Obstetric Ultrasound: What is it? Uses, Safety and Different Types of Studies Currently Carried Out

It is the use of ultrasound scans in pregnancy.

Since its introduction in the late 1950s, ultrasound has become a valuable diagnostic tool in Obstetrics.

Currently, used equipment is known as real-time scanners, with which a continuous image of the fetus in motion can be displayed on a monitor screen.

Very high-frequency sound waves of between 3.5 to 7.0 megahertz (that is, 3.5 to 7 million cycles per second) are generally used.

They are emitted from a transducer that is placed in contact with the mother’s abdomen and is moved to “look” (like a light shining from a flashlight) at any particular contents of the uterus. Repetitive ultrasound beams scan the fetus in thin slices and are reflected in the same transducer.

The information obtained from different reflections is recomposed back into an image on the monitor screen (a sonogram or ultrasonogram). Movements such as fetal heartbeat and fetal malformations can be assessed, and measurements can be made with precision on the images displayed on the screen.

These measurements form the cornerstone in evaluating gestational age, size, and growth in the fetus.


A full bladder is often required for the procedure when the abdominal examination is done in early precognition. There may be some discomfort from the pressure on a full bladder. The conductive gel does not stain, but it may feel slightly cold and damp.

There is no sensation at all from the ultrasound waves.

Why and when is ultrasound used in pregnancy?

Ultrasound is currently considered a safe, non-invasive, accurate, and cost-effective fetus investigation. It has progressively become an indispensable obstetric tool and plays a vital role in the care of every pregnant woman.

The primary use of ultrasound is in the following areas:

Diagnosis and confirmation of early pregnancy

The gestational sac can be visualized as early as four and a half weeks gestation, and the yolk sac at approximately five weeks. The embryo can be observed and measured in about five and a half weeks.

The ultrasound can also confirm that the pregnancy site is within the uterus cavity.

Vaginal bleeding in early pregnancy

Fetal viability can be documented in vaginal bleeding in early pregnancy. Pulsed Doppler ultrasound can detect a visible and detectable heartbeat at approximately six weeks and is usually clearly representable at seven weeks.

If this is observed, the probability of a continued pregnancy is better than 95 percent. Missed abortions and withered eggs usually give typical images of a deformed gestational sac and the absence of fetal poles or heartbeats.

The fetal heart rate tends to vary with gestational age in the early stages of pregnancy.

At six weeks, the average heart rate is around 90 to 110 beats per minute (bpm), and at nine weeks, it is 140 to 170 bpm. At 5-8 weeks, bradycardia (less than 90 bpm) is associated with a high risk of miscarriage.

Many women do not ovulate around day 14, so the findings after a single exam should always be interpreted with caution.

The diagnosis of failed abortion is usually made by serial ultrasound scans that demonstrate the lack of gestational development.

For example, if an ultrasound shows a 7 mm embryo but cannot demonstrate a clear heartbeat, a failed abortion can be diagnosed. It is reasonable to repeat the ultrasound in 7-10 days to avoid any errors in such cases.

The timing of a positive pregnancy test can also be helpful in this regard to assess possible dates of conception. A positive pregnancy test 3 weeks earlier, for example, would indicate a gestational age of at least seven weeks. Such information would help interpret the scans.

In the presence of bleeding in the first trimester, ultrasound is also essential in the early diagnosis of ectopic and molar pregnancies.

Determination of gestational age and evaluation of fetal size

Fetal body measurements reflect the gestational age of the fetus. This is particularly true in early gestation. In patients with uncertain menstrual periods, such measures should be done as early as possible during pregnancy to arrive at a correct date for the patient.

In the latter part of pregnancy, the measurement of the body parameters will allow the evaluation of the size and growth of the fetus. It will significantly aid in the diagnosis and treatment of intrauterine growth retardation (IUGR).

The following measurements are generally made:

The croup length of the crown (CRL)

This measurement can be done between 7 to 13 weeks and provides a very accurate estimate of gestational age. Appointments with the CRL can be within 3-4 days of the last menstrual period.

An important point to note is that when the expiration date has been set by a precisely measured CRL, it should not be changed by a subsequent scan.

For example, if another test was done 6 or 8 weeks later says that one should have a new due date further away; usually, the date should not be changed but should be interpreted as that the baby is not growing at the expected rate.

Biparietal diameter (BPD)

The diameter between the two sides of the head. This is measured after 13 weeks. It increases from approximately 2.4 cm at 13 weeks to about 9.5 cm at term.

Different babies of the same weight can have different head sizes. Therefore appointments in the latter part of pregnancy are generally considered unreliable. Dating using the BPD should be done as soon as possible.

The length of the femur (FL)

It measures the longest bone in the body and reflects the longitudinal growth of the fetus. Its utility is similar to the BPD. Increases from approximately 1.5 cm at 14 weeks to about 7.8 cm at term.

Like the BPD, FL appointments should be made as soon as possible.

Abdominal circumference (AC)

The most crucial step to do in late pregnancy. It reflects more the size and weight of the fetus than the age. Serial measurements help monitor the growth of the fetus. AC measurements should not be used to date a fetus.

Other necessary measures are discussed here.

The weight of the fetus in any gestation can also be estimated with great precision using polynomial equations containing BPD, FL, and AC. Computer softwares and search graphics are available. For example, a BPD of 9.0 cm and a CA of 30.0 cm will give a weight estimate of 2.85 kg.

Diagnosis of fetal malformation

Many structural abnormalities in the fetus can be reliably diagnosed by ultrasound and can usually be done before 20 weeks.

Some of these abnormalities can be:

  • Hydrocephalus
  • Anencephaly.
  • Myelomeningocele.
  • Achondroplasia.


  • Onanism.
  • Spina bifida
  • Exophalon.
  • Gastroschisis.
  • Atresia duodenal.
  • Fetal hydrops.

With newer equipment, conditions such as cleft lips and palate and congenital heart defects are more easily diagnosed at an earlier gestational age.

First trimester ultrasonic ‘soft’ markers for chromosomal abnormalities, such as the absence of fetal nasal bone and increased fetal nuchal translucency (the area at the back of the neck), are now in everyday use to allow detection of fetuses of the syndrome. Down.

Ultrasound can also aid in other diagnostic procedures in prenatal diagnoses, such as:

  • Amniocentesis
  • Chorionic villus sampling.
  • Cordocentesis (percutaneous umbilical blood sampling).
  • Fetal therapy.

Placental location

Ultrasound has become essential in locating the placenta site and determining its inferior borders, making a diagnosis or exclusion of the placenta previa.

Other placental abnormalities can also be evaluated under conditions such as:

  • Diabetes.
  • Fetal hydropesia.
  • Isoinmunización Rh.
  • Severe intrauterine growth retardation.

Multiple pregnancies

In this situation, ultrasound is invaluable in determining the number of fetuses, chorionicity, fetal presentations, evidence of growth retardation and fetal abnormality, placenta previa, and any suggestion of twin-to-twin transfusion.

Hydramnios y oligohydramnios

The excessive or decreased amount of amniotic fluid can be represented by ultrasound. Both conditions can have adverse effects on the fetus.

In both situations, detailed ultrasound should be performed to exclude intrauterine growth retardation and congenital malformation in the fetus, such as intestinal atresia, hydrops fetalis, or renal dysplasia.

Other areas

Ultrasonography is of great value in other obstetric conditions, such as:

  • Confirmation of intrauterine death.
  • Proof of fetal presentation in pending cases.
  • Assess fetal movements, tone, and respiration on the biophysical profile.
  • Diagnosis of uterine and pelvic abnormalities during pregnancy e.g., e.g., fibromyoma and ovarian cyst.

Transvaginal examinations

An ultrasound can be performed with the probe placed in the patient’s vagina with specially designed probes.

This method generally provides better images (and, therefore, more information) in patients who are obese and in the early stages of pregnancy.

The best images result from the closer proximity of the scan head to the uterus and the higher frequency used in the transducer array, resulting in higher resolving power. The fetal heart rate can be seen at six weeks gestation.

Vaginal examinations are also becoming indispensable in the early diagnosis of ectopic pregnancies. An increasing number of fetal abnormalities in the first trimester are also diagnosed by vaginal examination.

Transvaginal examinations are also helpful in the second trimester in diagnosing congenital anomalies.

Doppler ultrasound

The Doppler shift principle has long been used in fetal heart rate detectors.

Additional developments in Doppler ultrasound technology in recent years have allowed a significant expansion in its application in Obstetrics, particularly in the ​​evaluation and monitoring of the fetus’s well-being, its progression against intrauterine growth restriction, and the diagnosis of malformations. Cardiac.

Doppler ultrasound is currently the most widely used in detecting fetal heart pulsations and pulsations in the various fetal blood vessels.

The “Daptone” fetal pulse detector is a portable device for detecting fetal heartbeats using the same Doppler principle.

Blood flow characteristics in fetal blood vessels can be evaluated with »flow velocity waveforms» Doppler. Decreased flow, particularly in the diastolic phase of a pulse cycle, is associated with compromise in the fetus.

Various systolic-to-diastolic flow ratios are used as a measure of this compromise. Commonly interrogated blood vessels include:

  • The umbilical artery.
  • The aorta.
  • The middle cerebral arteries.
  • The arcuate uterine arteries.
  • The inferior vena cava.

The use of color flow mapping can represent the blood flow in the fetal blood vessels in a real-time scan; the flow direction is represented by different colors.

Color Doppler is particularly indispensable in the diagnosis of fetal heart and blood vessel defects and the evaluation of hemodynamic responses to fetal hypoxia and anemia.

A more recent development is Power Doppler (Doppler angiography). It uses the amplitude information from the Doppler signals instead of the flow velocity information to visualize slow flow in smaller blood vessels.

A perfusion color visualization of a particular organ, such as the placenta overlay on the 2D image, can be very well represented. Doppler examinations can be performed abdominally and transvaginally.

The power emitted by a Doppler device is more significant than that used in a conventional 2-D scan. Therefore, its use is advised early in pregnancy.

Doppler installations are generally an integral part of modern ultrasound scanners.

They would need to be turned on to function. One does not need to ‘go’ to another machine for Doppler investigations.

3-D and 4-D ultrasound

3D ultrasound can provide us with a three-dimensional image of what we are scanning. The transducer takes a series of images, thin slices, of the subject, and the computer processes these images and presents them as a three-dimensional image.

Using computer controls, the operator can obtain views that might not be available using ordinary two-dimensional ultrasound. Three-dimensional ultrasound is rapidly moving out of the research and development stages and is now widely used in a clinical setting.

The scans require unique probes and software to accumulate and render the images, and the rendering time has been reduced from minutes to fractions of seconds.

A good three-dimensional image is often imposed on parents. Other 2-D scans can be extracted from 3-D blocks of scanned information.

Volumetric measurements are more accurate, and both clinicians and parents can better appreciate a particular abnormality or the absence of a specific abnormality on a 3-D scan than in a 2-D scan. There is potential to increase the psychological bonds between parents and the baby.

A growing body of literature is accumulating on the usefulness of three-dimensional scans, and the diagnosis of congenital anomalies may receive renewed attention.

Current evidence has suggested that more minor defects such as spina bifida, puffy lips/palate, and polydactyl may be more lucidly demonstrated.

Other more subtle features, such as lowered ears, facial dysmorphia, or pulses in the feet, can be better assessed, leading to a more effective diagnosis of chromosomal abnormalities.

The study of fetal heart malformations is also receiving attention.

The ability to obtain an excellent 3-D image is still highly dependent on the operator’s skill, amniotic fluid liquor around the fetus, its position, and the degree of maternal obesity, so a good image is not always easily affordable.

More recently, there are dynamic 3-D or 4-D scanners on the market. The attraction of looking at your baby’s face and movements before birth was also enthusiastically reported in parenting and health magazines.

This is believed to have an essential catalytic effect on mothers bonding with their babies before birth. What is known as “security scans” and the rather poorly named “entertainment scans” have quickly become popular?

Most experts do not consider 3-D and 4-D ultrasound a mandatory evolution of our conventional 2D scanners but rather an additional tool like Doppler ultrasound.

Most diagnostics will continue to be made with 2D scanners. 3-D ultrasound seems to have great potential in research and the study of fetal embryology.

Whether 3-D ultrasound will provide unique information or information merely complementary to conventional 2-D scans, remain to be seen.


There is no hard and fast rule regarding the number of scans a woman should have during her pregnancy. An examination is ordered when an abnormality is suspected for clinical reasons.

Otherwise, a scan is usually done around seven weeks to confirm pregnancy, exclude ectopic or molar pregnancies, confirm heart rate, and measure crown length for appointments.

A second exam is done at 18 to 20 weeks, primarily to look for congenital disabilities when the fetus is large enough to perform an accurate study of the anatomy of the fetus.

Multiple pregnancies can be firmly diagnosed, and dates and growth can also be assessed. The position of the placenta is also determined. Additional scans may be needed if abnormalities are suspected.

Many centers now perform an earlier screening exam around 11-14 weeks to measure the clarity of the fetal nape and assess the fetal nasal bone (and, more recently, detect tricuspid regurgitation) to aid in the diagnosis of Down Syndrome.

Some centers will do biochemical tests or blood tests during the same visit.

Additional scans may be performed at 32 weeks or later to assess fetal size (to estimate fetal weight) and assess fetal growth. Or to keep track of any abnormalities seen in a previous scan.

The position of the placenta is further checked. The most common reason for having more scans in the latter part of pregnancy is fetal growth retardation. Doppler scans may also be necessary for this situation.

The total number of scans will vary depending on whether a previous scan has detected specific abnormalities that require follow-up evaluation.

A Level II scan often indicates a ‘targeted’ examination, which is performed when there is an indication or when an abnormality is suspected on a previous exam.

One should not dwell too much on the definitions or guidelines for a level II ultrasound. The prenatal sonologist should always strive to look for and evaluate any abnormalities that may be present in the fetus. It is not very meaningful to speak of level III or even level IV scans.

Having a pregnancy scanned between 18 and 20 weeks as the norm is gradually becoming standard practice.

What about security?

It has been more than 40 years since ultrasound was first used in pregnant women. Unlike X-rays, ionizing radiation is not present, and the embryotoxic effects of such radiation should not be relevant.

The use of high-intensity ultrasound is associated with “cavitation” and “heating” effects that can occur with prolonged insonation in laboratory settings.

The complexity of some of the studies has made the observations challenging to interpret. The harmful effects of ultrasound on the fetus appear in the news in newspapers and magazines from time to time.

Continuous surveillance is needed, particularly in areas of concern such as the use of pulsed Doppler in the first trimester.

The most significant risks arising from the use of ultrasound are possible over-or under-diagnosis caused by inadequately trained personnel, often working in relative isolation and using poor equipment.

Ultrasound scans are best done when there is a clear indication. Security considerations should not be an issue to avoid its prudent use when it exists.