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Ultrasound or menstrual datingFrom: David J R Hutchon (DJRHutchon@Postmaster.co.uk)Sun Jan 3 14:59:18 1999
Many thanks to Obgyn.net for the summaries from the Edinburgh meeting. Reynir Geirrson and Jason Gardosi and others contend there is evidence that, over all, scanning is a better predictor of gestation than the last menstrual period, even when this is apparently confidently known by the woman. They demonstrate this partly by demonstrating the relationship between the scan in these women and established scan charts, and partly by showing the relationship between the two methods and the actual date of delivery. If you look carefully at the figure 5 in the recent paper by Tunon and Eik-Nes shows that for those women with scan size within 7 days of menstrual gestation the LMP is at least as precise as ultrasound. In the opinion of Ole Olsen, a Danish statistician, however the studies are statistically flawed and although the numbers in some of the studies are very large, as he points out study size does not compensate for methodological faults. Olsen feels that the interval from LMP to EDC should be readjusted to 283 days. When one thinks about it is remarkable that the original work by Hermanni Boehaave was so accurate. The figure was largely based on folklore although the idea of an interval of about 40 weeks had been known for at least a thousand years. When we talk about the mean length of pregnancy we cannot be referring to the mean length of every pregnancy otherwise the figure would be hopelessly skewed if every miscarriage was included. We must be talking about the mean length of healthy pregnancy which by definition means a healthy baby. Mittendorf in Chicago, using a large database of menstrual data showed slight but significant differences in the length of pregnancy in women of different parity, ethnicity etc but what we dont know is how it translates to ultrasound dating. I think amongst most obstetricians the argument centres around whether scan dates should be used exclusively or the whether the menstrual dates should be maintained if the difference is small enough to be statistically not significant.(e.g.about 7 days) Here in the UK the majority of obstetricians and midwives (and a CESDI recommendation) keep the menstrual dates if the difference is small. I would argue that this is the logical approach but not the most practical approach and it is far easier, with modern ultrasound machines, to simply pick a parameter and allow the machine to generate a gestation and an estimated date of confinement from its internal chart. The LMP will always be an important landmark in a woman's pregnancy but its importance for providing an EDC can be gradually diminished, as women begin to appreciate that the scan is the easiest way to get and accurate result. I do not think there is any great advantage in changing from "gestation" to say fetal age as the change is likely to lead to at least as much confusion as there is with gestation. An important aspect of the discussion arises from the use of menstrual data in the generation of ultrasound scans. This leads to a normal range in the size of the parameter for each gestation week. Clinicians interpret the normal range in the same way as in other medical measurements. For example the patient has a healthy haemoglobin if it is in the normal range. No one expects it to be spot on the mean value. If the scan measurements are used as a landmark in pregnancy in the same way as the LMP, and the mean interval to normal labour and a healthy baby of a population of women with a specific scan measurement is plotted against each of the scan measurents, a single regression line of the range of the measurements can be generated. Since there is no reliance on LMP for this chart, there is no need to consider how the scan EDC relates to the menstrual EDC. This is the proposal I have put forward at http://www.obgyn.net/us/cotm/9807/cotm_9807.htm It seems to me that if there is a significant difference between and the menstrual EDC in women with certain dates and the scan EDC using conventional charts, this simply shows that the population from which the scan chart was derived is not the same as the population to which it is being applied, and as such is therefore an inappropriate standard. Reference Tunon K, Eik-Nes S H and Grottum P A comparison between ultrasound and a reliable last menstrual period as predictors of the day of delivery in 15000examinations. Ultrasound in Obstetrics and Gynaecology 8 (1996) 178-185
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