Ultrasound imagery is offered to most women between 16--20 weeks but it's up to the doctor's discretion.
Ultrasounds allow imaging of body parts using sound waves. Ultrasound uses sound waves that are above the range of human hearing to create an image of organs within the body. Sound waves are reflected off internal body structures and back to the ultrasound machine. The reflected sound waves are analyzed by computer and turned into pictures. This method of imaging results in less clear pictures than X-rays, CAT scans or MRI. However, there is no radiation risk with ultrasound and no confirmed adverse effects on the fetus or mother from diagnostic ultrasound examinations in pregnancy.
There are different types of ultrasound exams. They are differentiated by the purpose for which they are done and the level of detail obtained.
- Limited exams are focused studies used to answer specific questions about the fetus, mother, or both. This exam is often used when you go to your doctor or the hospital with an urgent problem related to your pregnancy.
- Basic exams are performed to survey for obvious malformations of the fetus and to estimate fetal age, the amount of amniotic fluid present, location of the placenta, and for other concerns. These are the kind of exams that you would likely receive in your doctor's office or in the hospital as a routine evaluation. They are typically performed at 18-20 weeks of pregnancy.
- Comprehensive exams are a more in-depth look at the fetus when there is reason to suspect something is wrong with the fetus or mother. They include a detailed examination that is often done as a response to an abnormal screening test such as the alpha-fetoprotein (AFP) screen. In some areas of the country this is called a Level II or Level III ultrasound. Technical difficulties and the need to image many different areas of the fetus may extend the length of this exam to 30 or more minutes.
Ultrasound can measure fetal size, the amount of amniotic fluid, estimate fetal gestational age, identify multiple fetuses, some fetal abnormalities such as microcephaly or Down Syndrome, and locate the location of the placenta. Although an ultrasound can usually determine gender of your baby, many families do not want to know this information before delivery and some ultrasound centers have a policy of not revealing the gender.