Even at twenty weeks, some abnormalities may not be clearly seen, and could be picked
up at later scans when the fetus is bigger. To get around this problem, when one
or more of the vital structures (such as the heart or spine) is not clearly seen,
the mother will probably be advised to have a repeat scan in two to four weeks, to
ensure everything is clearly seen.
What is the use of early (first trimester) scanning?
This may help to:
Confirm pregnancy (where there is doubt)
Confirm location of pregnancy (where there is possibility of ectopic pregnancy)
Confirm and document gestational age (where dates are uncertain)
Establish the number of fetuses
Rule out or diagnose abnormal pregnancies (such as blighted ovum or hydatidform
What is a blighted ovum?
This is where there is a gestation sac with fluid but no embryo (fetus). It is also
known as an anembryonic pregnancy. It is believed that things actually go wrong soon
after conception and no embryo develops. Light vaginal bleeding at around six to
ten weeks of gestation is usually the first warning. An ultrasound scan will confirm
the diagnosis. (This is further discussed in Chapter 3, "Early pregnancy loss".)
Can the ultrasoundscan help in diagnosing fetal anaemia?
Not directly. If this condition is suspected and if the obstetrician sees fit to
confirm the diagnosis, the scan may be used as an imaging tool while getting a blood
sample from the cord for analysis.
Blood sampling can also be obtained for the purpose of making diagnosis in cases
of suspected infection or chromosomal or genetic abnormalities.
Ultrasound: What can be seen
Can an ultrasound scan tell whether twins are identical or not?
Only if they share the amniotic sac or placenta (in which case, they are identical).
If each twin is in a separate sac and has a separate placenta, these could be either
identical or non-identical, and the scan cannot tell whether they are identical or
not. Of course, if they are of different sex, then it is obvious they are non-identical.
Is it possible to tell the sex of the fetus in the first trimester by using ultrasound
No. Probably the earliest you can tell with confidence is around sixteen weeks of
What is a yolk sac?
In the first trimester, on the ultrasound picture, there appears a distinctive ring-like
image within the gestational sac, adjacent to the fetus. This is the structure that
is responsible for feeding the fetus until the placenta takes over, at about ten
weeks of gestation.
The yolk sac can be seen on the scan quite early, even before the fetal heartbeat
At what gestation is a fetal heartbeat visible on the ultrasound scan?
With a trans-vaginal probe, at just over 5 weeks. If a trans-abdominal probe is used,
it may be up until a week later, at six weeks, before it is seen.
What are the advantages of a trans-vaginal scan (TVS) over a trans-abdominal one?
The image with a TVS is sharper and clearer. More detail can be discerned and some
subtle things that might be missed using a trans-abdominal probe may be easily picked
up. Moreover, while a trans-abdominal scan in early pregnancy requires a full bladder
for a good image, a TVS does not.
A trans-vaginal scan also circumvents the problem of obesity in the mother, which
tends to prevent good images - full bladder or not - if using a trans-abdominal probe.
In early pregnancy, many units use the trans-vaginal route for ultrasound scanning.
However, both routes may sometimes be necessary to discern things properly. The transvaginal
route is not necessary beyond the first trimester of pregnancy, except in special