All About Baby Weight Prediction & Third Trimester Ultrasound

For women who want to avoid a C-section, baby weight prediction can cause a lot of stress. That’s because one of the main reasons for C-section is a big baby. 

Hearing that your baby is too large for the birth you envisioned means you’ll have to make difficult choices. Even if you do everything you can to prepare for a successful VBAC, a factor like your baby’s weight is not something you can change.

But how do care providers decide that your baby is too big to deliver vaginally?

Third trimester ultrasounds are one of the ways care providers can estimate a baby’s weight. Although ultrasound is not an accurate way to predict weight, this measurement is frequently used by providers to recommend against vaginal birth.

This article is all about how providers predict baby weight and the factors that determine your baby’s size. 

If you’ve been told you may be having a big baby, keep reading for information that will help you make informed decisions about your birth options.

Why predicting a baby’s birth weight matters

Care providers tend to pay close attention to babies’ weight and size during pregnancy. The size of the baby matters in different ways than some may think. Primarily, it’s important to make sure that the baby is growing properly and in a healthy way. 

During your pregnancy, you will likely have your fundal height measured externally on your belly, in addition to ultrasound, so that your provider can measure your baby. 

On average, a baby will weigh 6-8 lbs. However, it is important to remember that we are all individuals and every baby and pregnancy is different. Babies’ sizes and birth weights can vary.

If a baby measures too small, lower than the 10th percentile, it could be something called IUGR (Intrauterine Growth Restriction). IUGR is considered lower than 5 lbs 8 oz. If your provider feels your baby has IUGR they may suggest an early induction to help the baby thrive on the outside. 

According to the American College of Obstetricians and Gynecologists, a baby over 8 lbs 13 oz OR 9 lbs 4 oz, no matter what the gestational age, is considered Macrosomic. 

Macrosomia means that a baby is considered above the normal birth weight. If a provider feels a baby measures on the larger side, they may suggest induction. Some providers worry a little more about VBAC induction, and may also suggest scheduling a repeat C section.

Ultrasound technicians look at many different measurements when measuring a fetus. They check for the overall well-being of the fetus by checking the heart, brain, bone lengths, and more. Besides checking size, they want to make sure to catch any abnormalities along the way.

How accurate are ultrasound weights?

Measuring weight by ultrasound is a way a provider can determine due dates and how large or small a baby may be in utero. 

The hardest part of measuring weight through ultrasound is it’s really only correct half of the time. Ultrasound weight estimates can vary 1-2 lbs either way from the baby’s actual birth weight. 

Although induction is often suggested, it is not advised to base a mode of delivery or scheduled induction based on ultrasound measurement alone. 

The most common measurements used to predict baby weight are the head, belly, and femur length via Biparietal diameter (BPD). If an ultrasound scan comes back outside of normal, your provider may suggest doing multiple scans to follow up.

Why a care provider may suggest a third trimester ultrasound

Although the 20-week ultrasound is standard practice, sometimes care providers request additional ultrasounds later in your pregnancy. These are some of the reasons you may need to have an ultrasound in your third trimester:

• Placenta covering the cervix at 20-week ultrasound: It is routine to check the position of the placenta during the 20-week ultrasound. If the placenta is covering the cervix (placenta previa), they will check later in pregnancy to make sure it has moved away from the cervix. 
• Confirm position: If your provider is not able to confirm your baby’s position or it feels like the baby may have turned, they may suggest an ultrasound as late as 41-42 weeks to confirm that the baby is head down. 
• Failed non-stress test: If your provider ordered a non-stress test for a medical reason, such as a non-reassuring heart tracing or lack of movement felt by the mother, they may order an ultrasound to check on the baby. 

Ultrasounds can be effective for monitoring a baby’s weight and any medical concerns. However, they can also be inaccurate, and just because something is being watched, doesn’t mean it will be an issue when the baby is born.

For example, we heard from a VBAC Link follower about an abnormal femur measurement on her 20-week ultrasound. She was told there could be something wrong with the baby, but there was no way to be sure other than to continue measuring. Her providers suggested she come in for weekly ultrasounds, and up to twice weekly at the end of her pregnancy. 

As you can imagine, she experienced a lot of fear during her pregnancy at the thought of something being wrong with her baby. One person even suggested she induce at 37 weeks, which she did not feel good about. Her provider still suggested early induction, which she agreed to. 

After an awesome induced VBAC she held a sweet healthy baby in her arms. The baby did not have anything wrong medically and is doing great. 

During pregnancy, it is hard to know what to do and what not to do. If your provider is suggesting a third trimester ultrasound, don’t be scared to ask questions. Verify the reason and whether it is an evidence-based decision. 

As always, educate yourself and follow your intuition when it comes to making decisions about your birth.

Baby weight prediction & your birth options

If your provider suspects that your baby is on the larger side, they may suggest a C-section for the mode of delivery rather than a vaginal birth. 

When it comes to large babies, providers often will fear something called shoulder dystocia. This is when the baby’s shoulders can get stuck in the process of coming out. 

Shoulder dystocia can be scary for providers because the baby can be “stuck” and need assistance to get out faster. When larger babies are measured, it is possible they may suggest a C-section, or even tell the mother she can’t have a VBAC.

Less than 10% of babies are actually born with macrosomia. Studies have shown that birth weights are not increasing over time — they are actually measuring smaller instead of larger. Even so, a high percentage of medical inductions and Cesareans are performed due to concern about the baby’s size. 

As Dr. Gene Declercq states on Birth By the Numbers, of the 30% of births that are induced in the US, 16% are due to size concerns. Suspected big babies are also cited as the reason for 9% of Cesarean births, making it the fifth most common reason.

If you’re being told that your baby is too large to deliver vaginally, you still have options. There are some things you can do to improve your chances of a vaginal birth, regardless of your baby’s size. 

We suggest the following for everyone, but these tips are especially beneficial if your baby has been given a “larger” diagnosis:

• Go to the chiropractor: Visiting with a chiropractor during pregnancy can assure alignment and help create space in the pelvis for your sweet little one to arrive.
• Hire a doula: There are so many benefits of hiring a doula. They can help you navigate through birth. They are trained in birth and know tips and tricks to help babies come down and get in an ideal position. We have amazing VBAC doulas here at The VBAC Link. 
• Prolong getting an epidural: When it comes to birth we recognize that not everyone wants to go unmedicated. Going natural or getting an epidural is a personal choice. When you do not have an epidural it can allow you to move easier into positions that can help the baby navigate through labor in a smooth way. With that said having an epidural can also be a great tool in our birth toolbox to help with relaxation of the pelvic floor and allow the baby to come down nicely. 
• Have faith in your body and the birth process: Sometimes all our bodies and babies need is faith. Our bodies are incredible, and birth happens safely every day. Mentally preparing for labor and for your VBAC starts as early as you can. Working through fears with a fear release activity is a great way to process your fears leading up to birth and even during birth.

What determines baby birth weight?

The main way a provider will determine birth weight is via ultrasound. They may even call this a weight calculator. 

However, there are other things a provider will look at that may cause their baby weight prediction to be on the larger side.

These are some reasons a care provider may request a third trimester ultrasound and even suggest a C-section:

• Mother’s weight gain or plus size pregnancy.
• If the mother tests positive for gestational diabetes. This is typically tested for and diagnosed if the mother’s sugar levels are high in the 24th-28th week of pregnancy. If you have gestational diabetes, you may be referred to a nutritionist or advised to focus hard on your nutrition.
• You simply look “big” or you’re measuring on the larger side.
• Previous C-section after pushing for an extended amount of time. 
• You are a petite person and someone in your close family had a C-section due to a large baby. Sometimes, providers will suggest that a large baby could be hereditary, so if your mom or sister had larger babies, they may be more concerned about your baby’s size. 

Overall, if you are being told that you need a third trimester ultrasound, be sure to ask why. And remember, when it comes to the size of the baby, ultrasound can vary and even be inaccurate. 

If you are trying to avoid a repeat cesarean, the good news is there are things you can do to encourage vaginal birth, even with a larger baby.

Our Ultimate VBAC Course for Parents covers everything there is to know about vaginal birth after a Cesarean — from finding supportive providers to preparing your body and mind. 

Check it out today and get all the support you need on your VBAC journey.

FAQ

How much weight should my baby put on each week?

The amount of weight your baby gains each week can vary. Each baby is different and each pregnancy is different. Providers will typically say that a baby will grow anywhere from ½ lb to a lb each week the last few weeks of pregnancy. Again this is a general guess, and all babies are different. 

What are the signs of a big baby?

There are no true signs of a large baby. The only way to know exactly how much a baby weighs is for a baby to be born and weighed. A provider may estimate a baby’s size via third trimester ultrasound or by the external measurement or looks of the mother. 

Is baby weight in ultrasound accurate?

The simple answer is no. Ultrasounds in the third trimester are known to be inaccurate and can vary up or down by a pound or more. If your provider is suggesting an ultrasound to check weight in the third trimester, this may be a red flag that they are less VBAC supportive or may start to question the best mode of delivery for you. Keep in mind, “big” babies are born vaginally all the time. 

Does parents birth weight affect baby birth weight?

Studies show that genetics may play a part in the size of the baby. Even with this though they show that a quarter of the babies did not share the same gene of the mother and weight was affected by the pregnancy, and amount of glucose.