Accuracy of estimated fetal weight below 1,000 g

Performance of fetal ultrasound and magnetic resonance imaging in predicting birthweight according to the test-to-delivery interval: A cohort study

OBJECTIVE

To assess the influence of the test-to-delivery interval (TDI) on the performance of ultrasound (US) and magnetic resonance imaging (MRI) for predicting birthweight (BW).

STUDY DESIGN

This is a secondary analysis of a prospective, single center, blinded cohort study that compared MRI and US for the prediction of BW ≥ 95th percentile in singleton pregnancies. Patients that were included in the initial study underwent US and MRI for estimation of fetal weight between 36 + 0/7 and 36 + 6/7 weeks of gestation (WG). The primary outcome of the current study was to report the changes of US and MRI sensitivity and specificity in the prediction of BW > 95th percentile, BW > 90th percentile, BW < 10th percentile, and BW < 5th percentile, according to the TDI. The secondary outcome was to represent the performance of both tools in the prediction of BW > 90th percentile when TDI is<2 weeks, between 2 and 4 weeks, and>4 weeks. Receiver operating characteristic (ROC) curves were constructed accordingly.

RESULTS

2378 patients were eligible for final analysis. For the prediction of BW > 95th or 90th percentile, the sensitivity of MRI remains high until 2 weeks, and it decreases slowly between 2 and 4 weeks, in contrast to the sensitivity of US which decreases rapidly 2 weeks after examination (p < 0.001). For the prediction of BW < 10th or 5th percentile, the sensitivity of both tools decreases in parallel between 1 and 2 weeks. The specificities of both tools remain high from examination till delivery. These findings are reproducible with the use of the antenatal customized and the postnatal national growth charts.

CONCLUSION

The performance of MRI in the prediction of BW, especially in large-for-gestational age, is maximal when delivery occurs within two weeks of the examination, decreasing slightly thereafter, in contrast with the performance of US which decreases drastically over time.

Factors influencing the accuracy of fetal weight estimation with a focus on preterm birth at the limit of viability: a systematic literature review

BACKGROUND

Fetal weight estimation (FWE) is an important factor for clinical management decisions, especially in imminent preterm birth at the limit of viability between 23(0/7) and 26(0/7) weeks of gestation. It is crucial to detect and eliminate factors that have a negative impact on the accuracy of FWE.

DATA SOURCES

In this systematic literature review, we investigated 14 factors that may influence the accuracy of FWE, in particular in preterm neonates born at the limit of viability.

RESULTS

We found that gestational age, maternal body mass index, amniotic fluid index and ruptured membranes, presentation of the fetus, location of the placenta and the presence of multiple fetuses do not seem to have an impact on FWE accuracy. The influence of the examiner's grade of experience and that of fetal gender were discussed controversially. Fetal weight, time interval between estimation and delivery and the use of different formulas seem to have an evident effect on FWE accuracy. No results were obtained on the impact of active labor.

DISCUSSION

This review reveals that only few studies investigated factors possibly influencing the accuracy of FWE in preterm neonates at the limit of viability. Further research in this specific age group on potential confounding factors is needed.

Which is the most accurate formula to estimate fetal weight in women with severe preterm preeclampsia?

OBJECTIVE

To identify the most accurate formula to estimate fetal weight (EFW) from ultrasound parameters in severe preterm preeclampsia.

METHODS

In a prospective study, serial ultrasound assessments were performed in 123 women with severe preterm preeclampsia. The EFW, calculated for 111 live born, normal, singleton fetuses within 7 days of delivery using 38 published formulae, was compared to the actual birth weight (ABW). Accuracy was assessed by correlations, mean (absolute and signed) (%) errors, % correct predictions within 5-20% of ABW and limits of agreement.

RESULTS

Accuracy was highly variable. Most formulae systematically overestimated ABW. Five Hadlock formulae utilizing three or four variables and Woo 3 formula had the highest accuracy and did not differ significantly (mean absolute % errors 6.8-7.2%, SDs 5.3-5.8%, > 75% of estimations within 10% of ABW and 95% limits of agreement between -18/20% and +14/15%). They were not negatively affected by clinical variables but had some inconsistency in bias over the ABW range. All other formulae, including those targeted for small, preterm or growth restricted fetuses, were inferior and/or affected by multiple clinical variables.

CONCLUSION

In this GA window, Hadlock formulae using three or four variables or Woo 3 formula can be recommended.

A review of sonographic estimate of fetal weight: Vagaries of accuracy

PURPOSE

To determine the factors that might influence the accuracy of sonographic estimated fetal weight.

STUDY DESIGN

A PubMed search (Jan 1975 to Jan 2003) of articles published in the English language was carried out and the inclusion criterion was that estimates were within 10% of birth weight. A Chi-square test for trend was used and odds ratio (OR) with 95% confidence intervals (CI) was calculated.

RESULTS

Over 28 years, 175 articles were identified but only 54 (31%) met the inclusion criterion. Overall 62% (8895/14 384) of the predictions were within 10% of the actual weight. The accuracy was significantly different in articles where <7 vs. >7 days were allowed to lapse between examination and delivery (OR 2.17, 95% CI 1.93, 2.45); where examinations were done by registered diagnostic medical sonographers (RDMS; 65%) versus physicians (59%) or residents (57%; p < 0.0001); in term vs. preterm patients (OR 1.97, 95% CI 1.67, 2.13); and in studies with >1000 vs. <1000 cohorts (OR 1.62; 95% CI 1.51, 1.74).

CONCLUSIONS

If feasible the sonographic examination should be done by RDMS and within a week of delivery.

Analysis of factors influencing the ultrasonic fetal weight estimation

OBJECTIVE

The aim of our study was the evaluation of sonographic fetal weight estimation taking into consideration 9 of the most important factors of influence on the precision of the estimation.

METHODS

We analyzed 820 singleton pregnancies from 22 to 42 weeks of gestational age. We evaluated 9 different factors that potentially influence the precision of sonographic weight estimation (time interval between estimation and delivery, experts vs. less experienced investigator, fetal gender, gestational age, fetal weight, maternal BMI, amniotic fluid index, presentation of the fetus, location of the placenta). Finally, we compared the results of the fetal weight estimation of the fetuses with poor scanning conditions to those presenting good scanning conditions.

RESULTS

Of the 9 evaluated factors that may influence accuracy of fetal weight estimation, only a short interval between sonographic weight estimation and delivery (0-7 vs. 8-14 days) had a statistically significant impact.

CONCLUSION

Of all known factors of influence, only a time interval of more than 7 days between estimation and delivery had a negative impact on the estimation.

Nutrient requirements for preterm infant formulas

Achieving appropriate growth and nutrient accretion of preterm and low birth weight (LBW) infants is often difficult during hospitalization because of metabolic and gastrointestinal immaturity and other complicating medical conditions. Advances in the care of preterm-LBW infants, including improved nutrition, have reduced mortality rates for these infants from 9.6 to 6.2% from 1983 to 1997. The Food and Drug Administration (FDA) has responsibility for ensuring the safety and nutritional quality of infant formulas based on current scientific knowledge. Consequently, under FDA contract, an ad hoc Expert Panel was convened by the Life Sciences Research Office of the American Society for Nutritional Sciences to make recommendations for the nutrient content of formulas for preterm-LBW infants based on current scientific knowledge and expert opinion. Recommendations were developed from different criteria than that used for recommendations for term infant formula. To ensure nutrient adequacy, the Panel considered intrauterine accretion rate, organ development, factorial estimates of requirements, nutrient interactions and supplemental feeding studies. Consideration was also given to long-term developmental outcome. Some recommendations were based on current use in domestic preterm formula. Included were recommendations for nutrients not required in formula for term infants such as lactose and arginine. Recommendations, examples, and sample calculations were based on a 1000 g preterm infant consuming 120 kcal/kg and 150 mL/d of an 810 kcal/L formula. A summary of recommendations for energy and 45 nutrient components of enteral formulas for preterm-LBW infants are presented. Recommendations for five nutrient:nutrient ratios are also presented. In addition, critical areas for future research on the nutritional requirements specific for preterm-LBW infants are identified.

Estimation of fetal weight by ultrasound prior to 33 weeks gestation

The aim of this study was to develop an accurate formula for the ultrasonic prediction of fetal weight for infants < 33 weeks gestational age and < or = 1500 g birthweight. The subjects comprised live births free of lethal malformations or chromosomal anomalies, < 33 weeks gestational age and with birthweights +/- 1500 g born in the Royal Women's Hospital between January 1990 and March 1996. All subjects had accurate gestational age confirmed by ultrasound prior to 20 weeks gestation and ultrasound measurements within 72 hours of birth of biparietal diameter (BPD), femur length (FL) and abdominal circumference (AC). A formula with the highest explained variance was computed by linear regression analysis using the three fetal variables in various combinations from 54 infants born between January 1990 and December 1993. The optimal formula was: Log(10)birthweight = 0.714627 + 0.077362.AC + 0.058758.BPD + 0.287037.FL - 0.011274.AC.FL. The new formula was more accurate compared with existing formulae when tested in a separate cohort of 39 infants born between January 1994 and March 1996.