Calculation Software

Most current sonographic instruments provide the sonographer with the ability to record a multitude of obstetric measurements and perform calculations during the examination. Because of the various regression models available, it is important to know which is being used to obtain consistency within the laboratory. With regard to diversity, it is also important to understand the obstetric population with which one is working and to make appropriate adjustments to the regression models when necessary.

Pregnancy Dating

Crucial information used in the proper dating of a pregnancy includes an accurate LMP date and the performance of an early baseline sonogram for later comparison. However, in about 20% to 40% of all pregnancies, the correct menstrual age is uncertain because of unknown or unclear LMP dates. Accurate assessment of the fundal height of the uterus during physical examination in cases of maternal obesity may be extremely difficult. In addition, pregnancy dating and the determination of estimated fetal weight involve the following basic assumptions:

These assumptions are averages calculated over time and across multiple populations. Just one of these assumptions may have a significant impact on the calculation of pregnancy dating. Taken together, a much wider potential range of error exists. A common source of LMP errors is due to implantation bleeding or spotting, which is mistaken for the LMP. This implantation bleeding often occurs 2 to 3 weeks after the normal LMP and leads to an estimated gestational age that is less than the true age, resulting in sonographic estimates that appear larger than dates. For these reasons, an early baseline sonographic examination is the most reliable indicator of fetal age that may be referenced throughout the duration of the pregnancy.

Risk Factors

The major risk factor for macrosomia is gestational diabetes, which accounts for 40% of all cases. The prevalence rate of macrosomia is 25% to 42% among diabetic mothers versus 8% to 10% among nondiabetic mothers. Despite the higher frequency in diabetic mothers, nondiabetic mothers account for 60% of macrosomic cases because of their majority compared with the smaller number of diabetic mothers. Macrosomia is associated with enlargement of the placenta (Fig. 19-1; see Color Plate 17). A placental thickness obtained at a right angle to its long axis that measures greater than 3 cm before 20 weeks’ gestation or greater than 5 cm before 40 weeks’ gestation is considered abnormal.

Time of delivery is an important factor to consider. One study showed an increased incidence of macrosomia from 1.7% at 36 weeks’ gestation to 21% at 42 weeks’ gestation. Chronic and progressing macrosomia is in direct proportion to an elevated risk of associated conditions, and women who have previously delivered a macrosomic infant are at an increased risk in future pregnancies. Primary perinatal complications include shoulder dystocia, soft tissue trauma, humeral and clavicular fractures, brachial plexus injury, facial palsies, meconium aspiration, prolonged labor, and asphyxial injuries. Shoulder dystocia (Fig. 19-2) occurs when the arm of the fetus prevents or complicates delivery and may result in serious traumatic injury. Because many of these injuries are unpredictable events, available evidence suggests that planned interventions on the basis of estimates of fetal weight may not significantly reduce the incidence of shoulder dystocia and the adverse outcomes attributable to fetal macrosomia. However, because evidence strongly suggests an increased risk of prenatal complications for pregnancies with fetuses weighing greater than 4500 g, the option for cesarean delivery should be considered with fetal macrosomia in diabetic patients with small pelvic structures and in any pregnancy where macrosomia is a concern.

The sonographic fetal AC measurement has been determined to be helpful in identification of potential macrosomic infants. One study reported a less than 1% incidence rate of infant birth weights greater than 4500 g with AC measured less than 35 cm. The incidence rate significantly increased to 37% in cases where the AC was greater or equal to 38 cm. Macrosomic infants of diabetic mothers usually have organomegaly, especially a disproportional enlargement of the heart, liver, adrenals, and adipose tissue.

Hydrops fetalis is also associated with macrosomia and may manifest sonographically with one or more of the following: increased placental thickness, increased thickness of scalp or body wall greater than 5 mm (Figs. 19-3 and 19-4), hepatosplenomegaly, pleural and pericardial effusions, ascites, and structural fetal anomalies.

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