An ultrasound (also called Sonography or Scan) is a routine procedure that is usually prescribed during the first and second trimester of your pregnancy. It is usually done to confirm that the placenta is healthy and attached normally and that the baby is growing properly in the uterus.

How it works
Medical ultrasounds work on the same principle as sonar (used in oceanography to map the sea bed). During an ultrasound, high-frequency sound waves, inaudible to the human ear, are transmitted through the abdomen via a device called a transducer to look at the inside of the abdomen.

An ultrasound can be used during pregnancy to show images of the baby, amniotic sac, placenta and ovaries. It is especially valuable during pregnancy as major anatomical abnormalities or birth defects can show up on an ultrasound. In a prenatal ultrasound, the echoes are recorded and transformed into video or photographic images of your baby - for future reference.

When is an ultrasound performed and why?
During pregnancy, an ultrasound is generally performed at around 20 weeks gestation. The baby's heartbeat and movement of its body, arms and legs can be seen - and the gender can also be determined at around this time. Please note, gender determination through ultrasound is banned in India.

An ultrasound is usually performed earlier in the pregnancy to determine:

• Presence of more than one fetus
• Due date or gestational age (the age of the fetus)

Later in pregnancy, an ultrasound may be used to determine:

• Health of the baby
• Location of the placenta
• Amount of amniotic fluid around the baby
• Position of the baby
• Baby's expected weight

Types of Ultrasound
There are many different types of ultrasound technology that can be used during pregnancy, for various reasons:

Abdominal Ultrasound
Also known as "trans-abdominal", this is the most common method used, especially after 12 weeks of pregnancy (when the uterus has grown up and out of the woman's bony pelvis). You may be asked to drink 4 to 6 glasses of water before the test, so your bladder is full - and told not to urinate till the test is over. This will help the Doctor view the baby better on the ultrasound.

Vaginal Ultrasound
The vaginal or "trans-vaginal" ultrasound is performed by placing a sterilized ultrasound probe inside the vagina. This method is often used during fertility treatment or early pregnancy (less than 12 weeks) as it enables the ultrasonographer to look more closely at the woman's uterus. Vaginal ultrasounds may also be more beneficial if the woman has a retroverted or 'tipped' uterus (up till 12 weeks of pregnancy) or if the ultrasonographer needs to view the woman's cervix or detect a low-lying placenta.

Doppler Ultrasound
'Doppler ultrasounds' are done in the same way as abdominal ultrasounds, but they study the movement of blood (usually through a blood vessel). They are used to assess the functioning of the placenta and the well-being of the baby.

3D & 4D Ultrasound
Though not widely used, 'three-dimensional' ultrasounds (also known as 'ultrasound holographs' are some times recommended to have a better look at an abnormality that may have been detected in a routine ultrasound.

3D ultrasounds work by taking thousands of image 'slices' in a series (called a 'volume of echoes'). The volumes are then digitally stored and shaded to produce 3 dimensional images of the baby that look more life-like. 4D (or 'four-dimensional' images) just means the images can be seen to move in 'real time' so the activity of the baby can be studied.

Ultrasounds during pregnancy can be very useful and provide a lot of information, but it is important to remember that they also have their limitations. The accuracy of a pregnancy ultrasound will very much depend on the qualifications, skill and experience of the ultrasonographer performing the examination, as well as the Doctor's skill at interpreting the images.

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