Mike Wlydes, Perinatal Institute
This area of the site is designed to inform
pregnant women about ultrasound scanning in pregnancy. I
am a consultant obstetrician & gynaecologist
with an interest and background in obstetric
ultrasound. I have been a consultant in a large
maternity hospital in Birmingham UK since 1994. I
have tried within the text to give an account
of my experiences, which I hope are shared by
the majority of people undertaking ultrasound
scans in pregnancy.
The text is divided into the different types
of scans undertaken at different times during
pregnancy.
Early pregnancy assessment scans
These scans are done following a problem, usually
either vaginal bleeding or abdominal pain. These
symptoms are often the first signs of miscarriage
or sometimes ectopic pregnancy, and the ultrasound
scan is very helpful in distinguishing between
different situations.
Scanning can be done in 2 ways in early pregnancy. Firstly
an abdominal scan, where the woman is asked to
fill up her bladder as much as possible, and
gel is applied to the maternal abdomen to achieve
images of the pelvic organs. Often this is helpful
but frequently the increased image quality of
a vaginal scan is required to make the diagnosis. A
vaginal scan involves a specialy designed ultrasound
probe being placed within the vagina to look
at the uterus and ovaries more closely. The
scan doesn’t hurt, and it cannot cause any damage
or injury to the pregnancy.
The common outcomes from an early pregnancy
assessment scan are:
- Normal on-going intra-uterine pregnancy. More
than 90% of pregnancies seen to have a fetal
heart beat present on ultrasound will end with
a healthy baby. A few will still miscarry,
but there is no proven benefit to bed rest
or other interventions. The best advice is
to do what feels right, but many women will
wish to continue with normal activities, although
vigorous physical exercise may be best avoided
until bleeding has settled.
- A miscarriage. If a woman is more than 6
or 7 weeks pregnant it is usually possible
to see a normal pregnancy within the uterus
using vaginal ultrasound. Ultrasound scanning
can sometimes be certain that a pregnancy has
miscarried, and sometimes the situation is
unclear. If there is any doubt then further
scanning 2 weeks later will give a normal pregnancy
a chance to declare itself.
- Ectopic pregnancy. A pregnancy outside the
uterus, usually in the fallopian tube can make
the pregnancy woman ill, sometimes very ill. Ectopic
pregnancies can mimic miscarriage with vaginal
bleeding, they may case abdominal pain or they
may give no symptoms. If a pregnancy is advanced
past 6 weeks and the pregnancy test is positive,
and the uterus looks empty on ultrasound scan
then an ectopic pregnancy is possible. It
may be necessary in some cases to undertake
an operation called a laparoscopy to inspect
the tubes directly if an ectopic pregnancy
is suspected.
The
11 to 13 week scan
This scan has been developed over the last 5
to 10 years to screen for fetal anomalies in “normal” pregnancies. The
first part of the scan assessment is a general
look around the structures of the baby, the arms
and legs, hands and feet, heart, stomach, bladder
and head are usually visible. Movements are
seen and the position and texture of the placenta. The
second part of the scan is to measure nuchal
translucency (NT). NT is fluid in the skin
at the back of the fetal neck. All fetuses have
some nuchal fluid, but some have more than others. It
has been found from studying thousands of cases
that the measurement of NT can give a risk for
Down’s syndrome. Recently the addition of a
first trimester blood test has made screening
for trisomy 21 more accurate. Using both the
blood test and the nuchal translucency scan together
it is possible to identify 85% of cases of Down’s
syndrome. Nuchal translucency scanning is currently
not available widely within the National Health
Service. Further information on having a nuchal
test is available from www.nuchal.info.
A
Dating scan
A dating scan aims to make measurements which
accurately predict the due date of the baby. This
can be done from 8 to 20 weeks, but the test
is probably more accurate before 14 weeks than
afterwards. The dating scan does not aim to
assess fetal anomalies, and can be completed
fairly quickly. Dating scans can be done using
relatively unsophisticated ultrasound equipment,
sometimes these scans can be done in GP surgeries,
or other community settings.
Mid pregnancy anomaly scanning
The most common time to do a fetal anomaly scan
has been 20 weeks (between 18 and 22 weeks). The
purpose of scanning can be considered under a
number of headings:
Confirming everything is OK
Most scans show a normal baby, and having this
reassurance is what most women want from their
scan. However scanning cannot pick up all problems
and you should be aware that about half of significant
problems cannot be identified with a scan.
Diagnosing serious fetal
anomalies
The person undertaking your scan will be going
through a mental checklist of normal and abnormal
findings. Many of the problems that are diagnosed
during pregnancy are serious, and in many cases
the only option to consider is whether to have
a termination of the pregnancy, or continue the
pregnancy. Many pregnant women at 20 weeks would
not consider a termination of the pregnancy,
and for these people having a scan should be
considered carefully, and possibly to enjoy the
pregnancy until the problem comes to light.
Minor problems and variations
of normal
It is quite common for minor problems to be
suspected on ultrasound scanning which have little
or no impact on the health of a child. Being
told there is a problem on scanning is always
stressful, and even with reassurance the anxiety
levels for the rest of the pregnancy are increased. Some
things which are seen on scanning increase the
risks of Down’s syndrome, and it may be that
the result of the scan is a discussion with you
about having additional tests done (such as amniocentesis). Scan
findings which increase the risk of Down’s syndrome
are called “markers”.
To get a picture and tell
the sex of the baby
These objectives seem to be quite common amongst
pregnant women, but neither are especially important
from the sonographer’s point of view. It is
often possible to tell the sex of baby at 20
weeks, at least 70%, but this will only be correct
95% of the time. Many sonographers find the
issue of fetal sexing difficult, it seems to
demonstrate that the woman involved does not
understand the purpose of the scan, and in some
extreme cases women have been reported to have
undergone termination of pregnancy based on the “wrong” sex
being found. Many departments operate a policy
of not telling women the sex of their baby, even
if it is known, but most units are happy to tell
women the sex of their unborn child if it does
not prolong the examination and the couple are
able to cope with the news. These decisions
are partly a matter of policy, but also of individual
judgement of the professionals involved in providing
the scanning service.
Other
findings on a 20 week scan
The position of the placenta can be diagnosed
on ultrasound
About 20% of placentas at 20 weeks have some
element within the lower segment of the uterus. 1%
will ultimately end up being a problem requiring
caesarean section (placental praevia). Many
centres have abandoned repeating ultrasound examination
for these cases, but in circumstances where the
placenta seems to be entirely within the lower
uterus, and potentially blocking the way out
for the baby it may be best to reassess in late
pregnancy.
Growth Scans
Sometimes it is thought necessary to follow
the growth of a baby during pregnancy. This
is usually undertaken from 28 weeks onwards,
and the scan measures the abdominal circumference
and head circumference to assess the baby’s size. These
scans can also look at the amount of liquor (amniotic
fluid) around the baby, and assess the blood
flow through the placenta or within the baby. These
additional tests are often important in trying
to decide whether there is a problem with the
baby, rather than just measuring. There is no
benefit to routine scanning in the last 3 months
of pregnancy, but for complex pregnancies, such
as twins it can be helpful.
Advice
for mothers to be
As you decide whether to have an ultrasound
scan you need to consider the good, and the bad
that might come from it. Most scans are normal,
and that is reassuring, but if your baby has
a major problem then the scan might allow you
to consider a termination of the pregnancy, but
this would always be your choice, even if the
problems are so serious that the baby can’t survive. If
minor problems are identified you risk being
very worried, and maybe having additional tests
done, when everything is perfectly normal. The
information on these pages helps to define the
risks of particular problems being present, or
being diagnosed on scan. The issue of whether
you should have a scan is an individual issue
for you to consider.
