Birth weight can be fairly accurately predicted in obese women at 34-36 weeks gestation, using a calculation that involves gestational age projection.

Birth weight can be fairly accurately predicted in obese women at 34-36 weeks gestation, using a calculation that involves gestational age projection (GAP).

The calculation and its accuracy rates are described in the July issue of Ultrasound in Obstetrics and Gynecology.

Principal investigator Dr. Loralei L. Thornburg and colleagues at the University of Rochester (New York) reviewed the records of 1,382 women with singleton pregnancies who had undergone fetal ultrasound examinations between 34-36 weeks gestation. The group consisted of 357 obese women and 1,025 controls.

There were 159 women with class I obesity (BMI between 30-34.9 kg/m²), 105 women with class II obesity (BMI between 35-40 kg/m²) and 93 women with class III obesity (BMI greater than 40 kg/m²).

"The GAP method was able to predict actual birth weight within 20 per cent for all groups in over 90 per cent of cases," the Rochester team reports. The mean birth-weight prediction error was within four per cent for all groups.

"For all groups, the GAP method correctly excluded the presence of macrosomia with 90 per cent or greater specificity," Dr. Thornburg and colleagues say. "Negative likelihood ratios for the prediction of macrosomia were between 0.4 and 0.6 for all groups, regardless of obesity."

"In general, ultrasound in pregnancy is one of the greatest tools available to the obstetrician, allowing us to objectively measure and evaluate the fetus," Dr. Thornburg commented during an interview with Reuters Health.

"Obesity, especially morbid obesity, significantly limits the visualization and evaluation of the fetus by ultrasound...The greatest value of the GAP method in obesity is the ability to exclude macrosomia in all groups of obese patients, and improve our ability to counsel patients regarding risks and delivery options," she said.

"Obese patients often require more ultrasounds during pregnancy than their non-obese counterparts, since monitoring and evaluation of fetal growth by physical examination in the obese patient is more difficult," Dr. Thornburg pointed out.

"Repeat ultrasound examinations may also be necessary to complete fetal anatomic evaluations as well," she added.

Ultrasound Obstet Gynecol 2008;32:66-70