top of page

Potential Big Babies: Understanding the Implications and Accuracy of Scans

Pregnancy is an exciting journey filled with anticipation, and one important aspect of prenatal care is monitoring the growth and development of the baby. In some cases, healthcare providers may identify a potential big baby during routine scans. In this blog post, we will explore what it means to have a potential big baby, how it can impact your care, and discuss the accuracy of scans in predicting a baby's size. Typical antenatal care in the UK offers 2 scans - the 12 week and the 20 week scan. If there are no concerns raised after these, then no follow up scans will be offered. At 28 weeks your midwife will start to measure your bump size - or fundal height - to track how baby is growing. This is a manual measurement that the midwife does using a tape measure and her hands, and it's a general measurement without a lot of accuracy. If your midwife feels that your bump is 'too big' or 'too small' they will advise you to follow up with a growth scan to check baby's size. If you choose to have a growth scan, you may be told that baby is measuring 'big' or 'small'.


Understanding the Term "Potential Big Baby"

A potential big baby refers to a fetus that is estimated to have a larger birth weight compared to the average. This estimation is usually based on ultrasound scans, where measurements such as abdominal circumference, femur length, and head circumference are taken to assess fetal growth. While these scans can provide valuable information, it's important to remember that they are only estimates, and the actual birth weight may vary.


Impact on Antenatal Care

When a potential big baby is identified, healthcare providers may take additional steps to ensure the well-being of both the baby and the mother. These steps may include:

  1. Monitoring: Regular check-ups and scans may be scheduled to closely monitor the baby's growth and assess any potential complications.

  2. Gestational Diabetes Screening: Diabetes during pregnancy (gestational diabetes) can contribute to a larger baby. Therefore, your healthcare provider may recommend a glucose tolerance test to rule out or manage this condition. Read our blog post on gestational diabetes here.

  3. Birth Planning: Knowing the estimated size of the baby can help in developing a birth plan. In some cases, healthcare providers may discuss the possibility of a vaginal birth versus a cesarean section (C-section) to ensure a safe delivery. It is worth noting that the NICE guidelines do not recommend early delivery based on a 'potentially large baby' if there are no other medical complications present.

Accuracy of Scans in Predicting Baby's Size: Ultrasound scans are commonly used to estimate a baby's size, but they have inherent limitations. There is a recognized 15% margin of error in ultrasounds for weight estimations. Factors that can affect the accuracy of these estimates include:

  1. Timing: The accuracy of size estimates can vary depending on the gestational age at which the scan is performed. Earlier scans tend to have higher accuracy rates compared to later scans.

  2. Operator Skill: The experience and expertise of the sonographer or healthcare provider performing the scan can influence the accuracy of the measurements.

  3. Fetal Position: The position of the baby during the scan can affect the measurements. For instance, if the baby is curled up or in an unfavorable position, it may be challenging to obtain precise measurements.


It is essential to remember that while estimates of a big baby can provide valuable information, they are not absolute predictions. Many factors can influence a baby's birth weight, including genetics, maternal health, and lifestyle. It is also worth remembering that identifying a potential big baby during routine scans can have implications for your prenatal care and birth planning. While ultrasound scans are commonly used to estimate a baby's size, it is important to recognize their limitations. Your healthcare provider will guide you through the process, ensuring the best possible care for both you and your baby. Remember to discuss any concerns or questions you have with your healthcare team, as they are there to support you throughout your pregnancy journey.

bottom of page