In the last few months of pregnancy, your baby is gaining weight and growing so much it doesn’t feel like there’s enough room in there.
In fact, you are probably wondering exactly how big your baby is going to be by the time you reach full term.
A common question a pregnant woman might type into a search engine is: Can an ultrasound tell me how big my baby is?
How Accurate Is Ultrasound For Weight?
Often a woman does this because her care provider has told her the baby is already 8 pounds, at 36 weeks, and she might need a c-section.
Ultrasound for fetal weight estimation is notoriously inaccurate for predicting the weight of babies. This makes it unreliable as a basis for informed decision making about induction or c-section.
Why Estimate My Baby’s Size?
One of the reasons for prenatal appointments is an opportunity for your care provider to check your baby’s growth. If there is a problem with your baby’s wellbeing, being too small or too big for gestational age can be an indication that further investigation is needed.
In the past, a woman would subtract her pre-pregnancy weight from her current pregnant weight to determine how heavy her baby was.
However this method didn’t take other factors into account – such as the weight of the amniotic fluid, the placenta, or even a woman's normal weight gain during pregnancy.
These days, fundal height measurement is used as a way of gauging your baby’s growth and size. This is the measurement from the pubic bone to the top of your uterus.
After you reach 20 weeks gestation, your fundal height will be in the same range as the number of weeks you have been pregnant.
Some care providers suggest having a routine third trimester growth scan, at around 36 weeks. Unless there is a concern about your baby’s size, there is no reason why you should agree to having this scan.
How Does Ultrasound Estimate Size?
Ultrasound is commonly used in pregnancy to look inside and get an idea of what is happening. Sonar waves are bounced off tissues, which then can be seen as images on a screen.
Measurements are taken from four areas, and then used to calculate the estimated size of your baby.
The four measurements most frequently used to estimate fetal size are:
- Head circumference, which is the distance around baby’s head
- Biparietal diameter, or the distance between sides of baby’s head. Babies of similar weights can have different head sizes, so this data is not always reliable later in pregnancy
- Abdominal circumference, which most accurately reflects baby’s size
- Length of the femur, or upper thighbone. This is the longest bone in the body and can be different lengths in babies of different weights, so is not always reliable
Your care provider can put this data into a computer software program, or use charts, to determine the estimated birth weight.
How Accurate Is Ultrasound Measurement?
There are many different mathematical formulae for estimating fetal weight; most of them were devised in the 1980s.
It’s important to know there is a recognised 15% error in ultrasound fetal weight estimation. This means your baby can be either 15% smaller or 15% larger than the estimated weight given at your ultrasound.
Ultrasound scans are generally more accurate for assessing your baby's size during the first half of pregnancy. As babies develop, they can grow in uneven spurts, and so the accuracy depends on whether your baby is small or average in size.
The closer you get to your estimated due date, and the bigger your baby becomes, the harder it will be to measure him.
What Does This Mean For Me?
If your care provider has concerns about your baby’s growth, it is important to know whether there are any underlying issues causing it.
Most commonly, an ultrasound scan will be performed, and then another 2-3 weeks later. The second scan helps to determine if there have been any changes in your baby’s growth.
If there is still an issue, your care provider should suggest you have a biophysical profile done, to check on baby’s wellbeing and to see if there are any reasons for the growth discrepancy.
Once this has been done, and if no reason is discovered, it might be that you are more closely monitored for the remainder of your pregnancy. Any further decisions will be based on how well your baby is doing.
My Care Provider Recommends…
There is concern that ultrasounds for weight estimation can increase the likelihood women will be induced, or have a surgical birth, because of the fear of having a large baby.
If your care provider has already declared your baby to be ‘big’ before birth, it is more likely you will go into labour with a set of preconceived ideas about giving birth to a big baby.
During labour, your care provider and staff might then make assumptions based on the idea you are pregnant with a big baby.
This study demonstrates how using ultrasound to estimate a baby is ‘big' can increase the likelihood that women will have a c-section. The women were given an ultrasound before induction, and then the labour outcomes of women with overestimated fetal weights were compared with those whose fetal weights were not overestimated.
Overestimation of fetal weight occurred in 9.5% of cases. The rate of c-sections for stalled labour was higher for those with fetal weight overestimation, although there were no differences in length of the induction duration.
If you and your baby are happy and healthy, there should be no reason to worry about how much your baby will weigh at birth.
If your care provider gives you a weight estimate, remember it is only a guess. There is no 100% reliable way to measure a baby’s weight until after birth.