Tests and diagnosis

By Mayo Clinic Staff

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.

April 16, 2015