Third Trimester Growth - Advanced Women's Imaging

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.