What is Obstetric Ultrasound Imaging?
Ultrasound imaging, also called ultrasound scanning or sonography, is a method of obtaining images of internal organs by sending high-frequency sound waves into the body. The sound wave echoes which have a frequency higher than people can hear are recorded and displayed as a real-time visual image. No ionizing radiation (x-ray) is involved in ultrasound imaging. Obstetric ultrasound refers to the specialized use of sound waves to visualize and thus determine the condition of a pregnant woman and her embryo or foetus.
What are some common uses of the procedure?
Obstetric ultrasound should be performed only when clinically indicated. Some of the indications include:
- To establish the presence of a living embryo/fetus.
- To determine the number of embryos/fetuses.
- To estimate the age of the pregnancy.
- To evaluate the position of the fetus.
- To diagnose fetal abnormalities.
- To evaluate the placenta.
- To evaluate fetal growth and wellbeing.
- To assess the cervix or mouth of the womb.
How should I prepare for the procedure?
If an ultrasound is required early in your pregnancy, you will be required to have a full bladder for the procedure. Air interferes with sound waves, so if your bladder is distended, the air-filled bowel is pushed out of the way and an image of the uterus and embryo or fetus is obtained. A full bladder is not necessary in most cases after the first 3 months of pregnancy. The radiologist or sonographer may elect to examine an early pregnancy or the cervix by means of transvaginal ultrasound. This requires an empty urinary bladder.
You will be provided additional instructions when you make your appointment.
What does the equipment look like?
The ultrasound image is immediately visible on a nearby screen that looks much like a computer or television monitor. The radiologist or sonographer watches this screen during the examination and captures representative images for storage. Often, the patient is able to see it as well.
How does the procedure work?
The ultrasound transducer functions as both a loudspeaker (to create the sounds) and a microphone (to record them). For obstetric ultrasound, when the transducer is pressed against the skin, it directs a stream of inaudible, high-frequency sound waves into the lower abdomen and pelvis. As the sound waves echo from the embryo or fetus and surrounding structures in the uterus, the sensitive microphone in the transducer records tiny changes in the sound’s pitch and direction. These signature waves are instantly measured and displayed by a computer, which in turn creates a real-time picture on the monitor. Still frames of the moving picture are “frozen” to capture a series of images. Conventional ultrasound displays the images as thin sections (like looking at single slices of bread in a loaf).
Doppler ultrasonography is the application of diagnostic ultrasound to detect moving blood cells and measure their direction and speed of movement. The Doppler effect is used to evaluate blood flow by measuring changes in the frequency of the echoes reflected from blood cells.
The movement of the embryo or fetus and the heart beat can be seen as an ongoing ultrasound “movie.” Most ultrasound devices also have an audio component that processes the echoes produced by blood flowing through the fetal heart, blood vessels and umbilical cord. This sound can be made audible to human ears and has been described by patients as a “whooshing noise.”
How is the procedure performed?
A complete obstetric ultrasound examination usually takes between 20-60 minutes.
What will I experience during the procedure?
Who interprets the results and how do I get them?
What are the benefits vs. risks?
- Ultrasound does not use x-rays to produce an image—neither the mother nor her unborn child is exposed to ionizing radiation.
- Ultrasound has been used to evaluate pregnancy for nearly four decades, and there has been no evidence of harm to the patient, embryo or fetus. Nevertheless, ultrasound should be performed only when clinically indicated.
- For standard diagnostic ultrasound there are no known harmful effects to humans.