In the third trimester, ultrasound is used to analyze fetal growth and fetal movement and ensure fetal and maternal well-being. In the late stages of pregnancy it allows assessment of the presentation of the fetus i.e. to determine if the fetus is lying breech. More recently, 4-D or dynamic 3-D scans have become available to look at the face and movements of your baby before birth. This is thought to have an important catalytic effect for mothers to bond to their babies before birth.

The main use of ultrasonography are in the following areas:

1. Diagnosis and confirm an early pregnancy.
The gestational sac can be visualized as early as 4.5 weeks of gestation. The embryo can be observed and measured by about 5.5 weeks. In the presence of first trimester bleeding, ultrasonography is also indispensable in the early diagnosis of ectopic pregnancies and multiple pregnancies.

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 ultrasound at about 6 weeks - transvaginal scanning is generally used to produce the best possible image.

3. Determination of gestational age and assessment of fetal size.
Fetal body measurements reflect the gestational age of the fetus. See list of abbreviations:

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.

b) The Biparietal diameter (BPD)
The diameter between the 2 sides of the head. This is measured after 13 weeks. Different babies of the same weight however can have different head sizes and therefore dating in the later part of pregnancy is generally considered unreliable.

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.

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.

4. Diagnosis of fetal malformation.
First trimester ultrasonic 'soft' markers for chromosomal abnormalities such as the absence of fetal nasal bone and an increased fetal nuchal translucency (the area at the back of the neck) are now in common use to enable detection of Down syndrome fetuses.

Many other structural abnormalities in the fetus can be diagnosed by an ultrasound scan, and can usually be made before 20 weeks. Examples include hydrocephalus, anencephaly, myelomeningocoele, and spina bifida, exomphalos, With improved ultrasound resolution, conditions such as cleft lips/ palate and congenital cardiac abnormalities are more readily diagnosed and at an earlier gestational age.

Ultrasound can also assist in other diagnostic procedures in prenatal diagnosis such as amniocentesis and chorionic villus sampling

5. Placental localization.
Ultrasonography has become indispensable 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.
Ultrasonography is invaluable in determining the number of fetuses, the fetal presentations, evidence of growth retardation and fetal anomaly

7. Hydramnios and Oligohydramnios.
Excessive or decreased amount of liquor (amniotic fluid) can be detected by ultrasound. In both these situations, careful ultrasound examination should is performed to exclude intraulterine growth retardation and congenital malformation of the fetus.