Genetics Northern California

Prenatal Ultrasound

Ultrasound uses sound waves to make an image of the developing baby on a video screen.

Ultrasound is done during pregnancy to:
    • check your due date
    • find out if there is more than one baby (twins or triplets)
    • confirm the position of the placenta and the baby
    • measure the amount of amniotic fluid
    • monitor the growth of the baby
    • detect some birth defects
    • recognize when a miscarriage happens

Ultrasound is also used during diagnostic procedures like CVS and amniocentesis.

First trimester ultrasound: Your doctor, nurse practitioner or nurse midwife usually performs an ultrasound during one of your early prenatal visits. This early ultrasound checks your due date and counts the number of fetuses. You may also be able to see a heartbeat.

Second trimester ultrasound: You will be scheduled for an ultrasound when you are about 18 to 20 weeks pregnant. This ultrasound is done in the ultrasound department. The visit includes a full evaluation of the fetus to check growth and development. The sonographer notes where the placenta is located and how much fluid is around the fetus (amniotic fluid). This ultrasound may also be able to predict the sex (boy or girl).

Level 2 (targeted) ultrasound: This type of ultrasound is done by a doctor who specializes in prenatal ultrasound. It is scheduled when a routine scan suggests a birth defect may be present or when part of the fetus is hard to see. This ultrasound is also offered when a prenatal screening test shows a higher risk for a birth defect.

Risks: Ultrasound has been routinely used in pregnancy since the 1960s. There are no known health risks to the fetus or mother with routine use of prenatal ultrasound.

Results: Most prenatal ultrasound results are normal. Your doctor, nurse practitioner or nurse midwife will discuss your ultrasound results with you. Ultrasound does have limitations. A normal scan is reassuring, but ultrasound cannot find all birth defects. And, ultrasound may suggest a birth defect is present when it is not. You will be told about any ultrasound findings that need follow-up and you may be offered additional tests.

Click here for an information sheet on ultrasound in pregnancy: Ultrasound Exam in Pregnancy (pdf) [Spanish version]

Common ultrasound findings:

Last reviewed: December 8, 2017
Reviewed by: Kimberly Barr, MS, LCGC