Third-Trimester Growth Scans in Ajax

Plenty of pregnancies reach the final trimester without any further ultrasound after the mid-pregnancy anatomy scan. Others include one or more growth scans in the last few months. If your provider has just handed you a requisition for one, a few common questions tend to follow. Is something wrong? What will they measure? How accurate is a weight estimate made through the abdomen?

Here is a quick checklist of reasons a third-trimester growth scan is often requested. Your own reason may be on this list, or it may be something specific to you:

• Your bump is measuring larger or smaller than expected at a prenatal check
• A previous baby was small or large at birth
• You have a health condition such as diabetes or high blood pressure, or one has developed during this pregnancy
• You are expecting twins or more
• The placenta was noted as low-lying earlier and needs a recheck
• Your provider wants to confirm the baby’s position as the due date approaches
• Your provider simply wants more information to plan the final weeks

Being on that list does not mean there is a problem. In many cases a growth scan is about gathering information so that decisions near the end of pregnancy are well informed. This article, for parents in Ajax, Pickering, Whitby and nearby, explains what the scan generally looks at and how to make sense of the terms you may hear afterward.

The four core measurements

A growth scan relies on a set of standard measurements called biometry. You may see these abbreviations on paperwork:

• BPD, the biparietal diameter, which is the width of the head
• HC, the head circumference
• AC, the abdominal circumference, measured around the baby’s tummy at a set level
• FL, the femur length, the length of the thigh bone

The sonographer finds a precise view for each measurement and places markers on the screen. These views have to be lined up carefully, so you may notice the probe being angled and re-angled several times. Late in pregnancy, the baby’s head is often low in the pelvis and the limbs are tucked, which can make some measurements take longer.

The abdominal circumference is often given particular attention because it tends to reflect the baby’s stores of fat and the size of the liver, both of which relate to how well the baby is being nourished.

From measurements to estimated weight

The four measurements are entered into a formula that produces an estimated fetal weight. It is a helpful number, but it is important to understand what kind of number it is. It is an estimate based on the size of certain body parts, not a weighing. Late-pregnancy estimates can differ from the actual birth weight by a meaningful margin, in either direction. Factors such as the baby’s position, the amount of fluid and the mother’s body tissue can all affect precision.

That is why providers rarely make decisions on a single estimated weight alone. They look at it alongside the other information they have about you and your pregnancy.

Percentiles and the shape of the curve

Your provider may describe the baby’s size as a percentile. A percentile compares the baby’s estimated size with other babies at the same gestational age. A baby at the 50th percentile is right in the middle; a baby at the 20th percentile is smaller than average but well within the range of normal.

Percentiles depend on an accurate gestational age, which is why a reliable early due date matters so much. Our article on the dating ultrasound explains how that age is usually set.

Why one scan is often not enough

Many healthy babies are simply small or large because their parents are. What tends to interest providers more than a single percentile is the trend over time. A baby who has measured around the same percentile on two or more scans is following a consistent curve. A baby whose growth slows or speeds up noticeably between scans may need closer attention.

Because babies grow gradually, repeat growth scans are usually spaced a couple of weeks apart or more. Measuring too soon would not show a meaningful change, given the margin of error. Your provider will suggest the interval that suits your situation.

Fluid, placenta and blood flow

Size is only part of the picture. A third-trimester growth scan may also include several other observations.

Amniotic fluid

The amount of fluid around the baby is commonly measured, either by finding the deepest single pocket of fluid or by adding together pockets in four sections of the uterus. Fluid levels can give clues about the baby’s wellbeing and the function of the placenta. Levels that are lower or higher than expected are something your provider will interpret in context.

The placenta

If the placenta was lying low earlier in pregnancy, a later scan can show whether it has moved away from the cervix as the uterus grew. This is common, and a recheck is a routine part of care in that situation.

Doppler blood flow

When your provider wants more information, Doppler ultrasound may be used to look at blood flow in the umbilical cord and, sometimes, in other vessels. It shows how blood is moving rather than what structures look like. Doppler is the same principle used in adult vascular ultrasound, applied here to a much smaller patient.

Position

The scan usually notes whether the baby is head-down, bottom-down (breech) or lying across the uterus. Many babies turn head-down on their own in the last weeks, so a breech position earlier in the third trimester is not unusual. Position becomes more relevant to planning as the due date approaches.

Staying comfortable on the exam bed

Late pregnancy brings its own practical challenges. Lying flat on your back for a while can press on a large vein and make some people feel faint, sweaty or nauseated. A few things that help:

1. Let the sonographer know right away if you start to feel unwell.
2. A slight tilt to one side, often with a wedge or pillow behind your back, is common and helpful.
3. Use the washroom just before the scan unless your instructions say otherwise.
4. Wear a two-piece outfit so you can uncover your bump without changing completely.
5. Bring a bottle of water and a snack for afterward.

Short breaks are fine. The sonographer would much rather pause than have you struggle through.

Reduced movement is not a reason to wait for a scan

This point deserves its own section. In the third trimester, many providers encourage parents to get to know their baby’s usual pattern of movement. If you notice that your baby is moving less than usual, or the pattern has changed, do not wait for a scheduled growth scan and do not try to manage it on your own at home. Contact your provider or your hospital’s labour and delivery unit right away. They may want to assess you the same day.

Likewise, seek urgent care for vaginal bleeding, fluid leaking, regular painful contractions before term, severe abdominal pain, a severe headache with vision changes, or sudden swelling of the face and hands. Call 911 if you feel very unwell. A community imaging clinic is not equipped for emergency pregnancy assessment.

Fitting growth scans into a busy final trimester

The last few months of pregnancy tend to fill up quickly. Prenatal visits often become more frequent, there may be classes or hospital tours, and many people are still working or caring for other children. When growth scans are added on top, a little planning goes a long way.

Some ideas that parents in Ajax and nearby communities often find useful:

• Book the next scan before you leave. If your provider has asked for repeat growth scans at a set interval, call to book as soon as you have the requisition so the timing lines up with their plan.
• Line up appointments on the same day where it makes sense, so you are not travelling back and forth more than you need to.
• Consider a Saturday. Our Saturday hours in Ajax can suit people who would rather not take time off work for each scan.
• Keep paperwork together. A folder or envelope with your health card, current requisition and preparation instructions saves hunting around on a tired morning.
• Plan the drive home. Late pregnancy can be tiring, and a longer scan on top of a busy day is a lot. If someone can come with you, it can make the day easier.

None of this is essential, but small adjustments can make the final weeks feel less rushed.

After the scan

The images and measurements are reviewed and a report is sent to the health-care provider who ordered the scan. The sonographer usually will not discuss findings during the appointment, which is standard practice. Our post on how imaging results reach your doctor describes that process in more detail.

When you speak with your provider, it can help to ask:

• Where is my baby on the growth curve, and how does that compare with earlier scans?
• Is the fluid level within the expected range?
• Do you recommend another scan, and when?
• Does anything in this scan change our plans for the final weeks?

Booking your growth scan

Third-trimester obstetrical scans are part of our ultrasound service at Rossland Diagnostic Clinic, Unit 102, 300 Rossland Road East, Ajax. Free parking close to the door and ground-floor access make visits easier late in pregnancy. We are open Monday to Friday from 9:00am to 5:00pm and Saturday from 8:00am to 3:30pm. Bring your health card and requisition, then call 905-426-3111 or request an appointment at a time that lets you take it slowly.