An algorithm for the estimation of gestational age at the time of fetal death

Abstract

Background: Accurate assignment of gestational age (GA) at time of fetal death is important for research and clinical practice. An algorithm to estimate GA at fetal death was developed and evaluated.

Methods: The algorithm developed by the Stillbirth Collaborative Research Network (SCRN) incorporated clinical and post-mortem data. The SCRN conducted a population-based case-control study of women with stillbirths and livebirths from 2006 to 2008 in five geographical catchment areas. Rules were developed to estimate a due date, identify an interval during which death likely occurred, and estimate GA at the time of fetal death. Reliability of using fetal foot length to estimate GA at death was assessed.

Results: The due date estimated for 620 singleton stillbirths studied was considered clinically reliable for 87%. Only 25.2% of stillbirths were documented alive within 2 days before diagnosis and 47.6% within 1 week of diagnosis. The algorithm-derived estimate of GA at time of fetal death was one or more weeks earlier than the GA at delivery for 43.5% of stillbirths. GA estimated from fetal foot length agreed with GA by algorithm within 2 weeks for 75% within a subset of well-dated stillbirths.

Conclusions: Precise assignment of GA at death, defined as reliable dating criteria and a short interval (≤1 week) during which fetal death was known to have occurred, was possible in 46.6% of cases. Fetal foot length is a relatively accurate measure of GA at death and should be collected in all stillbirth cases.

FIGURE 1. Combining Lmp and Ultrasound Data to Determine Estimated Due Date

This figure depicts the first step in the algorithm which is to determine the best estimated due date using clinical history and imaging findings. The first three rows (above the dashed line) show the determination of the EDD and apply equally to live births as well as stillbirths. Numbers and percentages are based on the 620 stillbirth cases. As indicated below the dashed line, the EDD was considered clinically reliable for purposes of determining GA at death if there was a dating ultrasound available. After application of the algorithm, the EDD was revised by case review (see text) for 13 cases with EDD originally determined as follows: by ultrasound that disagreed with certain LMP (4 cases); by ultrasound where the LMP was uncertain or certainty was not recorded (7 cases); by ultrasound when the LMP was not recorded (1 case); and by uncertain LMP when no ultrasound from a living fetus was available (1 case). EDD determined by case review was considered reliable. See Appendix B for formulas for calculating the EDD. ^a GA_U/S was derived from the earliest dated obstetric ultrasound when the baby was alive. ^b “Uncertain” included cases in which certainty was not recorded. ^c See text and Table 1 for details. Abbreviations: LMP = Last menstrual period, U/S = Ultrasound, EDD = Estimated due date, GA = Gestational age, GA_U/S = Gestational age at the date of the dating ultrasound examination

FIGURE 2. Estimating Gestational Age at Fetal Death

This figure depicts the rules for determining the timing of fetal death, based on the length of the timing of death interval; the days between DoDx and delivery and presence/absence of maceration; the clinical reliability of the EDD, and the availability of the foot length measurement. Arrows show the direction of the computation of either the GA at Death estimate or the Date of Death estimate; where no arrow is shown, the two estimates were derived separately. Percentages are based on the 620 singleton stillbirths. See Appendix B for formulas for calculating the estimates. In the first box, there were 49 cases for which the TOD interval was >1 day but the days between the date of diagnosis of fetal death and the date of delivery were ≤1 and there was no fetal maceration; of these 27 had TOD interval >7 days. ^a Date of diagnosis of fetal death minus the date last documented alive. ^b Excluding cases with both {Ddel minus DoDx ≤1 day} and no fetal maceration, which are contained in the first box. ^c The clinical GA at the date of diagnosis of fetal death, as reported by the clinical site. Because this value was recorded in completed weeks in our chart abstraction, we used the clinical GA in completed weeks + 3/7 for calculations, which assigned the middle day (day 3) of days 0 through 6 for the completed week. [The date of diagnosis as determined by the site was not recorded.] ^d If GA by foot length fell outside the GA interval for TOD, the GA at death was estimated by the gestational age at the TOD interval boundary that was closest to the GA by foot length. Abbreviations: GA = Gestational age, TOD = Timing of death, EDD = Estimated due date, FtL = Foot length, DLA = Date last documented alive, DoDx = Date of diagnosis of fetal death, DDel = Date of delivery

FIGURE 3. Distribution of Timing of Death Interval for Stillbirths <24 Weeks’ Gestation Versus ≥24 Weeks’ Gestation

This figure demonstrates the distribution of the length of the TOD interval in stillbirth cases with age at fetal death estimated as <24 weeks’ gestation by the algorithm compared to those with death ≥24 weeks. Numbers at the base of each bar show the percentages represented by bar height. Abbreviation: TOD = Timing of Death

FIGURE 4. Frequency and Cumulative Frequency Distributions in Completed Weeks for Gestational Age at Death Estimates by the Scrn Algorithm and by the Clinical Gestational Age for Singleton Stillbirths

This figure compares the distributions of the GA at fetal death estimated by the algorithm (“Final GA”) and estimated by the clinical gestational age at diagnosis of death reported by the clinical site (“Clinical GA”). The left axis shows the number of stillbirths with each gestational age for the frequency curves. The right axis shows the total number of stillbirths with gestational age at a given week or earlier for the cumulative distribution curves. Abbreviation: GA = gestational age