A sonography is a procedure that can be used to evaluate the fetus, placenta and amniotic fluid during a pregnancy. It is performed using an instrument called a scanner that works on the echo principle. The machine sends out sound waves that are reflected back from the surfaces that it scans. These echoes or reflected sound waves form black-and-white images on a screen, similar to an X-ray. 

The sonography is a painless procedure. The doctor will request you to drink a lot of fluid so that the procedure can be performed on a full bladder. This enables the doctor to get a clearer picture. It is a painless procedure save for the discomfort of a full bladder. In the abdominal technique, the doctor coats the woman's abdomen with mineral oil or aqua gel and runs the hand-held scanner over her belly. Sometimes a vaginal probe can be performed early in the pregnancy that involves the insertion of a vaginal probe. The woman's body is exposed to sound waves for a fraction of a second. This procedure is not known to have any harmful effects.

If a woman is bleeding in early pregnancy (6-12 weeks), a sonography can be performed to detect fetal movement. If there is no sign of movement, it could indicate a miscarriage. A sonography can also identify ovarian cysts and ectopic pregnancy as the cause of pelvic pain in early pregnancy.

Chorionic villus sampling is a procedure that is performed for some women around 9-11 weeks into the pregnancy to determine whether the fetus has any birth defects. In these cases, a sonogram comes in handy to determine the site of the placenta from where the tissue sample should be obtained. Some women have an amniocentesis approximately 11-18 weeks into their pregnancy to screen for genetic defects. Again a sonography is helpful as it helps the doctor identify the amniotic fluid as well as the position of the placenta and fetus that have to be avoided by the needle.

If the mother has had a child earlier with a birth defect like anencephaly, spina bifida or hydrocephaly, a sonography performed between 14-25 weeks of pregnancy could detect these abnormalities.

In the case of high-risk pregnancies where the doctor might need to induce labour or do a cesarean section, a sonography performed at 16-25 weeks of pregnancy enables the doctor to measure the size of the fetus' head and calculate the gestational age and due date of the baby. A sonography which is done at 16-25 weeks can also detect if the mother is going to have twins, especially if the fetus seems to be inordinately big or there is a history of twins in the family.

Sonographies are normally also done late in the pregnancy (32-40 weeks) for high-risk pregnancies. This is to monitor fetal movement and respiration and check the volume of amniotic fluid.