Macrosomia: Being Pregnant With a Large Baby

Causes, prevention, treatment, and complications

During your pregnancy, your doctor will monitor your baby’s size. Growth is a good indicator of your baby’s health and well-being, but sometimes babies grow more than expected.

It can be a bit scary to find out that your baby might be big, and you may worry about what it’s like to carry and deliver a large baby. Here’s what you need to know about the causes, treatment, prevention, and complications of having a big baby.

Macrosomia Risk Factors
Illustration by JR Bee, Verywell

Fetal Macrosomia

The average newborn weighs around 7 to 7½ pounds (3200 grams 3400 grams), with most full-term newborns weighing between 5 pounds 11 ounces and 8 pounds 6 ounces (2600 grams to 3800 grams).

A baby is larger than average when his birthweight is over the 90th percentile or 8 pounds 13 ounces (4000 g). The medical term for this is macrosomia.

Approximately 3 to 15 percent of babies are born weighing over 8 pounds 13 ounces (4000 g).

The chances of having a more difficult delivery with injuries to both mom and baby go up with the baby’s weight. The possibility of complications is higher when the baby is over 9 lbs 15 ounces (4500 g), and the risk is highest when the baby weighs over 11 pounds (5000 g). Working closely with your doctor can help cope with and prevent any possible complications.

Causes

Many things can influence your baby's size. Sometimes the doctors do not know the reason that a baby is big. But, your weight, health, and genetics can all play a part. You have a greater chance of having a big baby if: 

  • You have high blood sugar. You are more likely to have a big baby if you have diabetes before you become pregnant or develop gestational diabetes while you're pregnant.
  • You have had a large baby before. If you’ve already had a child that was born large, your chances of having another one are higher.
  • You are overweight before your pregnancy. You are more likely to have a big baby if you are overweight or obese before you become pregnant.
  • You gain an excessive amount of weight during your pregnancy. What you eat during your pregnancy and how much weight you gain can affect your baby’s birth weight. 
  • You have been pregnant multiple times. It is not always the case, but in general, the more babies you have, the bigger they tend to be.
  • You are having a boy. Boys typically weigh a little more than girls, and larger babies are often male.
  • You are Hispanic or White. Your ethnicity can have an impact on the size of your baby. Caucasian and Hispanic mothers tend to have bigger babies than mothers from other backgrounds.
  • You are past your due date. Babies keep gaining weight and growing while they are inside the womb. So, the longer your pregnancy continues beyond 40 weeks, the bigger your baby will be.
  • You have a family history of big babies. Sometimes it’s genetic. If you or your partner come from a family of big babies, you can pass those genes down to your child. And, if you were a big yourself, then you have a greater chance of having a big baby.
  • You are of advanced maternal age. You are more likely to have a big baby if you are pregnant over the age of 35.

Symptoms and Diagnosis 

There is no way to know your baby’s real weight while you’re pregnant. So, it’s not possible to accurately diagnosis fetal macrosomia until after your child is born and placed on a scale.

Even though the doctors can’t get an exact measurement, they can still estimate your baby’s size.

Your doctor will determine the approximate size and weight of your baby by: 

  • Going over your risk factors: Your doctor will review your family history, health history, pregnancy history, current health, weight, and diet to find out if you might be at risk for macrosomia. 
  • Measuring your fundal height: Your doctor will measure your fundal height (the height of your growing uterus). If your belly is measuring larger than expected for how far along you are supposed to be, then you may be carrying a large baby.
  • Feeling your abdomen: The doctor or midwife will move her hands along your belly to feel the baby’s size and position.
  • Monitoring your weight: The doctor will keep track of your weight gain and talk to you about your diet since obesity, and excessive weight gain during pregnancy can lead to a larger baby. 
  • Ordering an ultrasound: An ultrasound can measure the size of the baby’s head, around the belly, and the length of the femur bone in the upper leg. These measurements are used to predict the baby’s weight. 
  • Checking your amniotic fluid: Having a lot of amniotic fluid is called polyhydramnios, and it’s associated with macrosomia.

    Your risk factors, the measurements of your belly and amniotic fluid, and the ultrasound are all clues. On their own, they cannot tell you how big your baby will really be. But, by gathering all this information and looking at it together, the doctor can get a good idea of your baby’s general size. 

    Prevention

    You cannot always prevent a big baby. Some babies are just genetically big, and that’s OK. But, since there can be complications when babies get too large, your doctor will monitor you, your pregnancy, and your possible risk factors to help you have the healthiest pregnancy, delivery, and baby possible.

    To prevent complications from fetal macrosomia, your doctor will:

    • Keep track of how much your uterus grows by measuring it at each visit
    • Monitor your weight gain
    • Monitor your baby’s growth through measurements and ultrasound
    • Order bloodwork and other prenatal tests to check you for any health issues 
    • Test you for gestational diabetes
    • Help you manage and control diabetes if you have it
    • Help you manage weight gain or obesity if it’s an issue
    • Recommend a nutritionist or dietitian if necessary
    • Refer you to other health professional such as an endocrinologist or perinatologist if necessary

    You can do your part by: 

    • Preparing for pregnancy by seeing your doctor for preconception advice
    • Working to get to a healthy pre-pregnancy weight
    • Going to all your prenatal check-ups
    • Getting all the bloodwork and prenatal testing your doctor orders
    • Staying within the recommended guidelines for weight gain during pregnancy
    • Eating healthy, well-balanced meals 
    • Getting a little exercise
    • Seeing a nutritionist, endocrinologist, perinatologist, or another specialist if necessary

    Treatment and Delivery

    When a doctor suspects a baby is large, there really isn’t any treatment. There is preparation.

    Your doctor will want to have as much information as possible about your baby’s estimated size, weight, and health so they can plan for a safe delivery. You can also prepare by learning as much as you can about having a big baby so you can work with the doctor to make the best decisions for you and your child.

    Your doctor should take the time to talk to you and answer all your questions about delivering vaginally or by cesarean section depending on:

    • The estimated size of your baby
    • The size of your pelvis
    • Your baby’s gestational age
    • Your pregnancy history
    • Your current situation and physical health
    • The risks to you and your baby 

    Vaginal Delivery

    Your baby's size isn't the only thing that your doctor looks at when planning your delivery.

    If the signs point to a large baby, it doesn’t automatically mean you have to have a C-section. You may still be able to try for a vaginal birth.

    The doctor will also take into consideration the size and shape of your pelvis, the baby's position in the birth canal, your health, and the baby's health.

    Cesarean Section

    A C-section may be necessary if there are concerns for your safety or the safety of your baby. Your doctor will recommend it if the baby could be larger than 11 pounds (5000 grams) or you have diabetes and the baby's estimated weight is more than 9 pounds 15 ounces (4500 grams).  

    Induction of Labor

    Your doctor does not have to induce your labor early if you’re carrying a large baby. Early induction of labor doesn’t necessarily prevent complications or birth injuries. However, your doctor might want to get your labor started if you are a week or two past your due date. 

    Complications 

    In general, complications from carrying a big baby are rare, even if your baby is in the 8 to 9-pound range.

    However, the chance of encountering a problem goes up when the baby is over 9 pounds 15 ounces (4500 grams), and even more so if the baby is over 11 pounds (5000 grams). When a baby is very large, there is a greater possibility of a difficult delivery and birth injuries.  

    The risks of macrosomia for the baby are: 

    The risks of delivering a large baby for moms are:

    Again, remember that complications are rare. Your doctor will do everything they can to minimize potential risks, and you should do your part, too.

    Post-Birth Recovery

    Women give birth to large babies all the time.

    A normal vaginal delivery and a quick recovery are always possible, even with a big baby.

    Of course, there is a chance that childbirth will be more challenging. If you have a difficult vaginal birth or a C-section, your recovery may take a little longer. 

    You should:

    • Follow the nurse or doctor’s instructions to care for an episiotomy, a tear, or your C-section incision site. 
    • Try to get enough rest.
    • Ask for help while you recover.
    • Go to all your follow up appointments. 
    • See any additional doctors you need to see, especially if you have diabetes. 
    • Take your time and don’t overdo it. 

    A Word From Verywell

    It is normal to worry about the size of your baby as you get closer to your due date. The thought of delivering a big baby or needing a C-section could make you nervous. But, remember, estimates of your baby’s size aren’t always accurate, and your due date could be off, too. Many moms who are expecting a big baby are surprised to find that their baby’s birthweight is within the average range when the nurse puts the baby on the scale.

    Even if your baby is big, you and your doctor can manage and plan for the delivery. With good information, care, and monitoring, most big babies are born safely and without serious, long-term complications.

    If you are anxious about the delivery or concerned about your health and your baby’s health, you can talk to your doctor, go to a childbirth class and look to your family and friends for support. You can also find encouragement and support from moms who’ve been there by visiting some of the online pregnancy forums.

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