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 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 ultra sonogram). 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. Such measurements form the cornerstone in the assessment of gestational age, size and growth in the fetus.

A full bladder is often required for the procedure when abdominal scanning is done in early pregnancy. There may be some discomfort from pressure on the full bladder. The conducting gel is non-staining but may feel slightly cold and wet. There is no sensation at all from the ultrasound waves.

2. When might Ultrasound be 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.  Some of the main usage of Ultrasound scan,

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. 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).

3.  Transvaginal Scans

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 obese and/ or in the early stages of pregnancy. The better images are the result of the scan head's closer proximity to the uterus and the higher frequency used in the transducer array resulting in higher resolving power. Fetal cardiac pulsation can be clearly observed as early as 6 weeks of gestation.

Vaginal scans are also becoming indispensible in the early diagnosis of ectopic pregnancies. An increasing number of fetal abnormalities are also being diagnosed in the first trimester using the vaginal scan. Transvaginal scans are also useful in the second trimester in the diagnosis of congenital anomalies.

4. Doppler Ultrasound

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