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‘Your baby is big'. Growth scans in your third trimester.

‘Your baby is big'. Growth scans in your third trimester has been a topic I have spoken to multiple clients about recently, and how the pressure of having an additional third trimester growth scan can start to build. This ultrasound is usually done at some point between 24-40 weeks gestation (presuming that your pregnancy has been low risk up until this point). Being told that “your baby is big” or is above the 90th centile can lead to further growth scans and being referred to consultant led care for the remainder of your pregnancy. Talk of induction prior to 40 weeks often follows this referral.

Man and woman at a growth scan with sonographer
What is measured at a growth scan?

So what happens at a growth scan?

These growth scans are usually in addition to your initial dating scan at around 12 weeks and your anomaly scan at around 20 weeks and may be requested if your pregnancy has been deemed higher risk due to a BMI over 35, gestational diabetes or due to other health reasons.

Along with having your fundal height measured (size of your bump from top to bottom), the sonographer will conduct the ultrasound in the same way as the others you may have had and measurements of the baby’s head circumference, abdomen, length of their thigh bone (femur) will be measured, along with measuring the fluid around the baby, their position within your womb along with your placenta’s position.


These growth scans are said to be around 85% accurate which leaves a 15% margin for error either way. So, if your baby is measuring on the 50th centile for example, they could actually be measuring 15% above or 15% below this measurement. This is the same for when baby might be measuring ‘big’ or ‘small’ (above the 90th centile, and below the 10th centile respectively). Baby is deemed ‘big’ if their predicted birth weight is said to be 4KG/9lb or more. However, baby’s predicted weight can vary by approximately 0.5KG/1lb either way too.


‘Your baby is big’ they say

This could be when the pressure starts to increase and talk of induction begins. As there is said to be a slightly high risk of shoulder dystocia and bone fractures in bigger babies during birth, having a baby show as ‘big’ in these growth scans will spark induction discussion. This may be because babies who are born after an induced birth have been reported to be on average 0.2KG/0.5lb smaller than babies who were born after spontaneous labour, therefore reducing the risk of fractures and shoulder dystocia.

Ultimately it is always your decision; it’s your body, your baby and your birth and you get to decide what happens (even if it does go against the grain). It has been reported that the advantages of induction for ‘big’ babies do not always outweigh the possible disadvantages of having and induced birth following a likely cascade of further intervention. It is always really important to do your own research on this so that you are able to make an informed decision about your birth. Using your BRAIN to gain the necessary information from your midwives and/or consultant will help with this.


BRAIN stands for; Benefits, Risks, Alternatives, Instinct, Nothing. You can find more information about this on my Instagram page @HypnobirthingWithAimee.


Remember that you are always in control of what happens to you and your baby, so try to be in the most educated and informed position possible.

**This article is not designed to replace any medical advice and you should always consult with your care team before making any decisions.

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