Scans and monitoring are a vital part of antenatal care, offering reassurance and insight into your baby’s development. By the time you reach the third trimester (weeks 28 to 40), most people have already had their routine 12- and 20-week scans. So what happens next?
While third trimester scans aren’t standard for everyone, they may be offered for specific reasons. Here’s what to expect, when extra scans are needed, and how these check-ups help ensure your baby’s wellbeing.
Are Scans Routine in the Third Trimester?
In a low-risk pregnancy, the NHS typically offers two standard ultrasounds:
- Dating scan around 8–14 weeks
- Anomaly scan around 18–21 weeks
Third trimester scans are not offered routinely, but many people will still have one or more extra scans during this time for medical reasons or reassurance.
More on routine scans from the NHS
Common Reasons for Third Trimester Scans (list is not exhaustive)
You may be offered an additional scan if:
- Your bump is measuring larger or smaller than expected
- You have a low-lying placenta detected at your 20-week scan
- You have gestational diabetes, high blood pressure, or a BMI over 35
- You’re carrying twins or more
- You are currently smoking
- You have fibroids
- It has been found you have Low PAPP-A
- Previous small baby
- There are concerns about baby’s growth or position
- You’ve experienced recurrent reduced movements
These scans are safe and non-invasive, and they can help your healthcare team decide if any additional monitoring or planning is needed.
What Happens During a Growth Scan?
A third trimester scan is often referred to as a growth scan. It usually takes place between 28 and 40 weeks, depending on your circumstances.
During the scan, the sonographer will:
- Measure your baby’s head, abdomen, and femur (thigh bone)
- Estimate your baby’s weight
- Check the amount of amniotic fluid
- Assess the position of the placenta
- Check baby’s position (head-down, breech, or transverse)
- Observe blood flow using Doppler ultrasound (in some cases)
You might be asked to come with a full bladder to help with image clarity. The scan usually takes around 20 minutes, and results are shared with you immediately or shortly after.
Can I Find Out My Baby's Size or Weight?
A growth scan provides an estimated fetal weight (EFW) and measurements plotted on a centile chart. This helps midwives and obstetricians assess whether your baby is growing as expected.
It’s important to remember:
- These measurements are estimates, not exact predictions
- A baby’s growth rate over time matters more than one-off measurements
What About Placenta Position Checks?
If you were told you had a low-lying placenta at your 20-week scan, you’ll likely have a follow-up scan around 32–34 weeks. This checks whether the placenta has moved up as your uterus expands.
In most cases, the placenta moves away from the cervix, allowing for a vaginal birth. If it remains low, your care team will discuss a birth plan, which may include a planned cesarean.
Will I Need More Scans Near My Due Date?
If your pregnancy goes past 40 weeks, your midwife may offer additional monitoring, which might include:
- A presentation scan to check baby’s position
- Assessment of amniotic fluid levels
- A membrane sweep or discussion about induction
Midwife’s Top Tips
- Bring your maternity notes to every scan so results can be recorded
- Ask questions during the scan if anything is unclear
- Track your baby’s movements and report any changes immediately
- Plan extra time around the appointment in case follow-ups are needed
- Relax and enjoy the moment - seeing your baby again is always special
While third trimester scans aren’t standard for everyone, they offer valuable insights when needed. Whether you’re checking your baby’s growth, placenta position, or movements, these scans are an important tool to keep you and your baby safe and supported.
Always speak to your midwife if you have concerns about your bump, movements, or symptoms - you’ll never be wasting anyone’s time.
