The accuracy of the fetal weight predictions generated by ultrasound equations depends on a wide variety of factors, including (but not limited to) (1) the accuracy of fetal biometric measurements obtained by ultrasound, (2) the gestational age of the fetus, and (3) maternal race. The first of these (the accuracy of the fetal biometric measurements) is most often dependent upon five factors: (a) the resolution of the ultrasound equipment that is being used, (b) the skill of the ultrasonographer, (c) the position of the fetus, (d) the amount of amniotic fluid surrounding the fetus, and (e) the size of the mother.

In general, different ultrasound equipment manufacturers incorporate only one (or a small subset) of these ultrasound equations into their equipment software packages. A full array of the representative ultrasound prediction equations is included in this website. This website uses 26 different ultrasound equations; the equations as well as their sources are listed under the " ULTRASONOGRAPHIC FETAL WEIGHT PREDICTION ALGORITHMS USED BY THIS WEBSITE FOR ESTIMATING FETAL WEIGHT" and "REFERENCES FOR FURTHER READING AND INFORMATION" headings.

In general, the accuracy of ultrasonographically-based equations for predicting fetal weight (i.e., birth weight) can be expected to range between ±8% to ±18% of the actual fetal weight, depending on the different factors listed above, as well as several others. This degree of intrinsic error is attributable, mainly, to the many assumptions that must be made in order to use this technique for the purpose of fetal weight estimation. The most important of these is that the sonographic measurements of multiple linear and planar dimensions of a fetus provide sufficient parametric information to allow for accurate algorithmic reconstruction of the three-dimensional fetal volume of varying tissue density. In reality, this is only an approximation because the human form is both irregular (i.e., non-spheroid) and comprised of varying tissue densities (e.g., bone is more dense than muscle, which is more dense than adipose tissue, etc.). In effect, the ultrasound technique, when used for this purpose, is analogous to what would occur if one tried to predict the weight of an adult by asking - and knowing the answer to - only three questions: (1) What is your hat size?, (2) How large is your waistline?, and (3) What is the length of your thigh? Obviously, if these were the only three pieces of information that were known about an adult, there would be a very wide range of guesses ("estimates") as to the most likely weight for that particular individual with those measurements. The same logic applies to the results that ultrasound can generate for estimating fetal weight by using only a handful of 1- and 2-dimensional fetal measurements that are typically obtained using this technique.

Although obstetrical ultrasonography has – in many ways – revolutionized the practice of obstetrics since its inception, most experts believe that the primary utility of such equipment revolves around the high degree of image quality and resolution that can be safely and conveniently obtained in order to image living tissues (especially those involving the maternal and fetal soft-tissues). Important obstetrical issues such as (1) visualizing the number of fetuses within the uterine cavity, (2) fetal position and presentation, (3) the location of the placenta, (4) the amount of amniotic fluid within the fetal cavity, and (5) whether there are identifiable fetal malformations – among others – are routinely answered in modern obstetrics by employing accurate and reliable ultrasound equipment. However, this is not the case with regard to the prediction of fetal weight by using the high-resolution images that are generated by the ultrasound technique. In studies that have investigated the accuracy of ultrasonographic methods and the associated ultrasound equations for predicting fetal weight, the results have varied widely depending upon gestational age, the range of fetal weights investigated, and the specific ultrasonographic algorithms employed. No single one of these ultrasound equations is the best or most accurate for predicting fetal weight in all circumstances. Which equation(s) are best in some particular circumstances versus others has been a topic of debate for decades, and is still unresolved. Thus, the particular "best" ultrasound equation to use under any given circumstance is as much a matter of "guessing" as anything else. In addition, fetal weight predictions made with ultrasonographic fetal measurements can be expected to apply only to the particular day that the ultrasound study was performed to obtain those measurements. Projecting these fetal weight estimates forward to future dates results in an additional level of imprecision being added to these already imprecise results, and this is not generally recommended.

If all four of the requested fetal measurements are provided (BPD, HC, AC and FL), fetal weight estimates can be generated using all 26 of the equations employed by this website. If not all of the fetal measurements are available then, depending on the type and number of the fetal measurements that are provided, only some of the equations will have the necessary input information available to generate results. Once the data concerning the fetal biometric measurements is entered, the results for each of the 26 equations that can be calculated will be displayed, together with the overall range of these predicted results. Please click on the START NOW button below to enter your ultrasonographic fetal biometric measurements.