Obstetric Ultrasound is the use of ultrasound scans in pregnancy.

Currently used equipments are known as real-time scanners, with which a continuous 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 fetus can be assessed and measurements can be made accurately on the images displayed on the screen.

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 indispensable 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 confirmation 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. The embryo can be observed and measured by about five and a half weeks. Ultrasound can also very importantly confirm the site of the pregnancy is within the cavity of the uterus.

2. Vaginal bleeding in early pregnancy.
The viability of the fetus can be documented in the presence of vaginal bleeding in early pregnancy. A visible heartbeat could be seen and detectable by pulsed doppler ultrasound by about 6 weeks and is usually clearly depictable by 7 weeks. If this is observed, the probability of a continued pregnancy is better than 95 percent. Missed abortions and blighted ovum will usually give typical pictures of a deformed gestational sac and absence of fetal poles or heart beat.

Many women do not ovulate at around day 14, so findings after a single scan should always be interpreted with caution. The diagnosis of missed abortion is usually made by serial ultrasound scans demonstrating lack of gestational development. For example, if ultrasound scan demonstrates a 7mm embryo but cannot demonstrable a clearcut heartbeat, a missed abortion may be diagnosed. In such cases, it is reasonable to repeat the ultrasound scan in 7-10 days to avoid any error.

The timing of a positive pregnancy test may also be helpful in this regard to assess the possible dates of conception. A positive pregnancy test 3 weeks previously for example, would indicate a gestational age of at least 7 weeks. Such information would be useful against the interpretation of the scans. Please read the FAQs for more comments.

In the presence of first trimester bleeding, 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).

a) 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) Dating using the BPD should be done as early as is feasible.

b) 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) Similar to the BPD, dating using the FL should be done as early as is feasible.

c) 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) AC measurements should not be used for dating a fetus.

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. computer softwares and 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. Diagnosis of 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..

5. Placental localization.
Ultrasonography has become indispensible in the localization of the site of the placenta and determining its lower edges, thus making a diagnosis or an exclusion of placenta previa.

6. 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.

7. Hydramnios and Oligohydramnios.
Excessive or decreased amount of liquor (amniotic fluid) can be clearly depicted by ultrasound.

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.

3-D and 4-D Ultrasound

3-D ultrasound can furnish us with a 3 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 3 dimensional image. Using computer controls, the operator can obtain views that might not be available using ordinary 2-D ultrasound scan. 3-dimensional ultrasound is quickly moving out of the research and development stages and is now widely employed in a clinical setting. It too, is very much in the News. Faster and more advanced commercial models are coming into the market.

More recently, 4-D or dynamic 3-D scanners are in the market and the attraction of being able to look at the face and movements of your baby before birth was also enthusiastically reported in parenting and health magazines. This is thought to have an important catalytic effect for mothers to bond to their babies before birth. What are known as 're-assurance scans' and the rather misnamed 'entertainment scans' have quickly become popular.

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 mainly to look for congenital malformations, when the fetus is large enough for an accurate survey of the fetal anatomy. multiple pregnancies can be firmly diagnosed and dates and growth can also be assessed. Placental position is also determined. Further scans may be necessary if abnormalities are suspected.

Further scans may sometimes be done at around 32 weeks or later to evaluate fetal size (to estimate the fetal weight) and assess fetal growth. Or to follow up on possible abnormalities seen at an earlier scan. Placental position is further verified. The most common reason for having more scans in the later part of pregnancy is fetal growth retardation.

What about Safety?
It has been over 40 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.

Although certain harmful effects in cells are observed in a laboratory setting, abnormalities in embryos and offsprings of animals and humans have not been unequivocally 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, brain damage 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. Every now and then ill effects of ultrasound on the fetus appears as a news item in papers and magazines. Continuous 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.