Fetal Macrosomia
Fetal macrosomia means your baby is large at birth. Some healthcare providers consider a large weight to be 8 pounds and 13 ounces (4,000 grams) or more. Some think it’s 9 pounds and 14 ounces (4,500 grams) or more. There’s no agreement among the scientific community on the exact definition of fetal macrosomia.
Another phrase you may hear is “large for gestational age” (LGA). This means your baby’s weight at birth is above the 90th percentile for the week in pregnancy that you delivered them. In other words, your baby is bigger than 9 out of 10 babies who are born at the same gestational age.
Fetal macrosomia can be unpredictable. Babies can be born larger than average with or without any known risk factors. It’s hard to know if a baby will be bigger than normal. Some babies are large even when there aren’t any clear reasons But if you have risk factors, your pregnancy care provider will talk with you about how it might affect your delivery.
There aren’t any symptoms of fetal macrosomia. It’s hard to know the exact size of the fetus until it’s born. Most of the pregnancy discomforts you feel are the same, no matter how big the fetus is.
There’s no single cause of fetal macrosomia. But several factors can increase your risk of having a big baby. Diabetes during pregnancy is a significant risk factor. This could be gestational diabetes or preexisting diabetes, like Type 1 or Type 2.
Your blood provides nutrients to the fetus through the placenta, including sugar (glucose). So, if you have high blood sugar, the fetus has it, too. In response, the fetus releases insulin and growth hormone.
These hormones can lead to the fetus having more body fat and an overall larger size. Newborns with fetal macrosomia due to maternal diabetes also tend to have larger shoulders. This can increase the risk of shoulder dystocia — when one or both of your baby’s shoulders get stuck in your pelvis during vaginal delivery.
Other risk factors for fetal macrosomia include:
- Obesity before pregnancy: If you have obesity, you’re 4 to 12 times more likely to have a baby with fetal macrosomia.
- Excessive weight gain during pregnancy: The fetus may receive excess nutrients, leading to a larger size.
- Previous babies with macrosomia: If you’ve had a baby with macrosomia before, you’re 5 to 10 times more likely to have another large baby.
- Having had multiple births: If you’ve given birth to three or more children, you’re more likely to have large babies in your next births.
- If you were bigger than 8 pounds at birth: You’re twice as likely to give birth to a baby weighing more than 8 pounds and 13 ounces if you were over 8 pounds at birth.
- Going two weeks past your due date: Being pregnant for more than 42 weeks increases the chance of macrosomia due to the continuous supply of nutrients to the fetus.
- Being tall: Women in the 80th percentile or higher for height are more likely to have a baby with fetal macrosomia.
It’s important to note that you could have none of these risk factors and still have a large baby. Many things contribute to your baby’s growth and size.
Having a large baby can increase the risk of certain complications during childbirth, including:
- Abnormal changes in the fetus’s heart rate during labor (fetal distress)
- Heavy bleeding or postpartum hemorrhage
- Longer labor, especially during the pushing phase
- Shoulder dystocia
- Uterine rupture
- Vaginal tears
After birth, a newborn with fetal macrosomia is at risk of the following complications:
- Breathing problems
- High red blood cell count
- Jaundice
- Low blood sugar
Your healthcare team will act quickly to manage any of these complications.
The only way to diagnose macrosomia is by weighing your newborn.
Your healthcare provider may suspect it during your pregnancy based on fundal height measurements and risk factors. They may recommend an ultrasound to take a closer look. Still, ultrasounds aren’t very precise at estimating fetal size.
An ultrasound can only estimate a fetus’s weight within about 10%. For example, if the ultrasound estimates the fetus is 9 pounds, that’s the “best guess.” The fetus could actually weigh somewhere between 8 and 10 pounds.
Your provider will combine information from your ultrasound with your pregnancy history to suggest the safest type of delivery. Don’t hesitate to ask questions or voice any concerns.
Your healthcare provider will discuss the risks of a vaginal delivery with you. In some cases, they may recommend a C-section to reduce the chances of a complication.
Delivering your baby early doesn’t seem to reduce complications. So, providers don’t recommend scheduling a delivery before 39 weeks unless there’s a medical need.
What can I expect?
Most deliveries of babies with macrosomia are uncomplicated. Still, issues could arise. Your baby may need to spend time in the NICU if they develop breathing problems or low blood sugar. Know that your baby’s care is in good hands if this happens.
If you deliver a large baby, your healthcare provider may recommend tests to screen you for undiagnosed diabetes. They’ll also monitor you more carefully during any future pregnancies.
Studies show that fetal macrosomia may have long-term effects on infants. These include obesity, Type 2 diabetes and cardiovascular disease later in life. But these conditions have many other causes and risk factors.
Can this be prevented?
Many factors contribute to fetal growth. So, you can’t completely prevent fetal macrosomia. But some steps you can take to reduce the risk include:
- Getting regular prenatal care and screenings: Frequent visits allow your healthcare provider to track your health and the fetus’s growth. It’s also important to do the glucose tolerance test to screen for gestational diabetes.
- Exercising during pregnancy: Aerobic and strength-training exercises during pregnancy can help reduce the risk of macrosomia. Discuss what types of physical activity are safe for you with your provider.
- Managing diabetes: If you have gestational diabetes, eating low-glycemic foods can help manage it and reduce the risk of macrosomia. If you have preexisting diabetes, keeping your blood sugar in range as much as possible can help reduce the risk of having a big baby.
- Managing obesity: Several strategies can help you reach a healthy weight before pregnancy. These include eating changes, exercise, weight loss medications and/or bariatric surgery.
References
- Akanmode AM, Mahdy H. Macrosomia (https://www.ncbi.nlm.nih.gov/books/NBK557577/). 2025 Feb 6. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2025 Jan-. Accessed 6/18/2025.
- Macrosomia: ACOG Practice Bulletin, Number 216 (https://pubmed.ncbi.nlm.nih.gov/31856124/). Obstet Gynecol. 2020 Jan;135(1):e18-e35. Accessed 6/18/2025.
- Nguyen MT, Ouzounian JG. Evaluation and Management of Fetal Macrosomia (https://pubmed.ncbi.nlm.nih.gov/33972073/). Obstet Gynecol Clin North Am. 2021 Jun;48(2):387-399. Accessed 6/18/2025.
- Scifres CM. Short- and Long-Term Outcomes Associated with Large for Gestational Age Birth Weight (https://pubmed.ncbi.nlm.nih.gov/33972069/). Obstet Gynecol Clin North Am. 2021 Jun;48(2):325-337. Accessed 6/18/2025.
- Wang J, Moore D, Subramanian A, Cheng KK, Toulis KA, Qiu X, Saravanan P, Price MJ, Nirantharakumar K. Gestational dyslipidaemia and adverse birthweight outcomes: a systematic review and meta-analysis (https://pubmed.ncbi.nlm.nih.gov/29786159/). Obes Rev. 2018 Sep;19(9):1256-1268. Accessed 6/18/2025.
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