These are the most common motivations for ordering a third-trimester pregnancy ultrasound:
The presence of bleeding during the second half of pregnancy may be due to placental problems. An ultrasound can assist your practitioner to find out what's going on.
In the later stages of pregnancy, if there are any concerns that your baby is not growing as it should, you may be sent for a 3D ultrasound (or a series of them) to take measurements of certain body parts.
Particular attention will be paid to measuring the size of your baby's head, the length of his thigh bone, and the distance around his midsection.
If you have been diagnosed with too much or too little amniotic fluid, you will probably be given regular ultrasounds during your third trimester to monitor your baby's condition.
If you’re over your due dates or have high blood pressure or diabetes, you may be asked to have a biophysical profile to check on your baby's movements and breathing and to measure the amount of amniotic fluid.
Cesarean section may be the preferred option if your baby is especially large (particularly if you have diabetes) or in an abnormal (breech) position, or if the position of the placenta prevents normal delivery.
All of these factors contribute to the reason as to why you should pursue a pregnancy ultrasound later on in your cycle.