What are Obstetric Ultrasound Scans?

Obstetric Ultrasound is the use of ultrasound scans in pregnancy. Since its introduction in the late 1950’s ultrasonography has become a very useful diagnostic tool in Obstetrics. Currently used equipments are known as real-time scanners, with which a continous picture of the moving fetus can be depicted on a monitor screen. Very high frequency sound waves of between 3.5 to 7.0 megahertz (i.e. 3.5 to 7 million cycles per second) are generally used for this purpose. They are emitted from a transducer which is placed in contact with the maternal abdomen, and is moved to "look at" (likened to a light shined from a torch) any particular content of the uterus. Repetitive arrays of ultrasound beams scan the fetus in thin slices and are reflected back onto the same transducer. The information obtained from different reflections are recomposed back into a picture on the monitor screen (a sonogram, or ultrasonogram). Movements such as fetal heart beat and malformations in the feus can be assessed and measurements can be made accurately on the images displayed on the screen. Such measurements form the cornerstone in the assessment of gestational age, size and growth in the fetus.

Why and when is Ultrasound used in Pregnancy?

Ultrasound scan is currently considered to be a safe, non-invasive, accurate and cost-effective investigation in the fetus. It has progressively become an indispensible obstetric tool and plays an important role in the care of every pregnant woman. The main use of ultrasonography are in the following areas:
1. Diagnosis and assessment of early pregnancy. The gestational sac can be visualized as early as four and a half weeks of gestation and the yolk sac at about five weeks.
2. Threatened miscarriage. The viability of the fetus can be documented in the presence of vaginal bleeding in early pregnancy. Fetal heart motion is usually clearly depictable by 7 weeks. If this is observed, the probability of a continued pregnancy is greater than 97 percent. Missed abortion and blighted ovum will usually give typical pictures of a deformed gestational sac and absence of fetal poles or heart beat. Ultrasonography is also indispensible in the early diagnosis of ectopic pregnancies and molar pregnancies.
3. Determination of gestational age and assessment of fetal size. Fetal body measurements reflect the gestational age of the fetus. This is particularly true in early gestation. In patients with uncertain last menstrual periods, such measurements must be made as early as possible in pregnancy to arrive at a correct dating for the patient. In the latter part of pregnancy measuring body parameters will allow assessment of the size and growth of the fetus and will greatly assist in the diagnosis and management of intrauterine growth retardation (IUGR).
The following measurements are usually made:
a) The Crown-rump length (CRL) This measurement can be made between 7 to 13 weeks and gives very accurate estimation of the gestational age. Dating with the CRL can be within 3-4 days of the last menstrual period. (Table)
b) The Biparietal diameter (BPD) The diameter between the 2 sides of the head. This is measured after 13 weeks. It increases from about 2.4 cm at 13 weeks to about 9.5 cm at term. Different babies of the same weight can have different head size, therefore dating in the later part of pregnancy is generally considered unreliable. (Chart and further comments)
c) The Femur length (FL) Measures the longest bone in the body and reflects the longitudinal growth of the fetus. Its usefulness is similar to the BPD. It increases from about 1.5 cm at 14 weeks to about 7.8 cm at term. (Chart and further comments)
d) The Abdominal circumference (AC) The single most important measurement to make in late pregnancy. It reflects more of fetal size and weight rather than age. Serial measurements are useful in monitoring growth of the fetus. (Chart and further comments) Other important measurements are discussed here. The weight of the fetus at any gestation can also be estimated with great accuracy using polynomial equations containing the BPD, FL, and AC. Lookup charts are readily available. For example, a BPD of 9.0 cm and an AC of 30.0 cm will give a weight estimate of 2.85 kg. (comments)
4. Placental localization. Ultrasonography has become indispensible in the diagnosis or exclusion of placenta previa, and other placental abnormalities as in diabetes, fetal hydrops, Rh isoimmunization and severe intrauterine growth retardation .
5. Multiple pregnancies. In this situation, ultrasonography is invaluable in determining the number of fetuses, the chorionicity, fetal presentations, evidence of growth retardation and fetal anomaly, the presence of placenta previa, and any suggestion of twin-to-twin transfusion.
6. Hydramnios and Oligohydramnios. Excessive or decreased amount of liquor (amniotic fluid) can be clearly depicted by ultrasound. In both these situations, careful ultrasound examination should be made to exclude intraulterine growth retardation and congenital malformation in the fetus such as intestinal atresia, hydrops fetalis or renal dysplasia. See FAQ and comments.
7. Fetal malformation. Many structural abnormalities in the fetus can be reliably diagnosed by an ultrasound scan, and these can usually be made before 20 weeks. Common examples include hydrocephalus, anencephaly, myelomeningocoele, achondroplasia and other dwarfism, spina bifida, exomphalos, duodenal atresia and fetal hydrops. With more recent equipments, conditions such as cleft lips/palate, congenital cardiac abnormalities and Down syndrome are more readily recognised. Markers for chromosomal abnormalities such as the fetal nuchal translucency (the area at the back of the neck) have also been defined to enable detection of these abnormal fetuses. Ultrasound can also assist in other diagnostic procedures in prenatal diagnosis such as amniocentesis, chorionic villus sampling, percutaneous umbilical blood sampling and in fetal therapy.
8. Other areas. Ultrasonography is of great value in other obstetric conditions such as:
a) confirmation of intrauterine death.
b) confirmation of fetal presentation in uncertain cases.
c) evaluating fetal movements, tone and breathing in the Biophysical Profile.
d) diagnosis of uterine and pelvic abnormalities during pregnancy e.g. fibromyomata and ovarian cyst.

The Schedule



There is no hard and fast rule as to the number of scans a woman should have during her pregnancy. A scan is ordered when an abnormality is suspected on clinical grounds. Otherwise a scan is generally booked at about 7 weeks to confirm pregnancy, exclude ectopic or molar pregnancies, confirm cardiac pulsation and measure the crown-rump length for dating. A second scan is performed at 18 to 20 weeks to look for congenital malformations, exclude multiple pregnancies and to verify dates and growth. Placental position is also determined. A third scan may sometimes be done at around 34 weeks to evaluate fetal size and assess fetal growth. Placental position is verified. Many centers are now doing a scan at around 13-14 weeks to measure the nuchal skin fold thickness for the purpose of evaluating the risk for Down Syndrome. The total number of scans will vary depending on whether a previous scan has detected certain abnormalities that require follow-up assessment. What is often referred to as a Level II scan merely indicates a "targeted" examination where it is done when an indication is present or when an abnormality is suspected in a previous examination. In fact professional bodies such as the American Institute of Ultrasound in Medicine does not endorse or encourage the use of these terms. A more "thorough" examination is usually done at an a perinatal center or specialised clinic where more expertise and better equipments may be present. One should not dwell too much on the definitions or guidelines for a level II ultrasound scan. The sonologist should always try very hard to look for and assess any abnormality that may be present in the fetus. It is not very meaningful to be talking about level III or even level IV scans. Whether a pregnancy must be scanned on a 'routine' basis at 18 to 20 weeks is still a matter of some controversy.

Transvaginal Scan

With specially designed probes, ultrasound scanning can be done with the probe placed in the vagina of the patient. This method usually provides better images (and therefore more information) in patients who are not pregnant or are in the early stages of pregnancy. Fetal cardiac pulsation can be observed as early as 6 weeks of gestation. Vaginal scans are becoming indispensible in the early diagnosis of ectopic pregnancies. An increasing number of fetal abnormalities are now being diagnosed in the first trimester using the vaginal scan. Transvaginal scans on the other hand are also useful in the second trimester in the diagnosis of congenital anomalies. Read one of my presentations at OBGYN.net-Ultrasound.

Doppler Ultrasound

The doppler shift principle has been used for a long time in fetal heart rate detectors. Further developments in doppler ultrasound technology in recent years have enabled a great expansion in it's application in Obstetrics, this time in the area of assessing and monitoring the well-being of the fetus. Blood flow characteristics in the fetal blood vessels can be assessed with Doppler 'flow velocity waveforms'. Diminished flow, particularly in the diastolic phase of a pulse cycle is associated with compromise in the fetus. Various ratios of the systolic to diastolic flow are used as a measure of this compromise. The blood vessels commonly interrogated include the umbilical artery, the aorta, the middle cerebral arteries and the uterine arcuate arteries. The use of color flow mapping can clearly depict the flow of blood in fetal blood vessels in a realtime scan, the direction of the flow being represented by different colors. 'Color' doppler is particularly indispensible in the diagnosis and assessment of congenital heart abnormalities. Another recent development is the Power Doppler (Doppler angiography). It uses amplitude information from doppler signals rather than flow velocity information to visualize slow flow in smaller blood vessels. A color perfusion-like display of a particular organ such as the placenta overlapping on the 2-D image can be very nicely depicted. Doppler examinations can be performed abdominally and via the transvaginal route. The power emitted by a doppler device is generally greater than that used in a conventional 2-D scan.

Color imaging

This is a recent addition to plain 2-D realtime scanning. Also known as "chroma" scans, user-selectable color hues are assigned to the shades of grey for better visualization of subtle tissue details. This clever enhancement is aimed at better interpretation of the scans. 'Color scans' do not imply that various parts of the same picture are depicted in different colors like what we see in a color photograph.

3-D Ultrasound




3 dimensional ultrasound is quickly moving out of the research and development stages and is very much in the News. Faster and more advanced commercial models are coming into the market. The scans requires special probes and software to accumulate and render the images, and the rendering time has been reduced from minutes to seconds. A good 3D image is often quite impressive and further 2D scans may be extracted from 3D blocks of scanned information. Volumetric measurements are more accurate and both doctors and parents can better appreciate a certain abnormality or the absence of a certain abnormality in a 3D scan than a 2D one and there is the possibility of increasing psychological bonding between the parents and the baby. A large volume of literature and documentation is expected to come out in the coming years and the diagnosis of congenital anomalies could receive revived attention. Present evidence has already suggested that even small defects such as spina bifida, cleft lips/palate, and polydactyl may be more lucidly demonstrated. Other more subtle features such as low-set ears, facial dysmorphia or clubbing of feet can be better assessed, leading to more effective diagnosis of chromosomal abnormalities. The study of fetal cardiac malformations is also receiving attention. The ability to obtain a good 3D picture is nevertheless still very much dependent on operator skill, the amount of liquor around the fetus, it's position and the degree of maternal obesity, so that a good image is not always readily obtainable. Other experts in this field have not considered that 3D ultrasound will be a mandatory evolution of our conventional 2D scans, rather it is an additional piece of tool like doppler ultrasound. Whether 3D ultrasound will provide unique information or merely supplemental information will remain to be seen. It's greatest potential is still in research and particularly in the study of fetal embryology. Click here for some good sample images and movie courtesy of Dr. Bernard Benoit. Visit his French site and the site on 3D ultrasound from Medison for more pictures and information. A short history of the development of 3-D ultrasound in pregnancy can be found in the History pages.

What about Safety?

It has been over 35 years since ultrasound was first used on pregnant women. Unlike X-rays, ionizing irradiation is not present and embryotoxic effects associated with such irradiation should not be relevant. The use of high intensity ultrasound is associated with the effects of "cavitation" and "heating" which can be present with prolonged insonation in laboratory situations. Harmful effects in cells of experimental animals or humans however have not been demonstrated in the large amount of studies that have so far appeared in the medical literature purporting to the use of diagnostic ultrasound in the clinical setting. Apparent ill-effects such as low birthweight, speech and hearing problems, and non-right-handedness reported in small studies have not been confirmed or substantiated in larger studies from Europe. The complexity of some of the studies have made the observations difficult to interpret. Nevertheless continual vigilance is necessary particularly in areas of concern such as the use of pulsed Doppler in the first trimester. The greatest risks arising from the use of ultrasound are the possible over- and under- diagnosis brought about by inadequately trained staff, often working in relative isolation and using poor equipment. A discussion on the various possible effects of ultrasound on the human fetus can be found here. Ultrasound scans should best be performed when there is a clear indication to do so. When there is, safety considerations should not be an issue to prevent it's prudent use.

If you are interested to find out more about a particular fetal anomaly, take a look at this compilation of Web pages which describe in some detail specific congenital anomalies that are diagnosable by ultrasound.