The term "fetal macrosomia" is used to describe a newborn who's much larger than average.
A baby who is diagnosed as having fetal macrosomia weighs more than 8 pounds, 13 ounces (4,000 grams), regardless of his or her gestational age. About 9% of babies worldwide weigh more than 8 pounds, 13 ounces.
Risks associated with fetal macrosomia increase greatly when birth weight is more than 9 pounds, 15 ounces (4,500 grams).
Fetal macrosomia may complicate vaginal delivery and can put the baby at risk of injury during birth. Fetal macrosomia also puts the baby at increased risk of health problems after birth.
Fetal macrosomia can be difficult to detect and diagnose during pregnancy. Signs and symptoms include:
Excessive amniotic fluid (polyhydramnios). Having too much amniotic fluid — the fluid that surrounds and protects a baby during pregnancy — might be a sign that your baby is larger than average.
The amount of amniotic fluid reflects your baby's urine output, and a larger baby produces more urine. Some conditions that cause a baby to be larger might also increase his or her urine output.
Genetic factors and maternal conditions such as obesity or diabetes can cause fetal macrosomia. Rarely, a baby might have a medical condition that makes him or her grow faster and larger.
Sometimes it's unknown what causes a baby to be larger than average.
Many factors might increase the risk of fetal macrosomia — some you can control, but others you can't.
Maternal diabetes. Fetal macrosomia is more likely if you had diabetes before pregnancy (pre-gestational diabetes) or if you develop diabetes during pregnancy (gestational diabetes).
If your diabetes isn't well controlled, your baby is likely to have larger shoulders and greater amounts of body fat than would a baby whose mother doesn't have diabetes.
Fetal macrosomia is more likely to be a result of maternal diabetes, obesity or weight gain during pregnancy than other causes. If these risk factors aren't present and fetal macrosomia is suspected, it's possible that your baby might have a rare medical condition that affects fetal growth.
If a rare medical condition is suspected, your health care provider might recommend prenatal diagnostic tests and perhaps a visit with a genetic counselor, depending on the test results.
Fetal macrosomia poses health risks for you and your baby — both during pregnancy and after childbirth.
Possible maternal complications of fetal macrosomia might include:
Possible complications of fetal macrosomia for your baby might include:
Metabolic syndrome. If your baby is diagnosed with fetal macrosomia, he or she is at risk of developing metabolic syndrome during childhood.
Metabolic syndrome is a cluster of conditions — increased blood pressure, a high blood sugar level, excess body fat around the waist and abnormal cholesterol levels — that occur together, increasing the risk of heart disease, stroke and diabetes.
Further research is needed to determine whether these effects might increase the risk of adult diabetes, obesity and heart disease.
You might not be able to prevent fetal macrosomia, but you can promote a healthy pregnancy. Research shows that exercising during pregnancy and eating a low-glycemic diet can reduce the risk of macrosomia.
Fetal macrosomia can't be diagnosed until after the baby is born and weighed.
However, if you have risk factors for fetal macrosomia, your health care provider will likely use tests to monitor your baby's health and development while you're pregnant, such as:
Ultrasound. Toward the end of your third trimester, your health care provider or another member of your health care team might do an ultrasound to take measurements of parts of your baby's body, such as the head, abdomen and femur. Your health care provider will then plug these measurements into a formula to estimate your baby's weight.
However, the accuracy of ultrasound for predicting fetal macrosomia has been unreliable.
Antenatal testing. If your health care provider suspects fetal macrosomia, he or she might perform antenatal testing, such as a nonstress test or a fetal biophysical profile, to monitor your baby's well-being.
A nonstress test measures the baby's heart rate in response to his or her own movements. A fetal biophysical profile combines nonstress testing with ultrasound to monitor your baby's movement, tone, breathing and volume of amniotic fluid.
If your baby's excess growth is thought to be the result of a maternal condition, your health care provider might recommend antenatal testing — starting as early as week 32 of pregnancy.
Note that macrosomia alone is not a reason for antenatal testing to monitor your baby's well-being.
Before your baby is born, you might also consider consulting a pediatrician who has expertise in treating babies diagnosed with fetal macrosomia.
When it's time for your baby to be born, a vaginal delivery won't necessarily be out of the question. Your health care provider will discuss options as well as risks and benefits. He or she will monitor your labor closely for possible signs of a complicated vaginal delivery.
Inducing labor — stimulating uterine contractions before labor begins on its own — isn't generally recommended. Research suggests that labor induction doesn't reduce the risk of complications related to fetal macrosomia and might increase the need for a C-section.
Your health care provider might recommend a C-section if:
If your health care provider recommends an elective C-section, be sure to discuss the risks and benefits.
After your baby is born, he or she will likely be examined for signs of birth injuries, abnormally low blood sugar (hypoglycemia) and a blood disorder that affects the red blood cell count (polycythemia). He or she might need special care in the hospital's neonatal intensive care unit.
Keep in mind that your baby might be at risk of childhood obesity and insulin resistance and should be monitored for these conditions during future checkups.
Also, if you haven't previously been diagnosed with diabetes and your health care provider is concerned about the possibility of diabetes, you may be tested for the condition. During future pregnancies, you'll be closely monitored for signs and symptoms of gestational diabetes — a type of diabetes that develops during pregnancy.
If your health care provider suspects fetal macrosomia during your pregnancy, you might feel anxious about childbirth and your baby's health — and worrying can make it hard to take care of yourself.
Consult your health care provider about what you can do to relieve stress and promote your baby's health. Also consider seeking information and support from women who've had babies diagnosed with fetal macrosomia.
If you have risk factors for fetal macrosomia, the topic is likely to come up at routine prenatal appointments.
Below are some basic questions to ask your health care provider about fetal macrosomia:
In addition to the questions you've prepared, don't hesitate to ask other questions during your appointment.
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