The vast majority of women have at least one ultrasound exam during their pregnancy. If your pregnancy is considered higher risk, you may be given a series of ultrasounds throughout your term. In the first trimester, ultrasound is often used to confirm and date a pregnancy (it is a very accurate method of determining fetal age early in gestation). Many health providers recommend an ultrasound exam between weeks 17 and 19 for evaluating fetal development and excluding abnormalities (this is often a more detailed exam, called an "anatomy scan," in which the baby is carefully measured for possible abnormalities). In the third trimester, ultrasound may be used to assess the health, position and size of the baby and the placenta. It is sometimes—though not always—possible to determine the gender of the baby from an ultrasound after about week 16.
An ultrasound exam is painless and non-invasive—and often extremely exciting for you and your partner. High-frequency sound waves (which cannot be heard by humans) are reflected off of the baby's body, creating an image of the baby—called a sonogram—that can be viewed on a small screen. To conduct the exam, the doctor or technician applies some oil or gel on your exposed abdomen, then moves a transducer across it. The transducer directs sound waves toward your uterus and the fetus inside the uterus. (Early in pregnancy, when the fetus is very small, the exam may be conducted with a transducer in the shape of a wand, which is inserted vaginally.) Ultrasound involves no radiation or X-rays, and is considered harmless for both mother and baby.
The findings of an ultrasound exam are available immediately. It's a good idea for your partner to come with you—during the exam, you'll learn important news about your baby's well-being. The accuracy of the exam's findings depends largely on the skill of the practitioner or the ultrasound technician who is interpreting the data and images, and, to some extent, on the position of the baby during the exam. An experienced user of ultrasound equipment will usually be able to give you an accurate assessment of what's going on inside your uterus.
All content here, including advice from doctors and other health professionals, should be considered as opinion only. Always seek the direct advice of your own doctor in connection with any questions or issues you may have regarding your own health or the health of others.