Drey, Eleanor A

Kang Dufour, Mi-Suk

English

eScholarship - University of California

UCSFUC San Francisco Previously Published WorksTitleFurther improving the accuracy of fetal foot length to confirm gestational duration: Additional dataPermalinkhttps://escholarship.org/uc/item/23r6q3w1JournalContraception, 101(1)ISSN0010-7824AuthorsDrey, Eleanor AKang Dufour, Mi-SukPublication Date2020DOI10.1016/j.contraception.2019.11.006 Peer reviewedeScholarship.org Powered by the California Digital LibraryUniversity of CaliforniaCommentaryFurther improving the accuracy of fetal foot length to confirm gestational duration: additional dataEleanor A. Drey, MD, EdM1, Mi-Suk Kang, MPH, PhD21Department of Obstetrics, Gynecology and Reproductive Sciences, Zuckerberg San Francisco General, University of California, San Francisco2Division of Prevention Science, University of California, San Francisco, and School of Public Health, University of California, BerkeleyCorresponding Author: Eleanor Drey, 1001 Potrero Ave, Ward 6D, San Francisco, CA 94110Manuscript word count: 1088The original study was supported by the Center for Reproductive Health Research and Policy, San Francisco Department of Public Health.1Further improving the accuracy of fetal foot length12345678910111213141516171819202122CommentaryIn both the context of obstetric and abortion care, accurate dating is critical. Happily, better ultrasound dating continues to improve providers’ ability to estimate gestational duration. Another approach to gestational dating is measuring fetal foot length directly, which is a routine procedure during pathologic examination of fetal specimens. Legal problems may arise when gestational dating based upon the fetal foot length is not concordant with other gestational age estimates.In 2005, we published data and reference tables based on 1099 procedural abortion cases (aspiration abortions and dilation and evacuations)between 10 to 24 weeks to improve the use of fetal foot length to confirm dating, with our complete methods described [1]. In comparing three modelsfor dating (last menstrual period [LMP] only, LMP confirmed by ultrasound, and ultrasound only), we determined that directly measured fetal foot lengths correlated closely with both gestational age estimated by LMP correlated with ultrasound and by ultrasound alone, with these two approaches leading to such extremely similar results that we published only the former. By contrast, dating by LMP alone proved less reliable, especially with advancing gestational duration.  Our table of fetal foot length values has been adapted as the standard by the National Abortion Federation. Table 1 updates our previous paper to correct a typographical error identified in the original table describing foot 2Further improving the accuracy of fetal foot length2324252627282930313233343536373839404142434445lengths based on LMP confirmed by ultrasound [1]. Table 2 includes additional ultrasound-only estimates, which do not improve the model significantly. However, given the larger ultrasound-only sample (1,099 vs. 491) and its basis using size measurements alone, calculations based on ultrasound alone allows us to tighten the standard error further (Table 2). Weoriginally had not included these ultrasound-alone values because of their similarity to those determined by LMP confirmed by ultrasound. The most important conclusion of our 2005 study was that the century-old Streeter [2] fetal foot length table commonly used by pathologists was outdated. Both Mokkarala [cite] and Stevens [cite] analyzed specimens in their institutions based on a similar concern. All three modern studies showed that measured fetal foot lengths are highly consistent with modern gestational age estimates, which allows us to rely on foot length measurements to estimate gestational age when necessary. All three of these studies found that ultrasound-based dating created the best fit between gestational age and fetal foot length.  Across the three populations, there was substantial variation in race and ethnic identity as well as age of participants.  With the addition of the 469 cases reviewed by Mokkarala and 610 cases reviewed by Stevens, we can see that the modeling results are consistent across more than 2,000 records.  Across the studies, modeling exercises found that the foot length ranges continue to perform well in a variety of populations, that the values were not modified by age, race or ethnicity--potential differences that were not assessed by Streeter. 3Further improving the accuracy of fetal foot length4647484950515253545556575859606162636465666768Mokkarala and Stevens also looked for variation in the model results by parity and body mass index and found no statistically significant relationship.These results give us confidence that one reference range can be applied across diverse settings without requiring adjustment for these characteristics.When comparing model results of the two more recent studies to the ranges predicted by our 2005 model, few values fell outside of our predictions.  In Stevens, all mean measurements were within 0-2 mm of our values. In Mokkarala, few data fell outside what our “best dating” model would predict and none of the predicted values from our model fell beyond the 24-week range. All three modern studies make the convincing argument that the Streeter measurements, favored by some pathologists, are less accurate. Current in utero dating standards allow for more precision given the ability toincorporate ultrasound measurements, a technology that was not available in 1920. The Streeter measurements are subject to several critical problems. In the pre-ultrasound era, Streeter necessarily relied upon LMP alone in his 704-specimen sample. Still more problematic is that Streeter obtained virtually allof his specimens after spontaneous abortions, which meant that any slowed fetal growth before demise and the exact timing of demise could not be known and thus could not be accounted for in his dating, along with his not excluding any cases with anomalies that might have led to demise and 4Further improving the accuracy of fetal foot length6970717273747576777879808182838485868788899091altered measurements. The three modern studies were careful in consideringconditions that would alter the relationship between gestational age and footlength and thus excluded cases of fetal demise and known fetal malformation.  Given Streeter’s reliance on spontaneous abortion specimens,where demise occurred at some unmeasured earlier time, Mokkarela, Stevens and Drey predictably found fetal foot lengths that were greater than Streeter’s means, most strikingly at later gestations. In fact, the discordance between using Streeter’s measurements to evaluate fetal foot length and theinutero ultrasonography-based gestational foot length age estimates is exactly what led us to do our original study.Similar to Streeter’s methods, in Stevens and Mokkarala, pathologists measured foot lengths after formalin fixation, whereas in Drey [1], providers made measurements before fixation. Despite these differences in measurement methodology, the values were similar across the three studies.Regarding concerns about fixed versus fresh specimens, Streeter commented that the concerns with changes caused by formalin fixation weremore an issue of specimen weight rather than length. It is unlikely that any significant change in foot length would be caused by formalin, or that changes due to formalin (if any) would outweigh the other inaccuracies in Streeter’s dating.  Although it seems unlikely that formalin fixation would alter measurements by more than a millimeter, such concerns could be eased by making the measurement prior to fixation. Pathologists could compare fetal foot length measurements before and after formalin fixation in5Further improving the accuracy of fetal foot length9293949596979899100101102103104105106107108109110111112113114order to assess whether any important differences emerge. The main advantage for pathologists in continuing to use Streeter’s table would lie in the third trimester, because none of the modern, more precisely dated tablesextend beyond 24 weeks, whereas Streeter’s includes values from up to 40 weeks.We can be reassured by the similar conclusions of Mokkarala, Stevens and Drey, which together make a powerful case to update fetal foot length standards based on modern pregnancy dating standards. Although the National Abortion Federation adopted our previous “best dates” data as theirstandard—that of LMP confirmed by ultrasonography--one could argue that the ultrasound-only dated model published here, with its larger sample size and smaller standard error values, should be used instead as the most precise standard for dating pregnancy duration by fetal foot lengths up to 24 weeks.6Further improving the accuracy of fetal foot length115116117118119120121122123124125126127128References[1] Drey EA, Kang MS, McFarland W, Darney PD. Improving the accuracy of fetal foot length to confirm gestational duration. Obstet Gynecol 2005;105:773–8. [2] Streeter GL. Weight, sitting height, head size, foot length, and menstrual age of the human embryo. Contributions to embryology. Washington, D.C: Carnegie Institution of Washington; 1920. p. 143–70. [3] Mokkarala S, Creinin MD, Wilson MD, Yee NS, Hou MY. Comparing preoperative dating and postoperative dating for second-trimester surgical abortions. Contraception 2020;101:5–9. [4] Stevens K, Elia J, Kaneshiro B, Salcedo J, Soon R, Tschann M. Updating fetal foot length to gestational age references: a chart review of abortion cases from 2012 to 2014. Contraception 2020;101:10–13.7Further improving the accuracy of fetal foot length129130131132133134135136137138139140141142143Table 1. Fetal foot length based on regression model using last menstrual period dating confirmed by ultrasonographyGestational duration (wk)Midpoint footlengthRange(beginningweek toend ofweek)Range +-1SD10 to 11 4 2–5 0–611 to 12 7 5–8 4–1012 to 13 10 8–11 7–1313 to 14 13 12–14 10–1614 to 15 16 15–17 13–1915 to 16 20 18–21 16–2316 to 17 23 21–24 19–2617 to 18 26 24–27 23–2918 to 19 29 27–30 25–3219 to 20 32 31–33 29–3620 to 21 35 34–37 32–3921 to 22 39 37–40 35–4222 to 23 42 40–43 38–4523 to 24 45 43–46 41–4924 to 25 48 47–49 44–52SD, standard deviation8Further improving the accuracy of fetal foot length144145146147148149* Weeks of gestational duration as measured by the “best estimate” (i.e., gestational duration by last menstrual period confirmed by ultrasonography within 1 standard deviation of last menstrual period).** Foot length calculated by the model at the midpoint of the week (e.g., midpoint of 10 to < 11 = 10 weeks, 3.5 days).*** Range represents the foot length values from the beginning to the end of the week (e.g., range of 10 to < 11 = values from 70 to 76 days).9Further improving the accuracy of fetal foot length150151152153154155156157158Table 2. Fetal foot length based on regression model using dating by ultrasonography alone*Gestational durationMidpoint footlengthRange (beginningweek to end ofweek)Range+- 1SD10 to 11 3 2-5 2-511 to 12 7 5-8 5-812 to 13 10 8-11 8-1113 to 14 13 11-15 11-1514 to 15 16 15-18 14-1815 to 16 19 18-21 18-2116 to 17 23 21-24 21-2417 to 18 26 24-27 24-2818 to 19 29 27-31 27-3119 to 20 32 31-34 30-3420 to 21 35 34-37 34-3721 to 22 39 37-40 37-4022 to 23 42 40-43 40-4423 to 24 45 43-47 43-4724 to 25 48 47-50 46-50* Weeks of gestational duration as measured by ultrasound, determined by biparietal diameter, using Hadlock values.10Further improving the accuracy of fetal foot length159160161162163164** Foot length calculated by the model at the midpoint of the week (e.g., midpoint of 10 to < 11 = 10 weeks, 3.5 days).*** Range represents the foot length values from the beginning to the end of the week (e.g., range of 10 to < 11 = values from 70 to 76 days).11Further improving the accuracy of fetal foot length165166167168169170171

Further improving the accuracy of fetal foot length to confirm gestational duration: Additional data

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Further improving the accuracy of fetal foot length to confirm gestational duration: Additional data.

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