Growth scans in pregnancy: key pros and cons you need to know

A growth scan is an ultrasound performed during the third trimester of pregnancy, usually around 28 weeks or later. Its primary aim is to check your baby’s size, weight, and overall health, comparing these factors to what is expected for your stage of pregnancy.

By measuring your baby’s head, abdomen, and thigh bone, the scan provides an estimate of your baby’s current size and projected weight at birth. Growth scans also assess the functioning of the placenta by examining blood flow, as well as checking the levels of amniotic fluid surrounding your baby. This information can help maternity staff to determine whether your baby is growing well and whether your placenta is providing adequate support. In almost all cases, placentas function well, but the NHS still operates a ‘worst case’ service where they screen to rule out the highest risks possible – even if you are not at risk.

From around 28 weeks of pregnancy, your midwife will start measuring your bump every 2-3 weeks using a tape measure to take the fundal height measurement. This is the distance from your pubic bone to the top of your uterus, which is then plotted on a customised growth chart to monitor your baby’s progress.

However, fundal height measurements aren’t always perfectly reliable. In fact, a tape measure of your bump should only ever be an estimated size. Factors like the position of your baby, your body type, and even the person measuring can affect the precision. If your midwife notices that your baby seems to be measuring smaller or larger than expected or if there appears to be little growth between assessments, they might suggest a growth scan for a more detailed evaluation.

Growth scans may also be offered in specific circumstances where fundal height measurements aren’t guaranteed to provide a true representation of your baby’s development. This includes pregnancies with multiples (like twins or triplets), or cases where the mother has conditions such as fibroids, a higher body mass index (BMI), or other factors that can make it difficult to measure the bump externally.

Accepting a growth scan is always your choice. They are not mandatory; however, you may want to attend to be sure there are no issues (which is very unlikely).

Growth scans are straightforward and similar to other ultrasound scans you may have had earlier in your pregnancy. During the scan, you’ll be asked to lie on your back, and the sonographer will apply gel to your tummy. They will then use a small handheld device called a transducer to take images and measurements of your baby. This typically takes around 20 to 30 minutes, although it can sometimes take longer if your baby is moving a lot or lying in a tricky position.

The sonographer is there to help make sure your baby is as healthy as possible, and they should guide you through the process every step of the way. They will measure key aspects of your baby, including the circumference of their head and abdomen, as well as the length of their femur (thigh bone). These measurements are used to estimate your baby’s weight and compare it to standard growth charts. The scan will also check the blood flow through the umbilical cord and placenta, as well as amniotic fluid levels, which can provide important clues about your baby’s well-being.

Growth scans are generally considered a reliable method for monitoring your baby’s growth and development. However, it’s important to remember that these scans provide estimates rather than exact measurements. The estimated foetal weight and size can have a margin of error, which some studies suggest can be as much as 10-15%. This means that while a growth scan can give you a good idea of how your baby is progressing, it’s not a perfect science.

It’s also worth noting that growth scans are just one tool among many that healthcare providers use to assess your baby’s health. They are most useful when combined with other assessments, such as your overall health, your baby’s movements, and the results of any other tests you may have undergone during your pregnancy.

If a growth scan suggests something unexpected, be sure to ask for more information. Your care team will support you in understanding the results and the best next steps.

Yes, you can. Growth scans are not mandatory, and it’s entirely up to you whether you wish to have one. Your midwife or doctor should explain why a scan is being recommended and what information it may provide. If you have any concerns or questions, please don’t hesitate to ask! It’s important that you feel fully comfortable with any decisions you make regarding your pregnancy care.

If you decide that you would prefer not to have a growth scan, that choice should be respected. Declining a scan does not mean that your healthcare providers will be unable to monitor your baby’s growth; they will continue to use other methods, such as fundal height measurements and assessments of your baby’s movements, to ensure that everything is progressing properly.

If a growth scan suggests that your baby is smaller than expected, your healthcare team will discuss the findings with you in the context of your overall pregnancy. Babies who are small for gestational age (SGA) are those who fall below the 10th growth centile but are otherwise healthy. These babies are typically just naturally small, and there may be no cause for concern.

However, if your baby is diagnosed with foetal growth restriction (FGR), it usually indicates that there may be additional factors affecting their growth, such as issues with the placenta or conditions like pre-eclampsia. FGR is associated with higher risks, including complications during pregnancy and labour, so babies with FGR are usually monitored more closely. In this situation, your healthcare provider will work with you to create a care plan that prioritises the health of both you and your baby.

Let’s list out the benefits of growth scans as well as their potential drawbacks, so you can make an informed decision that feels right.

Pros:

• More accurate estimates. Growth scans can provide a clearer estimate of your baby’s size and weight compared to external measurements alone, especially in cases where fundal height measurements are less reliable.
• Assessment of placental health. Growth scans evaluate the blood flow through the placenta and the levels of amniotic fluid, which can offer important insights into your baby’s wellbeing.
• Tailored care. For pregnancies with complications, such as multiples or medical conditions affecting growth, growth scans allow for more personalised and proactive care.

Cons:

• Margin of error. Although growth scans provide helpful estimates, they are not exact measurements and can be off by as much as 15%, which can sometimes lead to unnecessary concern.
• Increased anxiety. Being told that your baby is measuring smaller or larger than average can cause worry, even if your baby is healthy and thriving.
• Potential for over-medicalisation. Depending on the findings, growth scans may lead to increased monitoring or interventions, such as induction of labour or planned c-sections, which might not always be necessary.

Growth scans can be a valuable way to ensure that your baby is growing as expected, and are a source of reassurance to many mothers. However, it’s important to weigh the positives against the negatives, such as the margin of error and the possibility of increased anxiety or medical interventions.

As with your entire pregnancy journey, always feel free to discuss your concerns and preferences with your midwife. They are there to support you, and you have the right to ask questions, seek clarity, and make informed decisions that are aligned with the birth experience you’ve been hoping for.