Study design

This study was a secondary analysis of data obtained from the Effects of Aspirin in Gestation and Reproduction (EAGeR) trial, a block-randomized, double-blind, placebo-controlled trial conducted during 2006−2012. EAGeR was designed to study the effects of preconception low-dose aspirin on live birth in women with 1−2 prior pregnancy losses. Study design, methods, and inclusion and exclusion criteria have previously been described. ^, Briefly, women 18−40 years of age with no known major medical conditions were recruited from 4 medical centers in the United States. Inclusion was limited to women having regular menstrual cycles and attempting to conceive with no prior diagnosis of infertility. Women were randomized to daily low-dose aspirin (81 mg/d) or placebo. Women were followed up to the completion of 6 menstrual cycles while trying to conceive, and through pregnancy for those who became pregnant, with study treatment continuing through week 36 of pregnancy. The institutional review board at each study site and data coordinating center approved the trial protocol (University of Utah #1002521; Scranton, Pennsylvania # HHSN275200403394; University at Buffalo #SPM0900107A; University of Colorado #08-0982). All participants provided written informed consent prior to enrollment. The data coordinating center and each site obtained institutional review board approvals. The trial was registered on ClinicalTrials.gov (#NCT00467363 27 April 2007).

Exposure: ultrasound parameters

For women with evidence of an ongoing pregnancy (a positive urine pregnancy test with no clinical evidence of subsequent pregnancy loss), an early ultrasound targeted for approximately 6.5 weeks of gestation was performed to clinically confirm the pregnancy by visualization of a gestational sac. Gestational age in weeks and days was recorded, as well as crown–rump length, yolk sac diameter, subchorionic hemorrhage, and fetal heart rate. Transvaginal ultrasounds were conducted by American Institute of Ultrasound in Medicine (AIUM)−certified sonographers in AIUM-certified centers and recorded for study purposes. M-mode was used to measure fetal heart rate, which was recorded as the average of 2 measurements. Fetal sex was unavailable at the early ultrasound.

Gestational age at ultrasound

For this analysis, gestational age at the time of the ultrasound was calculated based on the difference between the date of the ultrasound and last menstrual period (LMP). The LMPs were determined in this population from high-quality prospective bleeding dates reported in preconception daily diaries and fertility monitors. Accordingly, gestational ages for purposes of this analysis were not re-dated based on gestational age from the ultrasound.

Outcome: clinical pregnancy loss

Clinical pregnancy loss was defined as a loss identified by the participant or clinician following clinical confirmation of pregnancy by sonographic identification of a gestational sac. Pregnancy follow-up included clinical or telephone visits every 4 weeks up to week 36 of gestation. A postpartum telephone visit was conducted 6−8 weeks after delivery or pregnancy loss. Medical chart abstraction was completed on each participant in the study, from which clinical pregnancy losses were verified.

Sample derivation

Of the 1228 participants in the study, there were 732 (59.6%) clinically confirmed pregnancies. There was no ultrasound for 25 of these pregnancies (3.4%), of which 15 women experienced a clinical loss and 10 had a live birth. Of the remaining 707 unique ultrasound examinations, 674 (95.3%) occurred between 6 weeks days and 8 weeks 6 days ( Figure 1 ) by LMP dating. Of those examinations, 617 (91.5%) had a detectable fetal heart rate.

Participant inclusion in the ultrasound characteristics and clinical loss Effects of Aspirin in Gestation and Reproduction (EAGeR) trial analysis: number of women who were randomized, had clinically confirmed ultrasounds between 6 weeks 0 days and 8 weeks 6 days gestation, and a detectable fetal heart rate

DeVilbiss et al. Early ultrasound prediction of pregnancy loss. Am J Obstet Gynecol 2020.

Base predictors

In addition to ultrasound features, we sought to identify the most informative predictors of clinical pregnancy loss from multiple biological, clinical, and demographic covariates. At baseline, participants completed questionnaires on demographics, lifestyle habits, socioeconomic status, and reproductive history. Covariates were selected for model consideration based on identified a priori relationships with pregnancy loss. ^, Baseline demographic covariates included study recruitment site (Scranton, PA; Denver, CO; Buffalo, NY; Salt Lake City, UT), treatment assignment (aspirin, placebo), age at study entry (years), and annual income (≥$100,000 USD; $75,000–99,000; $40,000–74,999; $20,000–39,999; ≤$19,999). Baseline clinical measures included gestational age at the first ultrasound, body mass index (kg/m ² ), systolic and diastolic blood pressure (mm Hg), exercise history (low, moderate, high), number of prior pregnancy losses (1, 2), number of prior live births (0, 1, 2), and time since the last pregnancy loss (≤4 months, 5−8 months, 9−12 months, >12 months).

Statistical analyses

Relationships between baseline measures and ultrasound parameters were examined using t tests, χ ² , or Fisher exact tests, as appropriate.

Study sample characteristics

Of the 617 women in this analysis, 64 women experienced a clinical pregnancy loss following the first ultrasound (10.4%), 7 women were lost to follow-up (1.1%), and 546 (88.5%) women had a live birth. Women most frequently had an ultrasound during gestational week 7 (7 weeks 0 days to 7 weeks 6 days; n = 281, 45.5%), followed by week 6 (6 weeks 0 days to 6 weeks 6 days; n = 259, 42.0%) and week 8 (8 weeks 0 days to 8 weeks 6 days; n = 77, 12.5%). Median (interquartile range [IQR]) fetal heart rate (of viable fetuses) was 124 (118−132), 137 (122−146), and 144 (125−163) bpm; median (IQR) crown–rump length was 6.0 (4.5−8.0), 8.6 (6.0−10.6), and 10.0 (6.6−16.1) mm; and median (IQR) yolk sac diameter was 4.0 (2.9−4.5), 4.0 (3.0−5.0), and 4.7 (3.2−5.0) mm for gestational weeks 6, 7, and 8, respectively. There were 63 documented cases of subchorionic hemorrhage (10.3%) ( Table 1 ).

Low fetal heart rate was more prevalent with younger gestational age. Women randomized to aspirin were more likely to have extreme yolk sac diameter measurements. Small crown–rump length and extreme yolk sac diameter were both associated with having no prior live births.

Principal Findings

Cut-points for predicting clinical losses using low fetal heart rate were found to be ≤122, 123, and 158 bpm for gestational weeks 6, 7, and 8, respectively. Cut-points for small crown–rump length were ≤6.0, 8.5, and 10.9 mm for gestational weeks 6, 7, and 8, respectively. Cut-points for abnormal yolk sac diameter were measurements of 2.3 mm (15th percentile) or greater than 5.1 mm (87th percentile).

Using these cut-points, of women who later experienced a clinical loss, 37 (59.7%) occurred among women having both low fetal heart rate and small crown–rump length, compared to 6 losses (9.7%) among women having neither characteristic. Following adjustment for gestational age, this corresponded to a 21% (15−27%) probability of clinical pregnancy loss among women having both characteristics compared to 5.0% (1.5−8.5%) probability with neither characteristic, equating to a 16% (9.1−23%) difference in adjusted absolute risk and a relative risk of 2.08 (95% CI, 1.24−2.91) ( Table 2 ).

In applying these cut-points to the test data, the pooled area under the AUC for prediction of pregnancy loss was 0.63 (95% CI, 0.47−0.79) for gestational age, which increased to 0.78 (0.65−0.90) after addition of low fetal heart rate and small crown–rump length ( Table 3 and Supplemental Figure 1 ). The addition of age or blood pressure, or yolk sac diameter and subchorionic hemorrhage, or continuous expression of fetal heart rate and crown–rump length did not meaningfully improve prediction of clinical pregnancy loss ( Table 3 and Supplemental Figure 2 ).

Principal findings

We identified gestational age specific cut-points for routinely measured ultrasound features that were most predictive of risk of pregnancy loss: low fetal heart rate ≤122, 123, and 158 bpm; small crown–rump length ≤6.0, 8.5, and 10.9 mm for gestational weeks 6, 7, and 8, respectively. Nonviability cut-points for low fetal heart rate and crown–rump length are lower than those that maximize predictive value. This study improves upon the prior allocation of cut-points for routinely measured features by basing them on clinical data for multiple sonographic characteristics, making them specific to gestational week, and determining them using methods that optimize prediction. These findings have implications for patient counseling, given that when both features are present, physicians could estimate a 15% increase in risk of clinical loss.

Comparison with other studies

While prediction of subsequent pregnancy loss could be improved through joint examination of multiple sonographic features, most existing studies investigated individual components of the first trimester ultrasound, including subchorionic hemorrhage, fetal heart rate, crown–rump length, and yolk sac diameter. The current findings differ from previous studies of subchorionic hemorrhage, which reported elevated risk of pregnancy loss with hemorrhage overall ^, and with increasing size of hemorrhage, ^, although subchorionic hematoma has not been shown to be an independent risk factor for pregnancy loss once vaginal bleeding has been taken into account. We were unable to further examine hemorrhage by size because of few observations with measured hemorrhage size (n = 48). Current findings also differ from studies of yolk sac diameter in which small, large, ^{, ,} or extreme ^, yolk sac measurements have previously been associated with increased risk of loss. In the current study, prediction was not improved beyond information provided by gestational age, low fetal heart rate, and small crown–rump length. This difference may be due to prior studies not having taken fetal heart rate and crown–rump length into account, which were more highly predictive of clinical loss in the current study. As expected, losses have been associated with lower heart rates ^{, , , ,} and smaller crown–rump lengths, ^{, , , ,} although when used, cut-points for these parameters are not based on prediction (fetal heart rate ^{, , ,} and crown–rump length ^, ); rather, when data-defined, cut-points have been based on the distribution of the population examined. ^, One study used data to define gestational week−specific cut-points and reported fetal heart rate cut-points of ≤115 bpm for 6−7 weeks and ≤145 bpm for 7−8 weeks for elevated risk of pregnancy loss (vs ≤122 for 6 to <7 weeks and 123 for 7 to <8 weeks in our study). Our findings expand on the prior data by using a large sample to define cut-points based on specific gestational week. We also combine multiple clinical measurements to improve pregnancy loss prediction, which has been performed on a limited basis. ^, We are not aware of any prior studies that used clinical data to first identify features and gestational age specific cut-points most predictive of pregnancy loss.

Maternal age is acknowledged as 1 of the strongest risk factors for pregnancy loss, and was identified as independently associated with pregnancy loss in a prior ultrasound study, even after accounting for gestational age based on LMP, vaginal bleeding, gestational sac diameter, and presence or absence of a yolk sac. In the current study, prediction of clinical loss did not improve with addition of maternal age, after accounting for gestational age, small crown–rump length, and low fetal heart rate.

Furthermore, prior evidence is aimed toward distinguishing nonviability. Diagnosis of nonviability carries cut points for fetal heart rate (<85 bpm) ^, and crown–rump length (≥7 mm and no heartbeat) that are intended to have 100% specificity. They are appropriately different from those identified in the current study that aimed to maximize prediction of subsequent loss, whereby sensitivity and specificity were considered equally important. However, the use of thresholds to determine nonviability with certainty does not allow for counseling many women who will go on to have a clinical pregnancy loss regarding their specific prognosis. In the current study, 95% of women who had neither low fetal heart rate nor small crown–rump length went on to have a live birth.

Clinical implications

Physicians use ultrasound indicators for patient counseling, and in some cases to decide upon the frequency of follow-up sonograms. These data can be used to reassure women without either low fetal heart rate or small crown–rump length, and to identify pregnant women with an elevated risk of pregnancy loss.

Strengths and limitations

This is the first study to identify routinely measured features and their gestational age–specific cut-points most predictive of subsequent risk of pregnancy loss. Because fetal heart rate and crown–rump length increase continuously with gestational age, cut-points require characterization on a gestational age–specific basis, and should be validated in other cohorts. These specified criteria should not be used to diagnose nonviability, as most pregnancies with low fetal heart rate and crown–rump length (79%) ended in live birth.

Analytic methods were used to reduce the potential impact of bias. To use all study observations and to improve efficiency, multiple imputations were used. Although rarely realized in practice, complete-case analysis assumes that the missing data are not related to missing or observed data. This assumption is relaxed in multiple imputation, which assumes that missing data can be related to observed data.

Although we used LMP dates, which may be considered unreliable in clinical obstetric populations, it is important to note that, in this setting, LMPs were determined prospectively from detailed follow-up using bleeding data from preconception daily diaries as well as fertility monitors. In addition, for trial inclusion, study participants were required to have regular menstrual cycle lengths between 21 and 42 days and to have discontinued contraception by the baseline visit. Furthermore, the use of LMP-based dating (rather than ultrasound-based gestational age dating) was necessary here to avoid the circular logic of calculating gestational week−specific cut-points by gestational ages determined by ultrasound measurements.

This study is limited by its generalizability to women who have an ultrasound between 6 weeks 0 days and 8 weeks 6 days since the beginning of their last menstrual period and have clinical confirmation with detectable fetal heart rate at this ultrasound. Furthermore, findings should be generalized only to women with a history of pregnancy loss. Although it should be noted that pregnancy loss is a relatively common event among women of reproductive age, and prior studies do not suggest that relationships between ultrasound parameters and risk of clinical loss differ by prior loss history despite a higher rate of subsequent losses among this population. Given the small number of study participants who experienced a clinical loss in the 25% test set, model performance is intended to be descriptive, and should not be considered a validation study. Further limitations are the relatively homogeneous, predominantly white population, the absence of data on causes of pregnancy loss such as phospholipid antibody syndrome, and insufficient data to examine genetic karyotype.

Conclusions

When both small crown–rump length and low fetal heart rate are present, physicians could estimate a 15% increase in risk of clinical loss. This study improves upon the prior allocation of cut-points for routinely measured features by basing them on clinical data for multiple sonographic characteristics, making them specific to gestational week, and determining them using methods that optimize prediction.