Macrosomia Workup

Updated: Dec 16, 2020
  • Author: Easha A Patel, MD; Chief Editor: Christine Isaacs, MD  more...
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Laboratory Studies

A glucose tolerance test at 24-28 weeks of gestation screens for gestational diabetes, a known risk factor for macrosomia. Identification and treatment of gestational diabetes has been shown in randomized controlled trials to decrease fetal birth weight, thereby reducing risk of macrosomia. [44, 45]

Early glucose screening is necessary for women with risk factors for the development of diabetes (eg, obesity, strong family history of diabetes, prior pregnancy affected by macrosomia or gestational diabetes).

According to an ACOG Practice Bulletin regarding gestational diabetes mellitus (GDM), antenatal testing should be performed when the mother has pregestational diabetes mellitus or poor glycemic control with GDM. However, the exact timing and the specific testing are based on local practice, rather than on an official recommendation. [46]

Neonatal evaluation for hypoglycemia, polycythemia, hyperbilirubinemia, and electrolyte abnormalities is indicated in all macrosomic newborns because maternal hyperglycemia is the most common cause and sometimes this diagnosis is not made in the mother prior to delivery of her child.

Long-term follow-up care of these infants is needed because they are at risk for obesity and perhaps diabetes in later life. [20]


Imaging Studies

Ultrasound measurements to obtain estimated fetal weights are indicated when clinical assessments indicate a uterine size greater than that expected for the gestational age. An examination within 1-2 weeks of delivery showing an abdominal circumference of 35 cm or larger should alert the clinician to anticipate a fetus with a birthweight of 4000 g or more. The definitive diagnosis can only be made after delivery of the neonate. [47, 48] It has been established that at higher fetal weights there is reduced accuracy of ultrasound in estimating fetal weight, most notable for birth weights over 4500g, thereby providing a greater challenge in care decisions and prediction of birth weight in suspected macrosomic infants. [49]

Jazayeri et al showed in a retrospective study that abdominal circumference measurements made within 2 weeks of delivery can be predictive of a birth weight greater than 4000 g. [50] Note the following:

  • A measurement of 35 cm or more identified more than 90% of newborns with a birth weight greater than 4000 g and occurred in only 18% of the population.

  • An abdominal circumference measurement within 2 weeks of delivery had sensitivity, specificity, and positive and negative predictive values of approximately 90%.

  • Abdominal circumference measurements in patients at risk for macrosomia can provide some clues to the size of the fetus and thus allow appropriate preparations for delivery (see Surgical Care).

Ben-Haroush et al [51] reported ultrasonography to be an accurate way of estimating birth weight as a screening measure. In patients suspected to have macrosomic fetuses, sensitivity was 75% and specificity was 65% resulting in a positive predictive value of 57% and a negative predictive value of 81%. In patients where macrosomia is not suspected, sensitivity was 32% and specificity was 92% resulting in a positive predictive value of 33% and a negative predictive value of 90%. In the overall population of 298 newborns, sensitivity was 56% and specificity was 88% resulting in a positive predictive value of 48% and a negative predictive value of 91%.

Most ultrasound machines have one or more estimated fetal weight equations in the software. Most of these equations are associated with significant errors. The Hadlock formula is commonly used, and it has a mean absolute percent error of 13% for birth weight of ≥4500g compared with 8% for non-macrosomic newborns. No single formula has been proven to be superior to another for detection of macrosomia of more than 4500 g. [49, 52, 53]

More recent studies have confirmed that appropriately performed abdominal circumference measurements by ultrasonography in the third trimester is the best way of predicting neonatal weight. Without doubt, the usefulness of this technique depends on the quality of image obtained in late third trimester and the cut off used to define the at-risk neonate. Studies using different cut-offs have come with a variety of positive and negative predictive values as well as sensitivities and specificities. [54, 55, 56]

Bicocca et al performed a multicenter, retrospective cohort study of all non-anomalous singletons with an estimated fetal weight of ≥4000 g by ultrasound scanning within 14 days of delivery. Cohorts were then divided into two groups based on time interval between the ultrasound scan and delivery (0-7 days and 8-14 days). The rate of detection and false-positive rate for the detection of birth weight of ≥4500 g were compared between the two groups. No significant difference in false-positive rate was found; however, the detection of birth weight of ≥4500 g was higher when the ultrasound scan was performed within 7 days of delivery, irrespective of maternal diabetes. [57]