According to researchers, birth weight projection based on ultrasound measurements earlier in pregnancy may be better than standard prediction methods for obese women.

by Crystal Phend, Staff Writer, MedPage Today

ROCHESTER, N.Y., July 11 — Birth weight projection based on ultrasound measurements earlier in pregnancy may be better than standard prediction methods for obese women, researchers said.

Action Points 
Explain to interested patients that ultrasound can be used toward the end of pregnancy to determine how large a baby is, which may affect complication risk and clinical decisions on Caesarean section.

Note that the study was retrospective and did not directly compare birth weight prediction methods.

One such method using ultrasound measurement at 34 to 36 weeks predicted birth weight within 20% accuracy in more than 90% of cases, reported Loralei Thornburg, M.D., of the University of Rochester Strong Memorial Hospital here, and colleagues in the July issue of Ultrasound in Obstetrics and Gynecology.

In their retrospective study, this gestation-adjusted prediction (GAP) method had lower sensitivity among morbidly obese women for the clinically important prediction of macrosomia, but could exclude macrosomia with more than 90% accuracy regardless of maternal body mass index.

Obesity is a risk factor for almost all obstetric complications as well as fetal macrosomia, or birth weight over 4,000 g, which is associated with both neonatal and maternal morbidity.

But obesity can make accurate predictions of birth weight difficult by obscuring fetal anatomy on ultrasound, the researchers said.

Particularly in obese patients, visualization is easier earlier in gestation, they said. “At term there is significant deterioration of resolution as the fluid-to-fetus ratio decreases, bony structures become increasingly calcified, and the vertex descends in the pelvis, making measurements of head circumference and biparietal diameter more difficult.”

Although there are other methods of predicting birth weight before term, the gestational-adjusted prediction method may be best because it is particularly easy and readily accessible to the clinician, Dr. Thornburg’s group said.

However, “whether any method of birth weight prediction can alter fetal outcome remains to be seen,” they noted.

The study included all women with singleton pregnancies who had ultrasound assessment between 34 and 36 weeks’ gestation at the University of Rochester from May 1994 through July 2000.

The cohort included 1,025 women with a body mass index below 30.0 kg/m2 as a control group and 357 obese women, of whom 159 had a BMI of 30 to 34.9 kg/m2, 105 had a BMI of 35 to 40 kg/m2, and 93 had a BMI above 40 kg/m2.

Standard ultrasound techniques were used along with the gestational-adjusted prediction method.

The method calculates the ratio between the current fetal weight estimated by fetal ultrasound and the median fetal weight for the gestational age at which the sonogram was performed.

This ratio is then multiplied by the median birth weight at the gestational age of delivery to give the predicted birth weight.

The method’s predictions were correct within 20% of the actual birth weight in over 90% of cases for all maternal BMI groups.

The predictions likewise were within 15% in more than 80% of cases and within 10% in at least 60% of cases for all groups.

Overall, the method performed as well for obese patients as for nonobese controls, but less well in those with a BMI over 40 kg/m2.

The method tended to overestimate actual birth weight among nonobese women and women with a BMI from 30 to 34.9 and 35 to 40 kg/m2 whereas it underestimated birth weight among those in the highest BMI category (mean error 4.2%, 3.2%, 3.1%, and -1.9%, respectively, P<0.0001).

“This may be owing to increasing maternal obesity, or it may be owing to relatively larger infant weights in the patients with [morbid] obesity,” the researchers said.

Birth weight increased with maternal BMI, as did the percentage of patients with macrosomia — defined as birth weight over 4,000 g (about 8.8 lb).

“Although birth weight prediction is important, prediction of macrosomia is the primary clinical use of many birth weight prediction methods,” the researchers noted.

For macrosomia, the gestational-adjusted prediction method had only moderate sensitivity (48% to 60%) and positive predictive value (45% to 70%) for all maternal BMI groups.

However, the specificity was high at 90% to 95%, as was the negative predictive value at 81% to 96%.

The “good accuracy in excluding macrosomia,” Dr. Thornburg’s group said, “is the more important clinical measure when counseling patients.”

They concluded that further study is warranted to compare this method with other prediction methods.

They cautioned that the study was limited by its retrospective design, particularly because women referred for ultrasound at 34 to 36 weeks’ gestation may have been more likely to have chronic disease and not be representative of the general population.

The researchers provided no information on funding or conflicts of interest.

Additional articles on this topic:

Primary source: Ultrasound in Obstetrics and Gynecology

Source reference: Thornburg LL, et al “Sonographic birth-weight prediction in obese patients using the gestation-adjusted prediction method” Ultrasound Obstet Gynecol 2008; 32: 66-70.

Reviewed by Zalman S. Agus, MD; Emeritus Professor, University of Pennsylvania School of Medicine.

Published: July 11, 2008 

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