Ultrasound: Third trimester examinations (week 27 to 40)
- First Trimester
- Second Trimester
- Third Trimester
- Obstetric Ultrasound Markers
Why am I having an ultrasound in the third trimester?
If your pregnancy is normal and straightforward, you will probably not have any more exams after 20 weeks. Your doctor will monitor the growth of your baby by measuring the fundal height (the distance from the front of your pelvis to the top of your uterus) and will make sure you are keeping healthy by taking your blood pressure, urine and blood samples. If any concerns arise or if you have had complications in a previous pregnancy, third trimester ultrasound exams are performed. Being sent for an exam does not necessarily mean that your doctor feels that there is a problem. Often, it is difficult to accurately measure the fundal height or feel the baby, so ultrasound is needed to ensure the baby is growing normally and is in the correct position to be delivered easily.
How does ultrasound evaluate growth of my baby?
The most common reason for an exam in the third trimester is to check that your baby is growing normally. If you have had complications in previous pregnancies or suffer from a medical condition, such as diabetes or high blood pressure, you will probably be offered exams at regular intervals throughout your pregnancy.
Sometimes your doctor may suggest an exam if your baby feels bigger or smaller than expected. Measurements of the baby’s head (HC and BPD), abdomen (AC), and limbs (FL) are taken and the volume of the amniotic fluid around the baby is measured. If all measurements are within normal limits, the baby is likely to be an average size. A second exam in two weeks may be advised because babies grow in spurts so serial exams can tell more about your baby’s growth than just a single exam.
A baby with an average HC but a large AC has a fat tummy! It may just be getting a good food supply from the placenta but it can sometimes indicate diabetes in the mother, especially if the amniotic fluid is increased. Your blood sugars will be checked to see if they are normal, as gestational diabetes does sometimes develop in the third trimester (and usually goes away after the baby is born). High levels of sugar in your blood mean that the baby is also getting more sugar, so you will be given dietary advice to bring the levels down.
A baby with an average head size and a small tummy may just be a small healthy baby - babies come in all shapes and sizes! Sometimes though, this pattern can indicate that your baby is not growing properly - known as intra-uterine growth restriction (IUGR) - especially if the amniotic fluid is reduced. If this is suspected, your doctor will continue to monitor you closely to determine whether the baby has to be delivered early or not.
Why am I getting an ultrasound to check my placenta?
This is very common because up to 1/3 of placentas are low (near the cervix or completely covering it) on the 18 week anatomy exam. If this occurs, you will be reexamined at about 30 weeks. By this time, most placentas will have moved up out of the way of the cervix but a few will need another exam a few weeks later. Sometimes a vaginal exam may be performed to see how close it is to the cervix. A placenta that is covering the cervix is known as placenta previa; in this case you will need to have a caesarean section. However, this is uncommon because almost all low placentas eventually move up and out of the way of the cervix by term.
Why am I getting an ultrasound to check the position of my baby?
Babies move around and change position frequently throughout pregnancy. The position usually doesn’t matter at all up until about 36 weeks. If your doctor thinks your baby is in the breech position (head up, bottom down) at this stage, you will have an exam to confirm this. Most breech babies are delivered by cesarean section; however, your doctor can still decide to attempt a vaginal delivery.
How does ultrasound assess the well-being of my baby?
It is possible to assess the wellbeing of a baby by watching its movements on the exam. A healthy baby stretches and flexes, moves its arms and legs frequently, opens and closes its hands and makes breathing movements by breathing the amniotic fluid into the lungs.
The baby is given a score out of eight to describe various types of movements. A score of eight usually means your baby is doing fine. The lower the score, the more concern there is about the baby. Absence of these features is a sign that your baby may not be getting enough oxygen from the placenta and early delivery may be indicated. However, often the score is just reduced because the baby is sleeping at the time of the exam so a score less than eight does not always mean there is any problem. It is quite common for a baby to only have a score of six out of eight and still be completely fine. If the score is very low, the technologist will bring this to the attention of the radiologist. Usually the radiologist will inform your doctor and possibly the radiologist will send you to the hospital to get a further evaluation, a non-stress test. This test monitors the baby’s heartbeat to see that it beats at a normal rate and variability - a healthy baby’s heart rate varies from beat to beat and increases when the baby moves.
Does my baby’s anatomy get evaluated on every exam in the third trimester?
The technologist does attempt to visualize all of the baby’s structures on every exam. However, as the baby gets bigger, it gets more compacted into your uterus and visualization becomes difficult. Often many of the structures are very poorly seen. Evaluating the anatomy is not the purpose of an ultrasound in the third trimester because it is not seen well and all attempts to visualize the anatomy should be performed in the 18 to 20 week period when it is possible to have genetic testing to investigate any suspected abnormalities.
This information has been prepared by Dr. Jay A. Zelazo, MD FRCPC (Diagnostic Radiologist) at Canada Diagnostic Centres.
