Most pregnant women will be offered a routine midtrimester ultrasound examination. It can be performed from 18 weeks onwards but is preferably done between 19 and 20 weeks. The ultrasound is performed transabdominally. A full bladder is not required.
The purpose of this ultrasound is:
- To confirm that the fetus is alive.
- To diagnose multiple pregnancy if no earlier scan was performed
- To confirm your due date if no earlier scan was performed
- To assess fetal growth by measuring the fetal head, abdomen and femur
- To assess the fetal anatomy. This involves a detailed examination of the fetal head, brain, face, lips, heart, stomach, lungs, abdominal wall, kidneys, bladder, spine, arms, legs, hands, and feet. Upon request, the fetal gender can be disclosed in most situations.
- To assess the position of the placenta. In approximately 3%of pregnancies the placenta is low-lying at the midtrimester ultrasound. A repeat ultrasound is then recommended around 32 weeks. In the majority of patients, the placenta will no longer be low-lying then.
- To assess the liquor volume.
- To assess the length of the cervix.
- To check for ovarian cysts or fibroids.
This examination is expected to detect the majority of major fetal malformations. It is important to appreciate however that even with the best ultrasound equipment not all abnormalities can be seen.
Sometimes abnormalities can be missed because suboptimal views are obtained. This can be due to difficult fetal position or the patient's weight. In these situations a repeat ultrasound is often organised a few weeks later when the fetus is bigger and/or in a different, hopefully better position.
Other times abnormalities may be missed despite good views. This may happen with conditions where the abnormality only becomes evident in later pregnancy or where there are in fact no structural changes in the fetus.