The customised ultrasound dating generator and calculator

The customsed ultrasound dating generator for BPD

Instruction on use

BPD of 40mm, mean interval = The second column shows the number of entries
BPD of 41mm, mean interval =
BPD of 42mm, mean interval =
BPD of 43mm, mean interval =
BPD of 44mm, mean interval = Go to customised EDD calculator
BPD of 45mm, mean interval =
BPD of 46mm, mean interval =
BPD of 47mm, mean interval =
BPD of 48mm, mean interval = Ensure that you enter data form each pregnancy only once. The pregnancy must be normal
BPD of 49mm, mean interval = and sinlgelton leading to spontaneous onset and deliver a normal healthy baby.
BPD of 50mm, mean interval = To enter data, click on BPD box and enter measurement, then TAB to next box
Enter BPD in mm (rounded to whole number) Enter scan date UK date style (dd/mm/yyyy)
Enter delivery date Interval = days

This calculator uses the data entered in the customised chart to determine the EDD for BPD 40 to 50mm

Name or other identifier
Enter the date of the ultrasound scan BPD measurement in mm (one decimal place)

The expected date of delivery from the customised data is
To print out this, first highlight the results section with the mouse, then go to FILE, PRINT and click SELECTION in the print box.
Return to data entry for chart generation

The charts have been generated from whole number values of the BPD in mm
However, since the accuracy of the EDD can never be more than one day, and the growth rate of the BPD between 40 and 50 mm is approximately 0.5mm per day, it is reasonable to measure the BPD to one decimel place and extrapolate the interval. The EDD is therefor accurate to one day, the entered BPD is rounded to the nearest 0.5mm.

Return to data entry for chart generation
Intructions for use. This web browser calculator will only work on IE 5.0 Once you have accessed the page from the Internet go OFFLINE and into HISTORY. From here the icon can be dragged to the desktop or any directory ready for use on your own computer off-line. No data is transmitted out of your computer and the data is stored in the XML userdata file. I have put in measurements which approximate to the charts of Altman & Chitty ( you can put in another published chart which you use if you wish) however if you already have a large data base to enter, these can be simply removed from each box. The chart should not be used however until for each measurment there are fairly confident numbers. I would suggest at least 30 for each measurement. At the end of each seesion entering the data ensure the results are saved using the "save and exit" button. At the start of each subsequent use always press the LOAD button before doing anything else. Only single pregnancies, with spontaneous labours resulting in a healthy infant should be included. If you are already using Altman and Chitty charts entering new data may gradually change the values to align with your own population. The rational is explained in the OBGYN.net paper on proposal for valid customised charts generation ("Back to the future for Hermanni Boerhaave" published by OBGYN.NET)

In the Hermanni Boerhaave story, I am trying to point out that using a scan measurement only provides a measure of the age of the fetus. Another set of data needs then to be applied to provide an estimate of the date of delivery. The true mean (or modal) interval from LMP to delivery may nowadays be closer to 282 days rather than the 280 proposed by Boerhaave. By drawing up the charts in the way I have proposed an estimate for the date of delivery is provided directly by the chart, using the same type of data which was used to generate the chart and from no other data. The precise interval from LMP to delivery becomes irrelevant. Not only this, but the data on which the chart is based is relatively free of error. The date of the scan, the scan measurement and the date of delivery are all known. There is another advantage in this method. Because relatively large numbers of secure data can be obtained, we can look for differences that might exist in sub-populations defined by ethnicity, parity, age, fetal sex and maternal size, all of which have been shown to affect the size of the newborn. This is what I showed at the BMFM society meeting in York, Ref1 with a difference of 4.4 days between the two extreme groups. Adding to the database since has made the differences more convincing.
Although it is possible to draw up a graphic chart using the methodology I have just described, I am not sure that this is a good way to use the data. For dating, there is no need to have the complete range of measurements throughout pregnancy. There are well recognised times when dating by ultrasound is the most accurate. The data from routine ultrasound services is going to be abundant at these times and therefore most confident at the same gestations as ultrasound should be used to provide an accurate EDD. Data for other gestations is unnecessary for dating.
There is an important difference in generating an ultrasound EDD using this methodology from the traditional ultrasound charts. Taking a BPD of say 40 mm, the data shows us that on average the woman will deliver a healthy normal baby after 150 days. Based on a very large population, which is quite feasible, the confidence interval for this could be very narrow.
With a traditional chart we can see that a baby with a BPD of 40 mm will on average be 130 days gestation. There will always be a range of gestations for each measurement. ( The traditional centile lines on these charts emphasises this point.) This range will be small if the population is large but it is just not feasible to get large numbers of women who are sufficiently secure in their menstrual data. Within this population there will remain a small number of women whose gestation does not relate as expected to their menstrual period. Finally to provide an estimated date of delivery, the interval from LMP still has to be known and this provides room for further error.
I believe it is this inherent error in traditional ultrasound dating which convinces the majority of obstetricians and midwives that they should favour an integrated use of ultrasound and menstrual dates. CESDI also recommends this approach. Ref2 Widespread adoption of the Boerhaave method for generating the EDD charts could be expected to speed up the general acceptance of pure ultrasound dating. We have recently shown that is really not possible to accurately date or assess growth using a chart and wheel Ref3 and these will always be necessary until we have either universal computerised maternity systems or programmes built into the ultrasound scanners.

Return to data entry for chart generation
References
1. Hutchon DJR. Customised ultrasound dating charts. British Maternal and Fetal Medicine Society. fourth Annual Conference, University of York. Abstartcs - Journal of Obstetrics and Gynaecology 1999 19:suppl 1;s57

2. Hutchon DJR. Routine ultrasound is the method of choice for dating pregnancy. Br J Obstet Gynaecol 1999:106;616
3. Hutchon DJ, et al. Clinical interpretation of ultrasound biometry for dating and for assessment of fetal growth using a wheel and chart: is it sufficiently accurate? Ultrasound Obstet Gynecol. 1999 Feb;13(2):103-6.
Altman D G and Chitty L S. New Charts for ultrasound dating of pregnancy. Ultrasound Obstet Gynecol. 1997;10:174-191
This calculator is for educational use. It is believed accurate but no responsibility for accuracy of the results is accepted by the author. David J R Hutchon BSc, MB, ChB, FRCOG Consultant Obstetrician, Memorial Hospital, Darlington, England. It is advisable to keep a hard copy printout at regular intervals. The data is stored on temporary files within the computer operating system and may be deleted by some procedures. If this happens the data can be easily restored by entering the results of the previous printout into the table.

Go to OBGYN.net paper on proposal for valid customised charts generation ("Back to the future for Hermanni Boerhaave" published by OBGYN.NET)

Comments and suggestions are welcome and will be included in the comments section.

To E-MAIL me CLICK HERE TO SEND COMMENTS AND PLEASE SEND ME YOUR NAME AND E-MAIL IF YOU INTEND TO USE THIS CUSTOMISED GENERATOR at DJRHutchon@Postmaster.co.uk

This Web page is at www.hutchon.freeserve.co.uk/

Complete intervals = Complete numbers=