What is ultrasound?
Ultrasound (or "ultrasonography") is a medical diagnostic tool that uses high frequency sound waves to produce images of structures within the body. This test is commonly used during pregnancy to evaluate the fetus and the mother. Ultrasound allows your doctor to evaluate both structural aspects (i.e. whether the fetus' body is developing normally) and functional aspects (i.e. whether the fetus and its organs are operating as they should). Because it is a "real time" imaging procedure, doctors can examine such processes as body motion, cardiac activity and breathing movements.
When in pregnancy is ultrasound performed?
Ultrasound may be performed at any time during a pregnancy depending on the indications (to be discussed later in this article).
What are we looking for?
In the first trimester (first 3 months), ultrasound is primarily performed to evaluate vaginal bleeding, assess the age of the fetus, and confirm that the fetus is alive. Chorionic villus sampling, a way to evaluate the genes of the fetus (described in more detail below), is also performed in the first trimester. Ultrasound is used to assist in this procedure.
In the second trimester (third through sixth months) ultrasound is generally performed to evaluate the fetus for anatomic or structural abnormalities. Abnormalities may be suspected because of a prior abnormal pregnancy, or because of a blood test. The best time to perform a second trimester ultrasound for structural anomalies is between 17 and 20 weeks of gestation. Genetic amniocentesis, the removal of a small amount of amniotic fluid from the womb (described in more detail below), may also be performed during the second trimester using ultrasound guidance.
Third trimester (sixth through ninth months) ultrasonography is performed to evaluate the fetus' growth, and to confirm its size if your doctor is concerned that it may be too small or too large. Previously diagnosed abnormalities can be reevaluated and followed in the third trimester. Amniocentesis to evaluate the maturity of the fetus is performed in the late third trimester. A weight assessment can also be performed in the late third trimester to help your doctor plan the optimal method of delivery.
Are there risks?
Ultrasound has been performed in the United States since the late 1960s and its safety has been studied for thirty years. To date, no adverse effects of ultrasound have ever been demonstrated in humans.
Who performs an ultrasound?
Guidelines have been developed for the safe and proper use of ultrasound. The American College of Obstetrics and Gynecology (ACOG), American College of Radiology (ACR), American Institute of Ultrasound in Medicine (AIUM), and National Institutes of Health (NIH) have published guidelines separately and jointly that outline the standards to which diagnostic ultrasound should adhere. They also have indicated the training requirements for ultrasound practitioners. The ACR and AIUM have recently begun accreditation of ultrasound practices to assure that these standards are met. Check to see that your ultrasound is being performed in an accredited facility. Many practitioners perform "basic" ultrasound examinations, but will refer you to a second facility if you require a "targeted" or more detailed ultrasound study.
Most examinations are performed by trained technologists called sonographers, who are certified by the American Registry of Diagnostic Medical Sonographers (ARDMS). These sonographers meet strict training and educational requirements. Check with your ultrasound facility to make sure that an ARDMS registered sonographer is performing your study!
What happens during the exam?
Your ultrasound examination should be a pleasant and painless procedure. You will need to remove part or all of your clothing, wear a gown, and lie comfortably on an examination table as the technologist covers your abdomen with warm jelly. For the most part, you and your husband will be able to watch the study on a television monitor attached to the ultrasound machine. The sonographer will be examining and freezing images of the fetus to assure that a thorough examination is performed. He or she will also measure several of the fetus' body parts during the examination, including the head, abdomen, and femur (long bone of the thigh). Many ultrasound departments will also give you a picture of your baby to take home.
Certain specialized examinations may require you to fill your bladder for part of the examination. Also, in the first trimester, it may be necessary to perform all or part of your examination with an internal (vaginal) probe. The sonographer will explain these to you if it becomes necessary.
After the radiologist evaluates the images from your examination, he or she may want to re-perform part of the examination personally. If all is well, at the end of the examination you will know the gestational age of the pregnancy and that your fetus is healthy. In many cases it will be possible to tell you if you are having a boy or a girl! Your doctor will be notified of your results in a timely fashion.
Does everyone need an ultrasound?
There are many reasons why your doctor would order an ultrasound during your pregnancy. Ultrasound has become a common tool for the evaluation of even the most normal of pregnancies, and many doctors believe that it should be routine. Routine ultrasound in pregnancy is not, however, the standard of care in the United States as it is in many other countries. In England, for example, a second trimester ultrasound is a routine procedure.
The American College of Obstetrics and Gynecology (ACOG), American College of Radiology (ACR), American Institute of Ultrasound in Medicine (AIUM), and National Institutes of Health (NIH) have developed guidelines for the use of ultrasound in pregnancy. Below is a partial list of the uses of ultrasound.
Estimation of the fetus' age for patients with uncertain dates
It is important to be very certain of where you are in your pregnancy. Many women have irregular menstrual cycles and are uncertain as to when they conceived. Knowing your "dates" may avoid unnecessary complications if your pregnancy exceeds 40 weeks gestation. Certain procedures such as the "triple screen" blood test, chorionic villus sampling, amniocentesis and fetal echocardiography need to be performed at precise times in the pregnancy.
Evaluation of fetal growth
Periodic ultrasound examinations are useful to assess the growth of the fetus. If your doctor suspects that your baby is not growing properly either because of a maternal or fetal problem, a series of ultrasounds can reliably compare the growth of your fetus to expected normal rates. If normal, this can be very reassuring.
Vaginal bleeding of unknown origin
There are many causes of vaginal bleeding during pregnancy. In the first trimester, bleeding can be caused by the implantation of the embryo into the wall of the uterus. In the second and third trimesters of pregnancy, bleeding may be caused by placental abnormalities such as placenta previa (a placenta that covers part or all of the cervix), or placental abruption (a placenta that gets torn from the uterine wall). Often, an ultrasound performed early in the second trimester may not be able to accurately detect a placenta previa, and a repeat examination in the third trimester may be needed.
Determination of fetal position
Towards the end of your pregnancy, it is important that the fetal position be the correct one for a vaginal delivery. Although your doctor can usually determine the fetal presentation by examining you, ultrasound may be necessary for a more accurate assessment. Ultrasound is able to diagnose a breech presentation, which is when the fetus presents rump or feet first.
Suspected multiple pregnancy
Ultrasound can easily diagnose multiple pregnancies. This is especially important if you have had fertility treatments, which increase the likelihood of multiple pregnancies.
Assistance with amniocentesis or chorionic villus sampling (CVS)
Amniocentesis and CVSare procedures used to determine the genetic makeup of the fetus and for diagnosing inherited disorders. CVS is done at about the 10th week of gestation and is performed by inserting a catheter (a plastic tube) through the cervix, and guiding it into the the placenta with the help of ultrasound. There is a small incidence of miscarriage occurring as a result of this procedure.
Genetic amniocentesis is performed between 14 to 18 weeks of gestation. Using ultrasound as a guide, and after carefully sterilizing your skin, your doctor places a thin needle into the amniotic sac that surrounds the fetus. Amniotic fluid is withdrawn and the fetal cells that it contains are examined for their chromosomal make-up. The amniotic fluid is also routinely analyzed for alpha-fetoprotein which is a marker for neural tube defects (which lead to abnormalities of the brain and spine). There are many other tests that can be performed on the amniotic fluid if your doctor thinks it is necessary. For example, if both parents are carriers of the genes for Tay Sachs disease, the amniotic fluid can be analyzed to determine if the fetus is affected.
Evaluation of appropriate size
Your doctor may be concerned that the fetus may be too large or too small for its expected size. Often the clinical assessment of the fetus' age is not accurate. Ultrasound can reassure you and your doctor that the fetal size is appropriate for its age. Measurements of the fetus are fairly accurate in the first and early second trimester. An ultrasound performed in the third trimester is not as accurate for predicting or confirming age, unless you have had a previous examination.
Benign tumors of the uterus, called fibroids, can often grow during pregnancy. On an early obstetrical visit, your doctor may suspect that you have a fibroid and refer you for an ultrasound examination for confirmation. The size of the fibroid can be monitored during the pregnancy. Benign and, very rarely, malignant tumors or cysts of the ovary may be suspected from your examination in the first trimester. Most ovarian masses are benign cysts that are normal in early pregnancy. Ultrasound can confirm the benign nature of these cysts and they can be followed to make sure they do not grow larger.
Suspected ectopic pregnancy
Ectopic pregnancy is a rare complication of early pregnancy. Instead of implanting in the uterus normally, the embryo develops in the fallopian tube. Risk factors for having an ectopic pregnancy are prior pelvic infections and previous ectopic pregnancy, among others. Patients with an ectopic pregnancy typically have abnormal vaginal bleeding and pelvic pain. They may feel dizzy and faint. Ectopic pregnancy may require emergency surgery, but newer, non-surgical treatments are now available. If you suspect you may have an ectopic pregnancy you should contact your doctor immediately! The ultrasound diagnosis is not always definitive, but a normal examination is reassuring.
Suspected fetal death
If your doctor suspects that the fetus is no longer alive, ultrasound can be used to confirm this.
Evaluating fetal health (biophysical profile)
In the late third trimester, it may be necessary to assess fetal well-being by performing a series of tests designed to evaluate fetal heart rate, movements, and breathing. One such test is the Biophysical Profile ultrasound. The biophysical ultrasound is a regular ultrasound that measures the fetus and estimates fetal weight. It also verifies fetal position, the maturity of the placenta, and the volume of the amniotic fluid (the fluid surrounding the fetus). The radiologist will also carefully evaluate the fetus for chest movements ("fetal breathing"), body movements, and tone. The character of blood flow in the umbilical cord is also evaluated; this can give us a clue as to whether the fetus is hypoxemic (i.e. has low blood oxygen). A score expressed as a total of ten or twelve points will be assigned. The biophysical profile is only one "piece of the puzzle", and needs to be correlated with your physical examination and other tests.
Assessing amniotic fluid
Amniotic fluid may be excessive in diabetics, or in maternal thyroid or heart disease. Excessive amniotic fluid may also indicate fetal anomalies. Decreased amniotic fluid may indicate fluid leakage or fetal growth abnormalities. Ultrasound can readily assess amniotic fluid volume in order to determine if it is normal.
Estimate fetal weight near the time of delivery
Estimation of fetal weight may guide your doctor in the management of your labor, or to plan a cesarean section. If for any reason you need to deliver your baby earlier than term, the estimated weight can guide your pediatrician in the proper care of your baby. The weight estimation is based on measurements of the fetal head, body and bones. The greater the weight estimation, the larger the range of error. There can be as much as a 15% error in the estimated weight at term.
Evaluating abnormal blood test results
Most pregnant women have blood taken at about 16 weeks gestation for what is called the "triple screen" blood test. This sensitive test may suggest that you are at a higher risk for a fetus with a chromosomal abnormality such as Down's syndrome or a nervous system abnormality such as neural tube defect. A "targeted" ultrasound performed to evaluate these results may be reassuring. There are specific ultrasound findings seen in both Down's syndrome and neural tube defects. Your doctor may recommend, however, that you undergo amniocentesis as not all abnormalities can be diagnosed by even the most meticulous of ultrasound examinations.
Evaluation and screening of congenital anomalies
Examinations performed in the second and third trimesters of pregnancy are designed to evaluate the many components of the fetal anatomy. Every ultrasound study, at the very minimum, must evaluate the following areas of fetal anatomy: the fetal brain; the spine, a "four chamber" view of the heart (seeing the 4 normal cardiac chambers rules out about 90% of all congenital heart disease), the stomach, the urinary bladder, the fetal umbilical cord and its insertion into the fetal abdomen, and the kidneys. It is often possible to determine the gender of the fetus after 15 weeks. The basic ultrasound examination also consists of measurements of the head, the abdomen, and the femur (the long bone of the thigh), as well as an assessment of the placenta, the amniotic fluid, and the fetal position.
Since it is not always possible to image all of the required anatomic areas in every examination, the radiologist will inform your doctor if any areas of the anatomic survey are incomplete and if the examination needs repeating. In addition, if an abnormality is suspected, a more complete, or "targeted," ultrasound can either be performed immediately, or you may be referred elsewhere. Certain abnormalities may result in a recommendation to perform other tests like amniocentesis or fetal echocardiography. Ultrasound cannot evaluate your fetus for absolutely every possible anomaly. If you have specific questions about what your ultrasound showed, please do not hesitate to ask questions of the sonographer and radiologist before you leave.
The ultrasound examination is part of almost every normal pregnancy. It is a safe and painless exam that can help reassure you and your doctor that all is going well. It may be used during each trimester to follow fetal growth and to look for fetal abnormalities. If you are pregnant, you should ask your doctor if an ultrasound is needed for you.