Ultrasound is considered to be a safe imaging modality for pregnancies. While there are no known effects on the fetus, ultrasound is used at the discretion of the obstetrician and is not recommended for any use other than medical. It can be used at any point in a pregnancy to determine if there are any risk factors. It is also used to determine fetal age and monitor growth.
First Trimester
Ultrasound is commonly used during the first trimester to determine the age and due date of a pregnancy. It can be done between six and 12 weeks of gestation. Miscarriages are most likely to be seen up to 10 weeks of gestation.
Subchorionic hemorrhages are frequently seen during the first trimester. These are areas of bleeding around the gestational sac, and they usually resolve by the end of the first trimester. A corpus luteum cyst is often seen on one of the ovaries. The corpus luteum releases a hormone that is responsible for maintaining the pregnancy throughout the first trimester.
Second Trimester
The second trimester ultrasound is usually a fetal anatomy survey done between 18 and 22 weeks of gestation. It is during this ultrasound that the sex of the baby is most likely to be seen. The arms, legs, head, thoracic and abdominal cavities are imaged.
If an abnormality is found during the second trimester ultrasound, the patient may be sent to a perinatologist for further testing. An amniocentesis may be performed to determine if there are any genetic abnormalities. Significant abnormalities may warrant surgery during the pregnancy or caesarean section with surgery following delivery.
Keepsake images are often given to the expectant parents during this ultrasound. These can include 3D images, which can be taken if certain conditions are met:
- The fetus must be in a good position.
- There must be a pocket of amniotic fluid around the face.
- There must not be too much distance between the transducer and the fetus.
Four-dimensional ultrasound is 3D ultrasound while watching the fetus move.
Third Trimester
Third trimester ultrasound is usually done to assess the growth and well-being of a fetus. This may be done because the mother has diabetes, hypertension, autoimmune disease, or other disorders.
The fetal well-being assessment is called a biophysical profile. The fetus is monitored for the flexion and extension of the arms and legs and other movements. Fetal breathing movements should be observed for periods of at least 30 seconds. The amniotic fluid index is measured. A non-stress test may be done in conjunction with the biophysical profile.
Depending on the outcome of an ultrasound examination, serial ultrasounds may be done at any point in the pregnancy to monitor any concerns. In many cases, the focus of concern resolves and a normal pregnancy is carried.
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