Ultrasound

You may have one or more ultrasound tests during your pregnancy. The first test may be done within the first four weeks to date the pregnancy and determine the due date. This is done by measuring the baby's crown to rump length. Accurate charts are available to determine the age of the fetus from these measurements.
By about weeks 11 to 13, ultrasound may be used to examine various aspects of your baby's developments, such as:

Is the neural tube closed?
Are the internal organs developing properly?
Does the heart beat OK, and at what rate?
By 20 weeks, you may be able to determine the sex of the baby by observing its sex organs with ultrasound.