Ultrasound in Pregnancy

Ultrasound is a technology that produces images, or "sonograms," that help your doctor follow how the fetus is developing. Sonograms are also used to assess the condition of the placenta and the uterus to make sure the pregnancy is proceeding normally. Sonograms are produced by low energy sound waves that harmlessly bounce off the fetus and surrounding organs. The ultrasound exam poses no known risk to mother or baby. Sonograms are done throughout pregnancy, more or less, depending on the medical history and how the pregnancy is going.

Confirmation of pregnancy

At the first prenatal visit, an ultrasound exam may be done to confirm the pregnancy, which requires that a living fetus be identified within the uterus. If you have a positive pregnancy test, you may have an ectopic pregnancy or else you may be earlier in your pregnancy than you thought. The sonographer then examines the fallopian tubes to look for a tubal pregnancy. If all is proceeding normally, an ultrasound exam should identify the tiny embryo within a gestational sac (that looks like a white ring) at six weeks gestation. A beating heart is observed as well.

Gestational age and development

The ultrasound cannot precisely date the pregnancy until the sixth week, by determining the distance between the head to the buttocks of the fetus (crown-to-rump length). The ultrasound-derived gestational age is then used to correct significant differences with the calculated due date. Ultrasound measurements are used on subsequent visits to follow the growth of the fetus. A too-large baby is more common in women with diabetes, while a baby that is too small may mean the uterus is getting less than the needed amount of blood flow.

Later in pregnancy

Toward the due date, ultrasound observations determine the position of the fetus, condition of the placenta and amount of amniotic fluid. These and other factors are considered when deciding if a C-section would be safer than a vaginal delivery. During labor, ultrasound is combined with other technologies to monitor the progress and well being of the fetus. In non-stress testing, ultrasound measures fetal heart rate while another external monitor measures your contractions. You will be asked to monitor fetal activity by pressing a button to indicate when you sense a movement. Results of non-stress testing will guide your doctor into further testing or intervention.