
Pregnancy
ultrasound plays a crucial role in antenatal care. Some tests which
are routinely done for diagnostic purposes are directly ultrasound
guided. Such tests include:
Amniocentesis:
This
is the most well-known and commonest invasive test used to make
a diagnosis of any condition that may be affecting the fetus. It
is commonly performed in the early second trimester (15 –
16 weeks) and is now routinely ultrasound guided.
An
amniocentesis is a test in which the cells that are floating in
the fluid surrounding the fetus are examined for the chromosomal
pattern of the fetus. The operator will have the ultrasound probe
on the abdomen and will guide the fine needle used under direct
ultrasound guidance into the womb. Once the tip is seen to be inside
a pool of amniotic fluid, a sample will be drawn for the test. Normally
this takes a few short minutes and in the vast majority, only one
attempt is sufficient.
Amniocentesis results are usually available two to three weeks after
the test. Amniocentesis carries a risk of losing the pregnancy of
about 0.5 - 1% (1 in 100-200). Age is probably the commonest indication
for requesting amniocentesis usually for the diagnosis of Down’s
Syndrome. This is because, all other factors being normal, the older
the mother, the higher the risk of Down’s and other chromosomal
disorders. A 21 year old mother has a 1:1700 risk of having a baby
with Down’s Syndrome, a 30 year old 1:950, at 40 the risk
is almost 1:100 and by 45 years of age it is 1:30.
As mentioned earlier while these statistics refer to the most recognized
chromosomal anomaly, there are other anomalies, some of them incompatible
with life. It is therefore important for any prospective mother
to be aware that the overall risk of all chromosomal anomalies is
significantly higher than that of Down’s alone at any stage
of life. A 201 year old will therefore have an overall chromosomal
abnormalities risk (for her baby) of about 1:500 and at 40 the risk
would have risen to 1:66.
CVS:
This stands for Chorionic Villus Sampling which simply means taking
a biopsy from the developing placenta. It is also ultrasound guided.
The procedure is also performed under direct ultrasound guidance.
It differs from amniocentesis in three major aspects:
-
It can and is usually performed earlier in the pregnancy. This
is towards the end of the first trimester.
-
Results are available much more rapidly with a normal turnaround
of 48 – 72 hours.
-
It has a significantly higher procedure associated miscarriage
rate which could be as much as twice that of amniocentesis.
Fetal
Echocardiography:
Congenital heart defects occur in about 1% of live births. Causes
are multiple and sometimes cannot be identified. Whilst a routine
detailed ultrasound scan performed at 18-20 weeks will focus on
the heart among other major organs, not all defects will be detected.
It is also sometimes necessary to perform a much more detailed study
of the baby’s heart. This specialised structural study using
ultrasound is called echocardiography or popularly just shortened
as ‘echo’. Situations where this may be recommended
include:
-
Afamily history of congenital heart disease
- An
abnormal fetal heart rhythm detected during routine examination.
- Fetal
heart abnormalities detected during a routine pregnancy ultrasound
scan
- Abnormality
of another major organ system such as the gut or lungs.
- Insulin-dependent
(type 1) diabetes
- Exposure
to some drugs in very early pregnancy. For example, some anticonvulsants
used to treat epilepsy can damage the developing heart.
- If
the mother has abused alcohol or drugs during pregnancy
- If
a mother has diabetes, phenylketonuria, or a connective tissue
disease such as lupus
- If
the mother has had rubella during pregnancy
- Diagnosis
of some chromosomal abnormaities
Cordocentesis
Sometimes it becomes necessary to obtain a sample of blood from
the umbilical cord of the unborn baby, usually for diagnostic purposes.
This requires direct ultrasound visualization. This would simply
not be feasible without the aid of ultrasound. In some condition,
transfusion of blood into the unborn baby is performed. Again, this
is directly dependent on ultrasound.
There are many other areas where ultrasound in pregnancy is not
only useful but indispensable. These include localization of the
placenta which could help determine the safest mode and timing of
delivery, checking pattern of blood flow in the cord, a useful feature
in monitoring fetal well-being in at risk fetuses, estimating fetal
weight which could be useful in making important decisions and many
more.
Biophysical
Profile
Sometimes there is concern about the unborn baby’s wellbeing.
This may be because the baby is not moving well, growth is unsatisfactory
or the pregnant mother is suffering from such conditions as pre-eclampsia,
diabetes etc. It may, in such circumstances, be deemed necessary
to monitor the baby closely and one of the strategies sometimes
adopted is to perform what is known as a biophysical profile >
With this one checks the baby’s movements, general muscle
tone, breathing movements and fetal heart pattern. Apart from the
latter, the rest are all done by using ultrasound.
Trans-vaginal
Ultrasound scanning
The idea of having the scan performed vaginally can and does provoke
anxiety for some women. It is important to say that this is a safe
procedure and does not in any way pose a risk to either mother or
baby.
In very early pregnancy, this may be the preferable method of scanning
as it produces sharper details and may give crucial information
otherwise unobtainable (using trans-abdominal scanning).
It is also the case that a trans-vaginal scan does not require a
full bladder, not the most comfortable of requirement especially
for an expectant mother struggling with perpetual nausea.
A
vaginal scan also circumvents the difficulties with obtaining a
good view of the embryo/fetus in early pregnancy for women who are
overweight or obese.
Amniotic
Fluid
Ultrasound scanning is quite useful in estimating the amniotic fluid
volume. This is easy to do and in most cases this will be normal.
However, where the volume is found to be reduced or increased, it
could be a very important gauge of the baby’s wellbeing. It
may also be the first warning that something is not quite right
with the baby and could trigger further, more specific, tests.
Serial
ultrasound monitoring of fluid volume is sometimes used to monitor
the progress of a pregnancy and together with other tests could
be used to make a decision on the timing and/or mode of delivery.
Placental
location
It
is not possible through physical examination to tell the exact location
of a placenta in the womb. For the majority of pregnant women this
is really of no importance. However, about 1 in 50 mothers (2%)
will have placenta praevia (low-lying placenta) at term. This is
a potentially dangerous condition for both mother and baby and most
of those affected will need close antenatal observation and delivery
by caesarean section. Ultrasound scanning does accurately locate
the placenta and aid in making these crucial decisions.
It is important to stress that almost one in five women will be
found to have a ‘low-lying’ placenta at their 20 weeks
pregnancy scan. This is no cause for panic as for the vast majority
of them, the placenta will be normally located by the time they
get to the third trimester. Many units offer a repeat ultrasound
scan at around 32-34 weeks as a form of reassurance.
Placental
abruption
Placenta
praevia (above) is not to be confused with Placental abruption (abruption
placenta). The two conditions are not related even though for both
the main concern is severe haemorrhage that is associated with them.
In placental abruption, the placenta gets partly or wholly detached
from the womb. That will cause bleeding and in most cases, pain.
The extent of the bleeding and therefore its effect on the baby
will depend on the degree of placental detachment. Ultrasound scanning
has a limited role in placental abruption. The diagnosis is mainly
secured on a clinical presentation rather than ultrasound findings.
Twins
and ultrasound scanning
In days gone by, it was not unusual for twins and rarely other forms
of multiple pregnancy not to be discovered until the time of delivery.
Ultrasound changed all that. It is exceedingly unusual for that
to happen in modern times in most developed countries. Over 95%
of mothers in the United Kingdom have antenatal ultrasound scans
and the figure is just over 70% in the USA. Even in those countries
where ultrasound scans are not offered as a matter of course, the
obstetrician or midwife looking after the pregnant woman will suspect
the presence of multiple pregnancy in the course of examining her
abdomen. This will usually trigger a recommendation for an ultrasound
scan. Confirming the number of fetuses is then, in most cases, a
straight-forward affair.
Identical
or not?
Many prospective mothers carrying twins are anxious to know whether
the babies are identical or not. In many (but not all) cases it
is possible to tell whether they are or not using ultrasound scanning.
If the twins are sharing the same gestation sac or even if they
are in different sacs but sharing the same placenta, they are then
definitely identical. This is, however, not that common (and a good
thing too).
Most twins have separate placentas and sacs. These could be identical
or non-identical. It is sometimes possible to tell with a fair degree
of confidence whether such twins are identical or not depending
on the timing of the ultrasound scan. Of-course if the twins are
of different sex, that is confirmatory that they are non-identical.
However, it is only possible to tell the gender of a fetus after
about 16-17 weeks. The babies have also got to be in a favourable
position to be able to do this.
Sharing
a sac or placenta not good?
This
is not the best form for twins as there are significant potential
complications unique to this type of twins. Some of these complications
can be life-threatening for the one or both babies. The subject
is discussed further in the section on multiple pregnancy.
Doppler Ultrasound
When there are problems or suspected complications, this special
type of ultrasound may be employed to try to get important information
which may aid in decision making. It takes its name from the 19th
century Austrian physicist Christian Doppler who was the first to
describe the phenomenon. So, what is it?
Doppler ultrasound is a form of ultrasound that can detect and measure
blood flow. Doppler ultrasound depends on the Doppler effect, a
change in the frequency of a wave resulting here from the motion
of a reflector, in this case the red blood cells.
If you are keen to know what the various types of Doppler Ultrasound
are, here we will attempt to explain. It is not a walk in the park.
Don’t say you haven’t been warned.
-
Colour Doppler -- This technique estimates the average velocity
of flow within a vessel by colour coding the information. The
direction of blood flow is assigned the colour red or blue, indicating
flow toward or away from the ultrasound probe (transducer).
- Pulsed
Doppler -- This method allows a sampling volume or "gate"
to be positioned in a vessel visualized on the grey-scale image,
and displays a graph of the full range of blood velocities within
the gate versus time. The amplitude of the signal is approximately
proportional to the number of red blood cells and is indicated,
not in colour, but simply as a shade of grey.
- Power
Doppler -- This device depicts the amplitude, or power, of Doppler
signals rather than the frequency shift. This allows detection
of a larger range of Doppler shifts and thus better visualisation
of small vessels, but at the expense of directional and velocity
information.
Color
Doppler depicts blood flow in a region and is used as a guide for
the placement of the pulsed Doppler gate for more detailed analysis
at a particular site.
Doppler ultrasound has many applications including, for example,
the detection and measurement of decreased or obstructed blood flow
to the legs as in suspected DVT. Colour Doppler ultrasound is done
first to evaluate vessels rapidly for abnormalities and to guide
placement of the pulsed Doppler to gain sample volume for detailed
analysis of velocities.
The bottom line:
It is very difficult to imagine modern obstetrics without ultrasound.
It is an integral part of the care of the pregnant mother and her
unborn baby and there is no doubt at all that millions of people
across the globe arguably owe their lives to this technology which
is now taken as a matter of course.
Twins
and other forms of multiple pregnancy
Frequency
About 1% of spontaneous conceptions among the Caucasian population
will be twins or other less common forms of multiple pregnancy.
The rate is slightly higher for those of black African ancestry
and somewhat lower for Japanese and others of oriental ancestry.
The rate of multiple pregnancy in assisted conception such as IVF
is significantly higher.
For
any individual woman, family history is a significant factor. If
a woman is a twin herself, she has a much higher chance than average
of having twins herself. The same applies to a woman who has first
degree twin relatives i.e. siblings or parents.
The
rate of identical twins is however constant and is not influenced
by age, race, family history or any other factors. Monozygotic twins
is another term to describe identical twins. The rate is roughly
4 per 1,000 births.
Twin
Issues
Twins may be exciting but these pregnancies also have their own
specific challenges. It is important to stress up-front that the
majority of twin pregnancies, upwards of 85%, have completely successful
outcome and dispel the impression that twin pregnancy is automatically
bad news.
That done, lets turn to the common saying, which happens to be largely
true, that doctors and midwives like repeating when speaking about
twin pregnancies. The saying goes “all pregnancy complications
are commoner in twins apart from prolonged pregnancy”. That
is an important and honest starting point. So, when you talk of
prematurity, pre-eclampsia, placental abruption, growth restriction,
antepartum haemorrhage, post-partum haemorrhage, it is a true general
statement that the risk of any of these is higher, the bigger the
number of babies a woman is carrying.
Unique
Twin Issues:
Increased
risk of pregnancy complications is not the only issue that needs
to be dealt with. There are problems that are unique to twin pregnancy.
The most important one of these is twin-to-twin transfusion. This
condition occurs only in identical (monozygotic twins) and specifically
those who share a placenta. The medical term for a common placenta
in twins is monochorionic. It is true that twins that share a placenta
have got multiple blood vessel communications within that placenta.
These communications are not always a problem and in many if not
most cases there is no problem as there is physiological equilibration
of the blood flow in both. However, for some, these communications
are a problem which is sometimes lethal for one or both. This is
brought about by an imbalance in the blood flow whereby one ends
up as a ‘donor’ and the other a ‘recipient’.
It means there is net flow of blood to the recipient and this is
the one at greatest risk as he/she gets overloaded.
Can
anything be done?
When
twin-to-twin transfusion is suspected or diagnosed, the care becomes
a specialist issue. There will be serial ultrasound scanning to
track the progress of the condition of the twins. Other tests will
also be done to keep a close eye on the babies. Sometimes intervention
is required to try to reduce severity of the condition. In most
cases, delivery is early to try to prevent further deterioration.
Not all cases of twin-to-twin transfusion are that severe. Some
are mild enough not to require any intervention.
Vanishing
twin
It
is true but probably not widely recognised that twin pregnancies
in the second and third trimester represent at most two third of
pregnancies that start as such. What does this mean? Basically it
means, a lot more pregnancies start as twins. For a variety of reasons
some embryos in twin pregnancies do not make it beyond the first
few weeks and these pregnancies continue as singleton pregnancies.
Some of these women would have had early ultrasound scans say at
6 or 7 weeks. Presence of twins would have been identified then
and a repeat scan a few weeks down the line a repeat scan shows,
not two, but one fetus. The other twin has ‘vanished’.
As mentioned earlier, this phenomenon is estimated to affect up
to a third of all twins that start as such. Mercifully, this vanishing
occurs fairly early before the first scan for most affected mothers
and therefore these will be oblivious of the fact that they started
off with twins. The loss of the one twin is usually accompanied
by very mild or no noticeable symptoms.
| Where
can I get further help? |
For
further
pregnancy
related questions please visit
the authoritative book by Consultant Obstetricians available online.
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