The tremendous feelings a mother has for her child
growing inside her womb are impossible to describe. The experience is
different, yet wonderful, for every mother. The sensation many mothers and
fathers feel when they first glimpse live ultrasound images of their fetal
babies brings a fascinating reality and a new dimension to the parental
experience of pregnancy and childbirth.
Benefits and Limitations of Prenatal
Ultrasound
Modern obstetric medicine (for guiding pregnancy and childbirth) relies heavily on
ultrasound to provide detailed images of the fetus and uterus. Ultrasound is an excellent
exam for ruling out concerns. However, ultrasound is very operator-dependent. When an
experienced physician uses advanced ultrasound equipment, the exam can provide detailed
information on the fetus.
However, in some cases, prenatal ultrasound can miss some fetal abnormalities. On
average, one third to one half of fetal structural birth defects are not detected with
ultrasound. Less commonly, ultrasound can sometimes indicate a fetal abnormality when no
abnormality is present, causing stress and worry among the parents. A number of studies
have shown that ultrasound is most effective when performed by an experienced physician at
a major medical center. When there is an increased risk of genetic or chromosomal birth
defects, the physician may order additional testing, such as amniocentesis (sampling of
the amniotic fluid around the fetus) or chorionic villus sampling (CVS; sampling the
chorionic villi, small tissues that attach the pregnancy sac to the wall of the uterus) in
addition to ultrasound imaging.
If ultrasound is needed very early
during pregnancy, transvaginal ultrasound, rather than transabdominal ultrasound, is
typically performed. Transvaginal ultrasound is performed by inserting a probe into the
vagina. Early during pregnancy, the probe is able to get close to the uterus and is
helpful in visualizing the fetus. As the pregnancy progresses, transabdominal ultrasound
becomes more beneficial than transvaginal ultrasound.
Transabdominal prenatal ultrasound is
used to check on the development of the fetus and look for abnormalities. Prenatal
ultrasound may be used for the following:
- Determining the age of the pregnancy
- Determining whether the uterus is growing
faster or slower than expected and pinpointing the reasons why this
might be occurring
- Examining the baby's physical development and
functions such as breathing, heartbeat, excretion, and body movement
- Measuring amniotic fluid late in pregnancy
- Imaging the limbs and spinal column to check
for proper formation and growth
- Imaging the development of the brain and other
major organs
- Determining whether the pregnancy is ectopic
(occurring in the fallopian tubes instead of the abdomen)
- Determining whether there is a multiple
pregnancy (i.e., twins, triplets, quadruplets)
- Determining the due date of the pregnancy
- Confirming a possible miscarriage (warning
signs may include bleeding early in pregnancy or cessation of fetal
heartbeat and movement)
- Determining the cause of bleeding during the
second or third trimester (causes may include problems with the
placenta, which may require a cesarean delivery or special care)
- Diagnosing some birth defects, such as missing
limbs, spina defida (a gap in the spine), or malformations of the
urinary tract. A special type of ultrasound exam called echocardiography
images the blood flow through fetal heart chambers and major blood
vessels to help detect heart irregularities.
- Guiding other obstetric/prenatal tests, such as
amniocentesis (sampling fluid from the womb) or the sampling of cells
from the chorionic villus (vascular projection from the embryo that
attaches the pregnancy sac to the uterine wall).
- Guiding prenatal surgery and the safe delivery
of medications to the fetus
- Determining whether a cesarean delivery is needed (instances may include when the fetus
is especially large or in an abnormal position, or when the placenta is blocking the exit
from the uterus)
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Ultrasound image of umbilical cord. Click on image to view larger
version. |
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Ultrasound image of intracranial anatomy and facial features. Click on
image to view larger version. |
Information obtained from prenatal
ultrasound exams can help determine prenatal care, thereby increasing a woman’s
chances of delivering a healthy baby. Certain problems detected by ultrasound, such as
fetal heart rhythms abnormalities or fetal urinary tract blockages, can be treated with
medications or surgery before the woman gives birth.
How Prenatal Ultrasound is Performed
Transabdominal fetal ultrasound is usually a straightforward process that is quick and
painless; a typical routine exam (called a basic or level 1 exam) takes approximately 15
to 20 minutes. The woman is usually examined while lying on her back with her belly
exposed. First, a jelly-like solution is applied to the skin to help improve the contact
between the ultrasound transducer and the skin, allowing clearer images of the fetus. The
woman may also be asked to have a full bladder during the ultrasound exam to help with
image clarity.
To perform the exam, the ultrasound
technologist passes a hand-held transducer across the woman’s belly. Inaudible, high
frequency sound waves are emitted by the ultrasound transducer and as these sound waves
pass into the body, they are reflected back at different strengths and frequencies by the
different anatomic structures of both the mother and the fetus. The same transducer
receives these reflected sound waves and a computer reconstructs them into real time
images.
During the examination, the ultrasound "movie" can often be recorded to
videotape or the movie can be frozen (similar to viewing a videotape in "freeze
frame" mode) and a still image can be recorded onto paper or film. Typically, moving
ultrasound images (images being acquired and displayed at multiple frames per second) give
a much better appreciation of the structure of the fetus or other anatomy being imaged
than a still image.
Types of Transabdominal
Prenatal Ultrasound Exams |
| Basic
or Level I Exam Routine exam (15 to 20 minutes) to determine pregnancy dates,
location of placenta, and check for birth defects. |
Targeted
or Level II Exam A longer exam (30 minutes to several hours) that typically uses
more advanced ultrasound equipment to further investigate a fetal abnormality. Three
dimensional ultrasound is sometimes used during a targeted exam. |
Panoramic Prenatal
Ultrasound
A recent advancement in the field of
ultrasound is the ability to scan a panoramic view of the fetus. As the transducer is
moved along a part of the body, the image continues to grow, showing the entire scanned
area (instead of just showing the area currently being scanned by the transducer). This
"panoramic landscape view" gives the obstetrician an appreciation for both fine
detail and the "big picture."
 |
Panoramic
ultrasound image of triplet gestation at 11 weeks. Click on image to view larger version.
|
Three-Dimensional (3D) Prenatal
Ultrasound
Three-dimensional (3D) ultrasound is
another recent advance that can produce 3D images of the fetus that are as detailed as a
photograph. This type of imaging may be used during targeted (also called level II) exams
when physicians are examining a particular fetal abnormality. Currently, the 3D ultrasound
technology is not available on a widespread basis.
 |
3D ultrasound image showing soft tissue of the fetal face. Click on image
to view larger version. |
