Estimating or predicting a baby's birth weight is difficult. A definitive diagnosis of fetal macrosomia can't be made until after the baby is born and weighed.

Keep in mind that any assessment of a baby's size during pregnancy depends on accurate knowledge of his or her gestational age. If a baby is large for his or her gestational age, it's important to confirm whether your projected due date is correct.

If you have risk factors for fetal macrosomia, your health care provider will likely use tests to monitor your baby's health and development, 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.

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If your health care provider suspects fetal macrosomia, a vaginal delivery isn't necessarily out of the question. However, you'll need to give birth in a hospital — in case forceps or a vacuum device are needed during delivery or a C-section becomes necessary.

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:

  • You have diabetes. If you had diabetes before pregnancy or you develop gestational diabetes and your health care provider estimates that your baby weighs 9 pounds, 15 ounces (4,500 grams) or more, a C-section might be the safest way to deliver your baby.
  • Your baby weighs 11 pounds or more and you don't have a history of maternal diabetes. If you don't have pre-gestational or gestational diabetes and your health care provider estimates that your baby weighs 11 pounds (5,000 grams) or more, a C-section might be recommended.
  • You delivered a baby whose shoulder got stuck behind your pelvic bone (shoulder dystocia). If you've delivered one baby with shoulder dystocia, you're at increased risk of the problem occurring again. A C-section might be recommended to avoid the risks associated with shoulder dystocia, such as a fractured collarbone.

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, after childbirth your health care provider will test you 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.

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Coping and support

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.

Preparing for your appointment

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:

  • What is likely causing the condition?
  • What kinds of tests do I need?
  • What needs to be done now?
  • Do I need to follow any restrictions?
  • How will fetal macrosomia affect my baby?
  • Will I need to have a C-section?
  • Will my baby need tests or special care after he or she is born?

In addition to the questions you've prepared, don't hesitate to ask other questions during your appointment.

May 19, 2018
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  7. Frequently asked questions. Pregnancy FAQ098. Special tests for monitoring fetal health. American Congress of Obstetricians and Gynecologists. http://www.acog.org/Patients/FAQs/Special-Tests-for-Monitoring-Fetal-Health . Accessed Feb. 10, 2015.
  8. Koyanagi A, et al. Macrosomia in 23 developing countries: An analysis of a multicountry, facility-based, cross-sectional survey. The Lancet. 2013:381:476.
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  10. Routine prenatal care. Bloomington, Minn.: Institute for Clinical Systems Improvement. https://www.icsi.org/guidelines__more/. Accessed Feb. 8, 2015.
  11. American College of Obstetricians and Gynecologists (ACOG) Committee on Practice Bulletins — Obstetrics. ACOG Practice Bulletin No. 173: Fetal macrosomia. Obstetrics & Gynecology. 2016;128:e195.
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