Abstract
Objective
To determine the association between epidural analgesia and perineal injury, including obstetric anal sphincter injury (OASI), in primiparous women.
Methods
We conducted a retrospective cohort study of primiparous women with singleton, term, vertex, and vaginal deliveries between 2012 and 2024 at a tertiary medical center. Primary outcomes were (1) any perineal injury, and (2) OASI. Propensity score matching (1:1) was used to adjust for maternal and obstetric confounders. Perineal outcomes were compared in matched and unmatched cohorts using standardized mean difference (SMD). An SMD <0.1 was considered negligible, 0.1–0.2 small, and >0.2 clinically significant. Multivariable logistic regression was used to evaluate associations between epidural use and outcomes.
Results
Overall, 45 132 women were included, of whom 36 799 (81.5%) received epidural analgesia. After matching (n = 5974 per group), baseline characteristics were balanced apart from fetal head station at full dilation, which was higher in the no‐epidural group (0.90 ± 0.9 vs. 0.66 ± 0.8; SMD = 0.239). Perineal injury occurred more often with epidural with a marginal difference (82.2% vs. 78.2%, SMD = 0.10). In a multivariable logistic regression analysis, epidural was associated with more perineal injury only in the absence of episiotomy (adjusted odds ratio [aOR] 1.321; 95% confidence interval [CI]: 1.225–1.425; P < 0.001), but not when episiotomy was performed. Conversely, epidural use was associated with lower OASI risk regardless of whether episiotomy was performed (aOR 0.492; 95% CI: 0.285–0.849; P = 0.011) or not (aOR 0.592; 95% CI: 0.424–0.825; P = 0.002).
Conclusion
In primiparous term vaginal deliveries, epidural analgesia modestly increased low‐grade perineal tears but reduced OASI risk, supporting its safety regarding severe perineal trauma.
Keywords: epidural analgesia, episiotomy, matched cohort, OASI, perineal injury, primiparous
1. INTRODUCTION
Obstetric perineal injuries are a common complication of vaginal birth, particularly among primiparous women. Approximately 90% of primiparous women experience some degree of perineal trauma. 1 Minor lacerations mainly cause transient postpartum discomfort, whereas obstetric anal sphincter injury (OASI) can result in long‐term complications including involuntary loss of gas, liquid, or solid stool known as anal incontinence, pelvic organ prolapse, dyspareunia, and chronic pain. 2 , 3 , 4 , 5 OASI occurs in approximately 0.5%–3% of all vaginal deliveries, 6 , 7 with higher rates reported among primiparous women reaching up to 6% in some cohorts. 7 , 8
Risk factors for perineal trauma include both maternal and intrapartum variables. These include fetal occiput‐posterior position, macrosomia, prolonged second stage of labor, midline episiotomy, operative vaginal delivery (OVD) and Asian ethnicity. 9 , 10 , 11 Maternal characteristics such as elevated body mass index (BMI, calculated as weight in kilograms divided by the square of height in meters), and advanced maternal age have been associated with increased risk, though less consistently. 12 , 13 Among these, OVD remains the most significant modifiable contributor.
Epidural analgesia is widely used in modern obstetrics, particularly among nulliparous women, and is considered highly effective for labor pain relief. However, its influence on perineal outcomes remains controversial. Early observational studies linked epidural use to longer second stage duration and higher rates of OVD, raising concerns that it might increase perineal trauma risk. 14 , 15 More recent research, however, has presented conflicting evidence. While some studies reported a higher incidence of perineal lacerations among women who received epidurals, 1 , 16 others found that this association diminished after adjusting for confounders such as parity and OVD. 17 , 18 , 19 Several reports even suggest that epidural analgesia has a protective effect on the risk of OASI. 20 , 21 , 22 , 23
The present study aimed to determine whether epidural analgesia independently affects the incidence of any perineal injury and the risk of OASI in primiparous women delivering vaginally at term.
2. MATERIALS AND METHODS
We conducted a retrospective cohort study at a large single, university‐affiliated tertiary medical center with approximately 13 000 deliveries annually. The research protocol was approved by the local Institutional Review Board (TLVMC‐0284‐08).
2.1. Study participants
We included all primiparous women aged 18–45 years who delivered vaginally at term (≥37 weeks) with a singleton, cephalic fetus between January 2012 and January 2024. Exclusion criteria included cesarean section or non‐cephalic presentation at birth.
2.2. Study variables
Variables were extracted from hospital electronic medical records and included demographic characteristics, obstetric history, and intrapartum factors. Throughout the study period our labor ward followed a selective mediolateral episiotomy policy, with episiotomy performed only when clinically indicated, at the discretion of the obstetrician or midwife. OVD was accomplished exclusively by obstetricians using vacuum extraction only. Perineal injury was defined as any of the following: first‐degree tear (involving vaginal mucosa or perineal skin), second‐degree tear (involving perineal muscles but not the anal sphincter), third‐degree tear (involving the anal sphincter complex), fourth‐degree tear (extending through the rectal mucosa) and episiotomy. Maternal outcomes included perineal injury, requirement for packed red blood cell transfusion, postpartum hemoglobin drop, and prolonged maternal length of stay (>72 h). Gestational weight gain categories were defined according to the Institute of Medicine (IOM) 2009 guidelines: 2.5–18 kg for underweight (BMI <18.5), 11.5–16 kg for normal weight (BMI 18.5–24.9), 7–11.5 kg for overweight (BMI 25.0–29.9), 5–9 kg for obesity (BMI ≥30).
2.3. Study outcomes
(1) The primary outcome was any perineal injury, defined as first‐ to fourth‐degree perineal laceration or episiotomy. (2) The secondary outcome was OASI, defined as third‐ or fourth‐degree tears involving the anal sphincter complex.
2.4. Statistical analysis
Categorical variables are described as frequencies and percentages. Continuous variables were evaluated for normal distribution using histograms and quantile‐quantile (Q‐Q) plots and are reported as mean and standard deviations (SD).
Standardized mean differences (SMD) were calculated to evaluate baseline characteristics between groups before and after propensity score matching. SMD provides a standardized metric independent of sample size. Following established guidelines, we interpreted SMD values as follows: SMD <0.1 indicated negligible difference, SMD 0.1–0.2 represented small difference, and SMD >0.2 suggested clinically significant difference requiring attention. 24 , 25 The two groups were matched according to the probability of a woman to undergo epidural anesthesia. The probability (propensity score) was calculated using a logistic regression model. The following parameters were used to calculate the propensity score: Maternal age, ethnicity, pre‐gestational or gestational diabetes mellitus, pre‐gestational weight, weight gain during pregnancy, gestational age at delivery and year of birth, occipito‐posterior presentation, spontaneous onset of delivery, and neonatal weight. Fuzzy matching without replacement was performed. An absolute difference (matching tolerance/caliper) in the propensity score of up to 5% (on a scale of 0%–100%) was considered acceptable for matching. Multivariable logistic regression was used to examine the correlation between epidural analgesia and two outcomes: (1) any form of perineal injury and (2) OASI.
To evaluate the modifying effect of episiotomy, each outcome was analyzed in separate models, and every model was performed twice, once for births without episiotomy and once for births with episiotomy. The regression contained two blocks. In the first block, anesthesia type was forced into the regression to evaluate the crude association and in the second block, other parameters were forced into the model. These variables were selected based on previous studies evaluating risk factors for perineal injury as well as parameters with a standardized difference ≥0.1 in the current study. All statistical tests were two‐sided and a P value less than 0.05 was considered statistically significant. Statistical analysis was performed with SPSS statistical software (IBM SPSS Statistics for Windows, version 29.0.2, IBM Corp., Armonk, NY, USA).
3. RESULTS
Overall, 45 132 primiparous women with singleton term vaginal deliveries were included in the study, of whom 36 799 (81.5%) received epidural analgesia. After propensity score matching, 5974 women remained in each group.
In the unmatched cohort (Table 1), women receiving epidural analgesia had higher rates of OVD (22.1% vs. 8.5%, SMD = 0.385), prolonged second stage of labor (14.4% vs. 8.0%, SMD = 0.204), and episiotomy (39.4% vs. 23.8%, SMD = 0.339). Perineal injury of any degree occurred more frequently among women receiving epidural analgesia (87.0% vs. 78.2%, SMD = 0.235), whereas the rate of OASI was not clinically different (0.8% vs. 1.1%, SMD = 0.027). Epidural analgesia was also associated with a higher mean birth weight (3236 ± 391 g vs. 3150 ± 381 g, SMD = 0.221) and a higher fetal head station at full dilation (0.66 ± 0.8 vs. 0.91 ± 1.0, SMD = 0.347). Spontaneous labor onset was notably less common among women with epidural (74.8% vs. 93.6%, SMD = 0.509), and fetal head station at full dilation was higher (0.6 ± 0.9 vs. 0.91 ± 1.0, SMD = 0.347). Other differences included higher rates of occipito‐posterior presentation (3.6% vs. 2.0%, SMD = 0.100), a greater proportion of Caucasian ethnicity (96.8% vs. 93.3%, SMD = 0.163), higher pre‐pregnancy BMI (22.22 ± 3.8 vs. 21.7 ± 3.4, SMD = 0.14), and a higher rate of excessive gestational weight gain (28.2% vs. 20.5%, SMD = 0.180).
TABLE 1.
Comparison of maternal characteristics and obstetric outcomes between women with and without epidural analgesia in unmatched and matched cohorts.
| Maternal characteristics | Unmatched cohort | Matched cohort | ||||
|---|---|---|---|---|---|---|
| No epidural N = 8333 | Epidural N = 36 799 | SMD | No epidural N = 5974 | Epidural N = 5974 | SMD | |
| Age | 31.11 (±4.4) | 30.78 (±4.4) | 0.073 | 31.28 (±4.1) | 31.12 (±3.8) | 0.039 |
| Smoking | 212 (2.5%) | 1326 (3.6%) | 0.061 | 163 (2.7%) | 199 (3.3%) | 0.035 |
| Alcohol | 12 (0.1%) | 43 (0.1%) | 0.008 | 11 (0.2%) | 9 (0.2%) | 0.008 |
| Drugs | 68 (0.8%) | 232 (0.6%) | 0.022 | 51 (0.9%) | 30 (0.5%) | 0.043 |
| Spontaneous pregnancy | 7499 (90%) | 32 394 (88%) | 0.063 | 5353 (89.6%) | 5225 (87.5%) | 0.067 |
| Diabetes (all forms) | 701 (8.4%) | 3870 (10.5%) | 0.072 | 377 (6.3%) | 356 (6.0%) | 0.015 |
| Pre‐pregnancy weight | 59.03 (±10.1) | 59.97 (±10.9) | 0.087 | 58.6 (±9.5) | 57.91 (±8.8) | 0.074 |
| Pre‐pregnancy BMI | 21.7 (±3.4) | 22.22 (±3.8) | 0.14 | 21.5 (±3.2) | 21.44 (±2.9) | 0.027 |
| Weight gain a | 12.5 (±4.7) | 13.29 (±5.1) | 0.156 | 12.7 (±1.6) | 12.86 (±4.7) | 0.036 |
| Inadequate weight gain | 2659 (38.6%) | 10 601 (32.0%) | 0.137 | 2242 (37.5%) | 2276 (38.1%) | 0.012 |
| Normal weight gain | 2823 (41.0%) | 13 170 (39.8%) | 0.023 | 2477 (41.5%) | 2478 (41.5%) | 0.000 |
| Excessive weight gain | 1411 (20.5%) | 9317 (28.2%) | 0.180 | 1255 (21.0%) | 1220 (20.4%) | 0.014 |
| Ethnicity | ||||||
| Caucasian | 7769 (93.3%) | 35 618 (96.8%) | 0.163 | 5806 (97.2%) | 5866 (98.2%) | 0.067 |
| Asian | 215 (2.6%) | 681 (1.9%) | 0.050 | 107 (1.8%) | 68 (1.1%) | 0.054 |
| African | 340 (4.1%) | 482 (1.3%) | 0.172 | 61 (1.0%) | 40 (0.7%) | 0.038 |
| High risk pregnancy follow‐up | 1342 (16.1%) | 7275 (19.8%) | 0.096 | 895 (15.0%) | 888 (14.9%) | 0.003 |
| Obstetric characteristics | Unmatched cohort | Matched cohort | ||||
|---|---|---|---|---|---|---|
| No epidural N = 8333 | Epidural N = 36 799 | SMD | No epidural N = 5974 | Epidural N = 5974 | SMD | |
| Gestational age at delivery (weeks) | 39.6 (±1.1) | 39.84 (±1.1) | 0.207 | 39.6 (±1.1) | 39.7 (±1.1) | 0.035 |
| Spontaneous onset of labor | 7762 (93.6%) | 27 534 (74.8%) | 0.509 | 5554 (93.0%) | 5499 (92.0%) | 0.035 |
| Induction of labor | 532 (6.4%) | 8889 (24.2%) | 420 (7.0%) | 475 (8.0%) | ||
| Occipito‐posterior presentation | 164 (2.0%) | 1330 (3.6%) | 0.100 | 57 (1.0%) | 65 (1.1%) | 0.013 |
| Head height at full dilation | 0.91 (±1.0) | 0.6 (±0.9) | 0.347 | 0.9 (±0.9) | 0.66 (±0.8) | 0.239 |
| Prolonged second stage | 666 (8.0%) | 5291 (14.4%) | 0.204 | 534 (8.9%) | 423 (7.1%) | 0.068 |
| Vacuum extraction | 705 (8.5%) | 8119 (22.1%) | 0.385 | 476 (8.0%) | 500 (8.4%) | 0.015 |
| Birth weight | 3150 (±381) | 3236 (±391) | 0.221 | 3165 (±378) | 3171 (±350) | 0.049 |
| Any form of perineal injury | 6514 (78.2%) | 32 018 (87.0%) | 0.235 | 4673 (78.2%) | 4910 (82.2%) | 0.100 |
| Intact perineum | 1819 (21.8%) | 4781 (13.0%) | 1301 (21.8%) | 1064 (17.8%) | ||
| Episiotomy | 1987 (23.8%) | 14 489 (39.4%) | 0.339 | 1387 (23.2%) | 1442 (24.1%) | 0.022 |
| Spontaneous perineal tears | 4730 (56.8%) | 19 701 (53.5%) | 0.065 | 3427 (57.4%) | 3620 (60.6%) | 0.066 |
| First degree | 2165 (26.0%) | 8419 (22.9%) | 0.072 | 1539 (25.8%) | 1620 (27.1%) | 0.031 |
| Second degree | 2337 (28.0%) | 10 076 (27.4%) | 0.015 | 1720 (28.8%) | 1836 (30.7%) | 0.042 |
| First/second not specified | 140 (1.7%) | 913 (2.5%) | 0.056 | 104 (1.7%) | 120 (2.0%) | 0.020 |
| Third degree | 81 (1.0%) | 276 (0.8%) | 0.024 | 58 (1.0%) | 40 (0.7%) | 0.033 |
| Fourth degree | 7 (0.1%) | 17 (0.0%) | 0.015 | 6 (0.1%) | 4 (0.1%) | 0.012 |
| Delta hemoglobin (g/dL) | −2.44 (±1.8) | −2.73 (±1.64) | 0.171 | −2.48 (±1.8) | −2.46 (±1.6) | 0.012 |
| Blood transfusions | 171 (2.1%) | 777 (2.1%) | 0.004 | 130 (2.2%) | 67 (1.1%) | 0.083 |
| Prolonged LOS | 1219 (14.6%) | 5759 (15.6%) | 0.028 | 817 (13.7%) | 767 (12.8%) | 0.025 |
Note: BMI, calculated as weight in kilograms divided by the square of height in meters. Values in bold indicate clinically significant standard SMD >0.2.
Abbreviations: BMI, body mass index; LOS, length of stay; OASI, obstetric anal sphincter injury; SMD, standardized mean difference.
According to the Institute of Medicine (IOM) 2009.
Following propensity score matching (Table 1—matched cohort), baseline characteristics were well balanced between groups, with SMDs below 0.1 for most variables, including operative vaginal delivery, prolonged second stage of labor, episiotomy, birth weight, spontaneous labor onset and occipito‐posterior presentation. One exception was fetal head station at full dilation, which remained clinically different between groups after matching (0.90 ± 0.9 in the no‐epidural group vs. 0.66 ± 0.8 in the epidural group; SMD = 0.239). Perineal injury was more common among women receiving epidural analgesia (82.2% vs. 78.2%), although the difference was marginal, with an SMD of 0.1 at the borderline between negligible and small effect size.
We next assessed the association between epidural analgesia and episiotomy using multivariable regression analysis model (Table 2). This analysis was performed separately for association between epidural and any type of perennial injury and for OASI. Epidural analgesia was significantly associated with a reduced risk of OASI regardless of whether episiotomy was performed or not. Among women who did not undergo episiotomy, epidural use was associated with a 41% reduction in the odds of OASI (adjusted odds ratio [aOR] 0.592; 95% confidence interval [CI]: 0.424–0.825; P = 0.002). Similarly, among women who underwent episiotomy, epidural analgesia was associated with a 51% reduction in the odds of OASI (aOR 0.492; 95% CI: 0.285–0.849; P = 0.011) (Figure 1).
TABLE 2.
Multivariable regression analysis—association between epidural analgesia and any perineal injury or OASI in women with and without episiotomy.
| Episiotomy | Tear type | Epidural | Rate | Crude OR (95% CI) | P value | Adjusted a OR (95% CI) | P value |
|---|---|---|---|---|---|---|---|
| No | Any | No | 71.3% | Ref. | Ref. | ||
| Yes | 78.6% | 1.465 (1.362–1.575) | <0.001 | 1.321 (1.225–1.425) | <0.001 | ||
| OASI | No | 1.10% | Ref. | Ref. | |||
| Yes | 0.90% | 0.773 (0.564–1.060) | 0.11 | 0.592 (0.424–0.825) | 0.002 | ||
| Yes | Any | No | 22.7% | Ref. | Ref. | ||
| Yes | 21.3% | 0.918 (0.805–1.047) | 0.203 | 0.978 (0.854–1.119) | 0.743 | ||
| OASI | No | 1.00% | Ref. | Ref. | |||
| Yes | 0.7% | 0.550 (0.326–0.930) | 0.026 | 0.492 (0.285–0.849) | 0.011 |
Note: Ref. refers to the reference group against which the comparison was made.
Abbreviations: BMI, body mass index; CI, confidence interval; OASI, obstetric anal sphincter injury; OR, odds ratio.
Adjusted to maternal age, ethnicity, pre‐pregnancy BMI, weight gain category, gestational age, induction of labor, occipital‐posterior position, fetal head height at full dilation, vacuum assisted delivery and newborn weight. Values in bold indicate statistical significance (P<0.05).
FIGURE 1.
Rates of perineal injury and obstetric anal sphincter injury (OASI) according to epidural use and episiotomy status. Grouped bars display the crude percentages of any perineal injury and OASI among primiparous term vaginal births. Within each episiotomy stratum, orange bars represent women who received epidural labor analgesia and yellow bars represent those who did not. In the absence of episiotomy, epidural use was associated with a higher rate of any perineal injury (78.6% vs. 71.3%) but a lower rate of OASI (0.9% vs. 1.1%). When episiotomy was performed, epidural analgesia had no appreciable effect on minor injury rate (21.3% vs. 22.7%) and remained protective against OASI (0.7% vs. 1.0%).
When analyzing the outcome of any degree of perineal injury, epidural analgesia was associated with a significantly increased risk among women who did not undergo episiotomy (aOR 1.321; 95% CI: 1.225–1.425; P < 0.001). However, no significant association was observed between epidural use and the rate of any perineal injury among women who underwent episiotomy (aOR 0.978; 95% CI: 0.854–1.119; P = 0.743).
4. DISCUSSION
In this large propensity‐matched cohort of primiparous term vaginal deliveries, we aimed to determine the association between epidural analgesia and perineal injury, including OASI, in primiparous women. Our main findings were: (1) In the matched analysis, groups were similar overall, and the sole clinically meaningful difference was a higher fetal head station recorded at the time of full dilation in the epidural group. (2) In the matched analysis epidural analgesia was found to be clinically associated with only a marginal increase (SMD = 0.100) in the risk of overall perineal injury, without an impact on the likelihood of OASI. (3) Using a multiregression model for stratifying births by episiotomy status, epidural analgesia was associated with a modestly higher incidence of minor perineal injuries in deliveries without an episiotomy, yet epidural was protective against OASI regardless of whether an episiotomy was performed or not.
These findings offer additional insight into the complex relationship between epidural analgesia and perineal outcomes during birth, suggesting that while epidural analgesia may predispose to minor tears (possibly in an indirect manner), it does not exacerbate and may mitigate the risk of the most severe lacerations.
The absolute OASI rate in our study (~1%) was lower than that reported in many earlier series, emphasizing the influence of contemporary second‐stage management and selective mediolateral episiotomy on anal sphincter preservation. Early observational studies linked epidural use to a doubling of OASI, largely because the analgesic was accompanied by longer second stages and liberal OVD. 15 Subsequent large cohorts that controlled for those factors have produced mixed results: some reported neutral associations, 18 , 19 whereas others demonstrated a protective effect. 19 , 22 Our findings confirm the latter pattern. After adjustment, epidural reduced OASI odds by 41% without episiotomy and by 51% when episiotomy was performed, like other studies. 17 , 21
Taken together, a growing body of evidence indicates that epidural analgesia per se is not a major contributor to OASI, and it may even have a protective correlation when modern obstetric management is applied. Two explanations may account for the opposing effects on minor tears versus OASI. One hypothesis involves the more controlled delivery that epidural analgesia can facilitate with adequate pain relief, the obstetrician or midwife can guide the mother through a slower, more measured expulsion of the fetal head while applying manual perineal support.
Garcia‐Lausin et al. showed a similar reduction in third‐ and fourth‐degree tears when epidural analgesia was used with modern obstetric management. 19 At the same time, relaxation of pelvic‐floor tone may predispose to superficial tissue tearing, producing the observed rise in first‐ and second‐degree lacerations in a subgroup of women with no episiotomy. The relatively low OASI rate (~1%) in this study compared to other studies 21 may be attributed to variations in the study population, as well as differences in obstetric practices.
The impact of epidural analgesia on the rate of fetal head descent during labor has been consistently observed and warrants further investigation, as the speed of descent may play a critical role in allowing perineal tissues to gradually adapt to the expulsion of the fetus.
In the matching process we did not include certain labor related factors known to be associated with the outcomes, such as prolonged second stage or OVD, and yet, the rates of these interventions were similar between the two matched groups. Therefore, another important observation of our study, although not directly related to pelvic injury, raises a relevant point. It questions whether epidural analgesia is truly associated with these outcomes once confounding factors are properly controlled.
4.1. Strengths and limitations
Strengths of the present study include the large sample size and the focus on a homogeneous group (primiparous, term singleton deliveries), which allowed us to isolate the effect of epidural analgesia with minimal confounding factors. Another strength of the present study is the use of a propensity score‐matched design, which helped reduce confounding by balancing baseline characteristics. Additionally, we assessed differences using SMD, allowing for a more clinically meaningful interpretation of the magnitude of effect.
By using a multivariable regression analysis model and stratifying results by episiotomy status we sought to separate the effect of the analgesic itself from that of associated obstetric interventions, which gives a wider view of maternal perineal morbidity.
Nevertheless, we acknowledge several limitations such as the retrospective nature of the study. Additionally, our results may not generalize to settings where different episiotomy practices or OVD methods other than vacuum are used.
4.2. Clinical implications
From a patient counseling perspective, these data are reassuring. Epidural analgesia does not appear to worsen, and may actually mitigate, the most severe form of perineal trauma when modern delivery techniques are employed. The modest increase in minor tears is clinically acceptable given their straightforward repair and low long‐term morbidity compared with OASI. Clinicians should therefore focus preventive efforts on modifiable factors that remain strongly linked to perineal injury such as prolonged second stage, and OVD rather than withholding maternal analgesia.
5. CONCLUSION
Among primiparous women who delivered vaginally at term, epidural analgesia was associated with a modest increase in low‐grade perineal tears, while independently associated with a reduced risk of OASI regardless of episiotomy status. These findings suggest that, under a selective episiotomy regimen, epidural analgesia does not increase and may in fact reduce the risk of high‐grade perineal trauma in primiparous women.
AUTHOR CONTRIBUTIONS
YB: Conception and design of the study. OD: Prepared the main draft of the manuscript. UA, RG and CG: Responsible for reviewing. EA: Performed data analysis. YY: Contributed to the critical review and editing of the manuscript.
CONFLICT OF INTEREST STATEMENT
The authors declare that they have no conflict of interest.
DATA AVAILABILITY STATEMENT
The datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request.
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Associated Data
This section collects any data citations, data availability statements, or supplementary materials included in this article.
Data Availability Statement
The datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request.