Ultrasound refers to sound which the human ear cannot detect, which is generally sound at frequency of 20 KHz or higher (the exact figure varies among individuals). Below this frequency, sound is detectable to the human ear and is known as infrasound.
Perhaps the most well-known use of ultrasound technology is during pregnancy, where it is used as a safe, non-invasive way of checking up on the health of an unborn child.
When carried out for obstetric use, ultrasound uses a piece of equipment called a transducer (a sort of probe) to build up a picture of a fetus and the uterus in which it lies. The transducer is placed in contact with the abdomen, and emits sound. The transducer detects echoes which are emitted by body tissues, and a computer then uses the frequency and location of the echoes to build up a picture of the fetus and its environment.
Ultrasound for Obstetric Use
When used in this way, ultrasound technology is used to gather important information about the health of the fetus and the uterus. Ultrasonography is used for several different purposes
- To confirm the pregnancy. The gestational sac can be viewed by ultrasound as early as four and a half weeks into a pregnancy. By five and a half weeks, the embryo is viewable. An early ultrasound also confirms that the pregnancy is located within the uterus—rather than a fallopian tube, ovary, or other dangerous location. Early ultrasound will also confirm whether multiple pregnancies have occurred.
- To determine the age of the fetus, assess its growth rate, and estimate a due date for the birth of the child. Measurements such as abdominal circumference, femur length, and the “crown-rump” length (the distance between the crown of the head and the top of the buttocks). These measurements can give a highly accurate estimate of the due date, and over time can give an overall picture of the growth rate of the fetus.
- To detect early pregnancy problems such as vaginal bleeding, absence of a fetal heartbeat, or deformed gestational sac, which may indicate the advent of a miscarriage.
- To detect fetal growth abnormalities. The increasingly sophisticated equipment used now allows for the detection of congenital cardiac abnormalities and chromosomal abnormalities such as Down syndrome (which was previously only detectable prior to birth via an amniocentesis).
- To determine the location of the placenta. Locating the placenta prevents or excludes a diagnosis of placenta previa, a disorder which may in serious cases kill the fetus, the mother, or both. Other placental abnormalities may be caused by conditions such as diabetes, and can be monitored over the course of the pregnancy.
- Ultrasound can also determine whether there is too little or too much amniotic fluid present in the uterus, either of which can present problems for the fetus.
- Finally, ultrasound is used to determine the general health of the fetus, including its movement and breathing, and the uterine and pelvic health of the mother during pregnancy.