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.

A full bladder is often required for the procedure when abdominal scanning is done in early pregnency. 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.

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:

    • 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.
    • 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.
      Fetal heart rate tends to vary with gestational age in the very early parts of pregnancy. Normal heart rate at 6 weeks is around 90-110 beats per minute (bpm) and at 9 weeks is 140-170 bpm. At 5-8 weeks a bradycardia (less than 90 bpm) is associated with a high risk of miscarriage.
      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.
      In the presence of first trimester bleeding, ultrasonography is also indispensible in the early diagnosis of ectopic pregnancies and molar pregnancies.
      The following measurements are usually made:
      • 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) An important point to note is that when the due date has been set by an accurately measured CRL, it should not be changed by a subsequent scan. For example, if another scan done 6 or 8 weeks later says that one should have a new due date which is further away, one should not normally change the date but should rather interpret the finding as that the baby is not growing at the expected rate.
      • 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.
      • 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.
      • The Abdominal circumference (AC) :
        he 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.
    • 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. Common examples include hydrocephalus, anencephaly, myelomeningocoele, achondroplasia and other dwarfism, spina bifida, exomphalos, Gastroschisis, duodenal atresia and fetal hydrops. With more recent equipment, conditions such as cleft lips/ palate and congenital cardiac abnormalities are more readily diagnosed and at an earlier gestational age.
    • Placental localization : ltrasonography 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. Other placental abnormalities in conditions such as diabetes, fetal hydrops, Rh isoimmunization and severe intrauterine growth retardation can also be assessed.
    • 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.
    • Hydramnios and Oligohydramnios : Excessive or decreased amount of liquor (amniotic fluid) can be clearly depicted by ultrasound. Both of these conditions can have adverse effects on the fetus. 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.