The 3rd trimester ultrasound is generally performed between 28 and 40 weeks of pregnancy and is often done to monitor the final stages of fetal growth, confirm the baby’s position, and assess the health of the placenta and amniotic fluid.
While the 3rd trimester ultrasound is not always routinely done unless medically necessary, it can be extremely helpful in confirming the baby’s position, growth, and overall health, especially if there are concerns or complications. It’s typically performed abdominally, and the scan is painless and non-invasive.
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The ultrasound measures the baby’s weight, head circumference, abdominal circumference, and femur length to estimate how much the baby weighs and whether it is growing at a normal rate. This helps determine if the baby is at risk for being too large (macrosomia) or too small (intrauterine growth restriction).
The ultrasound is used to determine the baby’s position—whether it is head-down (cephalic), breech (feet or bottom first), or transverse (lying sideways). This is important for planning the delivery method (vaginal birth or cesarean section).
The ultrasound checks the condition of the placenta, including its location (whether it's still in a safe position away from the cervix) and its function (whether it is providing enough nutrients and oxygen to the baby). The amount of amniotic fluid is also assessed to ensure the baby has enough space to move but not too much fluid, which could indicate complications.
The scan may also check for signs of fetal distress, such as reduced fetal movement or an abnormal heart rate. Monitoring the baby’s heartbeat provides reassurance that the baby is doing well.
In some cases, particularly for women with a history of preterm labor or other risks, the ultrasound may include a measurement of the cervix to check for signs of preterm birth. A shortening cervix can be an indicator of early labor.
