Your Pregnancy Matters

Worried about having a big baby? Four things to know about birth weight

Your Pregnancy Matters

Although methods exist to estimate birth weight, it is difficult to accurately predict a baby’s weight before birth.
As my patients approach their final weeks of pregnancy, they typically start asking questions about how much the baby will weigh and what that means for delivery.

Doctors think about this too. We try to accurately determine the weight of the unborn baby so we can make the best possible birth plan. But predicting birth weight is actually very difficult to do.

In a recent study, one-third of women reported their OB provider told them their baby might be getting “quite large” near the end of pregnancy. In the end, however, only one in five of those women had a baby that weighed more than 8 pounds, 13 ounces or 4000 grams, a common threshold for labeling a baby “large.”

It would be nice to know the exact birth weight of a baby before it’s born – it makes predicting some rare, but serious complications like birth trauma easier. But that’s just not possible, and our estimates of your baby’s size can potentially cause unnecessary stress on mom and lead doctors to intervene when nothing is really necessary.

Here are some things to think about as you approach the end of pregnancy and are wondering what your baby might weigh and how this could impact your labor and delivery experience:

1. What is considered a ‘big baby’?

Picking an absolute cut-off is tricky. Typically, we consider estimated weights of babies that weigh more than 4500 grams (10 lbs.) as larger than normal (or “macrosomic”). But what we really want to know is whether your baby is too big for your pelvis.

Weight is just one factor doctors consider when we’re estimating the chances of a patient successfully having a vaginal delivery. There are three parts to the equation that determine this: “The power, the passenger, and the passage.”

The “power,” or force of uterine contractions, is something we can only assess when labor begins.

The “passenger” refers to the baby. His or her weight isn’t the only factor – the exact position of the baby within the birth canal also plays an important part in making it out successfully. The direction the head is facing can make all the difference in how easily the baby descends in the birth canal.

“Passage” refers to the anatomy of your pelvis. When doctors do vaginal exams toward the end of the pregnancy, that’s what we’re trying to evaluate: How narrow is the pelvis? Can we reach the tailbone? Do we think this baby will fit through this pelvis?

Like the estimation of fetal size, this isn’t an exact science, but it can help us determine a birth plan for the day of delivery.

2. Ultrasound is not very reliable for estimating fetal weight near term.

For a 9-pound baby, an ultrasound’s predictive accuracy is typically 15 to 20 percent off. Which means we may over- or underestimate by more than a pound. Why is there such a wide range?

Ultrasound uses volumes to calculate fetal weight. It takes measurements of the head, waist circumference and some bones and comes up with an estimate. But it can’t measure the density of the fetal tissues directly.

I explain it to my patients like this: I can measure something with ultrasound the size of a brick. That structure will look the same whether made of Styrofoam or stone – but something made of stone obviously weighs a lot more than something made of Styrofoam.

There is a formula for calculating fetal weight based on standard measurements – but not all babies follow the rules for getting the weight accurate! There is no way to know exactly how much a baby is going to weigh until after the baby is born.

3. Who is really at risk for having a big baby?

Certain health and history factors put mothers at increased risk for having a large baby:

  • Diabetes, including gestational diabetes, is a concern, especially if the mom’s sugar levels have not been controlled well during pregnancy. High glucose levels in mom can cross the placenta and lead to high levels in the fetus. As a response to these high sugar levels, the fetus produces insulin, which stimulates its own growth.
  • Maternal obesity is another big risk factor. The rate of macrosomia has increased over time with rising obesity rates, so we expect to actually see more macrosomic babies in the future.
  • History of a previous big baby. The trend is for successive babies to get bigger, not smaller, so this is something we would take into consideration. 
If you are not in one of these categories and your health care provider has expressed concern about the size of your baby, I recommend having an honest conversation with him or her about their worries and recommendations.

4. What are the concerns about having a large baby?

We have to consider the possible risks to two patients – both mother and infant. The most serious risk of fetal macrosomia is birth trauma for the baby, especially something called a shoulder dystocia, where after the baby’s head is delivered the rest of his or her body does not deliver easily.

I’ve been in this situation before – it can be pretty frightening, and I completely understand why doctors are quick to recommend a C-section when worried about the possibility for a large baby – it’s an easier way to get the baby out safely.

At the same time, C-sections come with additional risk to the mom and result in additional hospital days, longer recovery periods and added costs. And since our estimates of fetal weight are frequently off, these risks may be unnecessary.

Making a plan that works for you

It is challenging to identify moms and babies that we really need to be concerned about. When doctors label a woman as having a possible “big baby,” we increase the possibility of using interventions during labor such as scheduled C-sections or labor inductions. The average weight of babies that were predicted to be “large” in the study I mentioned earlier was only 7 pounds, 11 ounces – not so big after all.

The recommendations from the American Congress of Obstetricians and Gynecologists are clear. ACOG says ultrasound is no better than a provider’s exam in estimating fetal weight, suspected macrosomia should not be an indication for induction of labor, and planned C-sections shouldn’t be performed unless the estimated fetal weight is 10 pounds or more in diabetic women or 11 pounds or more in other women.

I recommend having an open and frank discussion with your doctor about the clinical information available to you. Discuss the estimated size of your baby, your obstetrical history, your physical exam, and your risk factors. Balance the potential risks and benefits to both you and your baby when making your final decision.

For more information about pregnancy, labor, and delivery, sign up to receive Your Pregnancy Matters email alerts when we publish new stories. You can also make an appointment to see one of our specialists by calling 214-645-8300.