Third Trimester Growth


A third trimester ultrasound is performed in the last part of the pregnancy, usually after 22-24 weeks gestation.

A third trimester ultrasound is performed using transabdominal ultrasound.

Transabdominal ultrasound involves scanning through your lower abdomen.  A small amount of ultrasound gel is put on the skin of the lower abdomen, with the ultrasound probe then scanning through this gel. The gel helps improve contact between the probe and your skin.

Sometimes a transvaginal ultrasound is also needed during a third trimester ultrasound. This may be to check for a low lying placenta, to look at the length of the cervix or there may be another indication to have this type of scan. Transvaginal ultrasound during pregnancy, including the third trimester, is safe and will not harm either you or your baby. Your sonographer will be experienced at performing these ultrasounds during pregnancy.

Transvaginal ultrasound is an internal ultrasound. It involves scanning with the ultrasound probe lying in the vagina. The transvaginal ultrasound probe is thin, about 2cm diameter. The probe is covered with a disposable protective sheath. A small amount of ultrasound gel is placed on the end of this probe. The probe is then gently inserted a short distance into the vagina by the sonographer. All transvaginal probes have been cleaned and sterilised according to recommended protocols.

Performing the transvaginal ultrasound usually causes less discomfort than a pap smear. No analgesia is required for this ultrasound.

Your privacy will always be respected during your ultrasound, especially the transvaginal examination. You will have a large towel covering your lower body, in addition to wearing a gown during the transvaginal ultrasound.

You will always have a choice about whether transvaginal ultrasound is performed. If you have concerns about transvaginal ultrasound, please discuss this with your sonographer.

We usually get better images during transabdominal ultrasound if the bladder is partially filled, so to help your examination we ask you to drink water prior to the assessment. Please empty your bladder 1 hour before your appointment, drink 1-2 glasses of water and try not to empty your bladder again until after your appointment.

A full bladder moves bowel out from the pelvis into the abdomen, helping visualisation of the pregnancy, uterus and cervix.

Your bladder should not be so full that it causes pain. If your bladder is very full and painful, you should empty a small amount so you are more comfortable.

You will be able to empty your bladder after the transabdominal ultrasound is completed and before the transvaginal ultrasound begins (if transvaginal ultrasound is required).

As the baby grows in the last part of the pregnancy, the uterus becomes larger and heavier. Lying down for this ultrasound can sometimes cause expectant mothers to become faint and nauseous, especially with a multiple pregnancy or large baby. This faint feeling is due to a fall in blood pressure, caused by compression of the veins that run at the back of the mother’s abdomen (the inferior vena cava) by the uterus and baby.

Please tell the sonographer if you begin to feel faint or nauseous at all during your scan. It is no trouble to change your position. Your position can be changed before you feel worse – lying more on your side or more upright will usually help resolve your symptoms.

Not all women need to have an ultrasound in the later part of the pregnancy.

Your doctor may request a third trimester ultrasound for many reasons including:

  • Assessment of the baby’s size and welfare (well-being). Your doctor may be concerned that your baby is too small, not growing well, or too large. Perhaps you have a condition that may affect the growth of the baby, such as high blood pressure or diabetes.
  • Review of the placenta. Your doctor may wish to review the position of the placenta if there were concerns of a low lying placenta on your 18-20 week morphology ultrasound or if you have unexplained vaginal bleeding. Most women with a low-lying placenta seen at 18-20 weeks will not have a problem by the time they get to the third trimester. As the uterus gets bigger, it tends to pull the placenta up and away from the cervix.
  • Your doctor may be concerned if you have symptoms such as pain, contractions, vaginal bleeding or reduced fetal movements. A third trimester ultrasound can help provide reassurance that baby is well.
  • Review of the baby’s anatomy. Your doctor may wish to review a change, concern or abnormality in your baby which was noted at your 18-20 week morphology ultrasound.
  • Assess the position of the baby. Your doctor may be uncertain about the position of the baby (for example, if baby is lying in a breech position). This becomes more important towards the end of the pregnancy when the delivery of the baby gets closer.
  • You have a twin/multiple pregnancy. Twins are at higher risk of growth problems during the pregnancy. Depending on the type of twin pregnancy, your babies may also be at increased risk of other complications (such as twin to twin transfusion syndrome with monochorionic twins).

A third trimester ultrasound will usually include each of the following components however some ultrasounds may focus more on particular areas.

Your ultrasound is always performed in the context of your clinical history and the results of previous ultrasounds and investigations.

Measurements of the baby

The third trimester ultrasound will commonly measure:

  • baby’s head – biparietal diameter (BPD) and head circumference (HC).
  • baby’s abdomen  – abdominal circumference (AC).
  • baby’s leg – femur length (FL).

Each measurement is compared to a normal reference range, which varies with gestation. Every individual baby has its own characteristics (for example, some babies have bigger head measurements or shorter femur measurements). These characteristics are often similar to the baby’s parents (for example, one parent has a larger than average head size).

Measurements outside the normal range are not always significant, especially if the difference is minimal. Your baby will be carefully assessed if there are concerns about significant deviations from normal.

These head, abdominal and femur measurements are combined in a special formula to estimate the weight of your baby.

The size of the baby (the estimated fetal weight).

The estimated fetal weight is compared to the size of other babies at the same gestation.
This is often expressed as a percentile:

  • An EFW on the 50th percentile is an average sized baby.
  • An EFW less than the 10th percentile is a small baby.
  • An EFW more than the 90th percentile is a big baby.

This ultrasound weight is an estimation of your baby’s size only – there is a small recognised % error in this estimation, so that your baby may be either smaller or larger than the estimated weight on ultrasound. While we recognise there is this error present in our estimation of your baby’s size, ultrasound remains the best way your doctor has of checking the size of your baby.

Serial ultrasounds (more than one ultrasound) are required to assess the growth pattern of your baby over time. Every baby has its own individual growth pattern, and this can be monitored if there are concerns.
Your doctor will usually leave 10-14 days between such ultrasounds to allow more accurate assessment of growth.

The amount of amniotic fluid around your baby.

This amount of fluid is usually expressed as the “amniotic fluid index” (AFI). This index is calculated by measuring the maximal vertical distance of fluid in each quadrant (or corner) of the pregnancy sac. There is a wide range for the normal volume of amniotic fluid in a pregnancy, and this range will vary with gestation.

Sometimes, the volume of fluid around your baby may be increased above normal (polyhydramnios), or perhaps the volume of fluid around your baby is below the normal range (oligohydramnios).Changes in the fluid volume are not always significant, especially if the difference is minimal. You and your baby may be checked for other possible problems (for example, polyhydramnios can be associated with gestational diabetes, and oligohydramnios can be associated with small babies).

The blood flow in the umbilical cord (the umbilical artery).

The blood flow in the umbilical artery (which is in the baby’s umbilical cord) will be measured. Sometimes blood flow in the baby’s brain (the middle cerebral artery) and liver (the ductus venosus) are also checked. Such measurements of blood flow in the baby’s blood vessels are known as doppler studies.

They help assess the function of the placenta and the health, welfare and well-being of your baby. These measurements are expressed using different terms, including resistance index (RI), systolic/diastolic ratio (S/D ratio) and pulsatility index (PI).

Babies that are not growing normally (known as growth restricted) may show progressive changes in the blood flow of these vessels. Changes in these measurements are not always significant, especially if the difference is minimal. Monitoring such changes can help your doctor decide if your baby needs to be delivered early.

While serial ultrasounds to check the baby’s growth are usually done at intervals of at least 10-14 days, ultrasounds to review doppler studies and amniotic fluid volumes can be performed more frequently if required.

The baby’s heart rate and rhythm.

Your baby’s heart rate will vary, just as it does in adults. Most babies have a heart rate between 120-180 beats per minute.

The position of the baby.

This ultrasound will tell us what position the baby is lying in:

  • Head down (cephalic)
  • Bottom down, with the head at the top of the uterus (breech)
  • Sideways, across the uterus (transverse)

The position of the baby is more important towards the end of the pregnancy, when the baby is due for delivery.

The position of the placenta.

Your doctor will want to know that the lowest edge (inferior margin) of the placenta is not lying too close to the cervix. This is known as placenta praevia or a low-lying placenta.
Transvaginal ultrasound may be required during your third trimester ultrasound if there are concerns about the position of the placenta, to get a better look at the cervix area and lower edge of the placenta.

The anatomy of the baby.

As your baby grows during the pregnancy, it fills up the space inside the uterus, pressing its body against the wall of the uterus. This means some parts of the baby may be more difficult to see in the third trimester, especially hands and feet. The baby’s position will also affect how well some structures are seen, including the heart, face, and spine.

Some of the structures which we try to routinely review in a third trimester ultrasound include the baby’s kidneys, bladder and face.

The length of the cervix.

This is especially important if you have premature labour, vaginal bleeding or pain. The length of the cervix is not as important for us to know as you get closer to your due date (full term).
Sometimes a transvaginal ultrasound may be required to get a better view of the cervix.

The uterus

The uterus is checked for problems such as uterine fibroids.
If fibroids are present, their size and location will be noted.

3D/4D images of your baby.

Many parents enjoy the bonding experienced when they see their baby on 3D/4D imaging.
We aim to obtain good 3D/4D pictures of the baby’s face, but we also think it is important to make sure that your baby is otherwise healthy and growing well.
For this reason, the 3D/4D images of your baby are usually done after the baby has been fully examined with all routine measurements and other images completed.
The best time to obtain 3D/4D images of baby is between 24 and 34 weeks.

Beautiful and clear images of your baby, especially baby’s face, are often seen on this third trimester ultrasound.

It is natural for many parents to think that as their baby grows bigger, it is always easier to see the baby on ultrasound. This is unfortunately not always true. In fact, many parents find it more difficult to understand what they are looking at! The third trimester ultrasound usually focuses on one part of the baby at a time (for example, baby’s head) rather than giving an overview of the whole baby at once (for example, like the images of the baby’s body seen on the nuchal translucency ultrasound).

Many factors influence what parts of the baby can be seen and how well it is seen. These factors include the position of the baby, the volume of amniotic fluid (low fluid makes it harder to see) and the size of the mother’s abdomen (increasing skin thickness makes it harder to see). The sonographer will attempt to obtain the best possible images of your baby, and to explain these images as they scan.

Good 3D/4D imaging requires a co-operative baby and adequate amniotic fluid in front of the structure being imaged. Some babies press against the wall of the uterus or placenta, or they may have their arms or hands lying in front of the face. This will make obtaining 3D/4D images difficult or impossible.

It is good for parents to anticipate seeing their growing baby, but not to be too disappointed if this ultrasound proves difficult to understand or the images of baby’s face are impossible to get.