The ultrasound exam is a non-invasive procedure in which high-frequency sound waves are directed into your uterus and are reflected back to the receiver. It is commonly used to:
detect ectopic pregnancy
determine the number of babies you are carrying
locate the placenta
determine age of the baby (within a 10 day window)
detect any gross deformities or structural malformations
assess your baby’s growth and health
evaluate risks such as placental implantation
determine sex of baby (about 75% accurate)
determine the presentation and position of baby
assess fluid volume
What you need to know
Ultrasound benefits change based on when the procedure is done and why. For example, the most accurate dating of pregnancy is done during the first trimester of pregnancy. This means you are less likely to be faced with an early due date that forces you to be induced to prevent a post-date pregnancy. Unfortunately, there is incomplete research on the safety of first trimester ultrasound. In contrast, there appears to be a small increase in cesarean surgery rates for mothers who receive a late (third trimester) ultrasound. This rise in cesarean rate comes without any improvements in mother or baby outcomes.
There is some debate about routine ultrasound (routine means every woman has the test during pregnancy regardless of medical need). Although many care givers feel the test is safe, there are a significant number who feel the technology is not thoroughly tested and should not be used when there is no benefit. They site research that shows no improvement in outcomes for mothers or babies from routine ultrasound. The concern is a technology that may pose risks should only be used when necessary, especially when there is no improvement of outcomes.
Not all ultrasounds are done for routine reasons. For example, when ultrasound is used in cases of maternal hypertension or impaired fetal growth there is a reduction in fetal deaths and interestingly a reduction in induction of labor and hospital admissions without a rise in fetal distress or cesarean rates.
The decision to use ultrasound is complicated by the excitement families feel about seeing their baby before birth. The key to making a good decision appears to be in understanding why the test is being offered and what information is expected to be gained. In this way families can avoid unnecessary or unhelpful imaging.
An ultrasound may be either transvaginal or abdominal. Early in pregnancy ultrasounds will be transvaginal because the placement of the uterus prevents good viewing with an abdominal ultrasound. With transvaginal ultrasound, a probe will be inserted into the vagina to obtain an image of the uterus and its contents.
For an abdominal ultrasound, the technician will place a gel (it may be warmed or cold) on your abdomen, and then use a transducer to send the sound waves and collect information as they return. As this happens, an image of your baby and uterine contents can be seen on the ultrasound screen. Depending on the magnification, position of your baby and placement of the transducer the image may be recognizable as body parts or be an unrecognizable blur. A standard ultrasound exam will last about 20 minutes. An advanced ultrasound will last around one hour. In many parts of the United States, ultrasound has become a routine part of prenatal care.
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Alfirevic, Z., Neilson, J.P. (1996) Doppler ultrasound for fetal assessment in high risk pregnancies. Cochrane Database of Systematic Reviews 1996(4).
Bricker, L., Neilson, J.P., Dowswell, T. (2008) Routine ultrasound in late pregnancy (after 24 weeks' gestation). Cochrane Database of Systematic Reviews 2008 (4).
Caughey, A.B., Nicholson, J.M., Washinton, A.E. (2008) First- vs second-trimester ultrasound: the effect on pregnancy dating and perinatal outcomes. American Journal of Obstetrics & Gynecology 198(6):703.e1-6.