Relationship Between Ultrasonographic Findings at the Time of Breeding and Pregnancy Outcome

Grade 3 and 4 uterine edema at ovulation did not adversely affect fertility of mares. Authors’ addresses: Gluck Equine Research Center, Department of Veterinary Science, University of Kentucky, Lexington, KY 40546 (Squires, Ball, Troedsson); and Rood and Riddle Equine Hospital, PO Box 12070, Lexington, KY 40580-2070 (Bradecamp, Schnobrich, Riddle); e-mail: edward.squires@ uky.edu.

1. Introduction

It is a routine practice to examine the mare’s uterus and ovaries during estrus in order to assess follicular development of the ovaries and edema in the uterus as an aid in scheduling breeding. Several reports have described an increase in uterine edema in response to the secretion of estrogen from the pre-ovulatory follicle.^1–3  The peak in circulating estrogen occurs about 24 h prior to ovulation. Thus, the degree of uterine edema increases as the mare develops a pre-ovulatory follicle and approaches ovulation. Near the time of ovulation, the amount of edema decreases and when the mare is in diestrus, edema in the uterus should be minimal.⁴  Based on ultrasound examination of the uterus, the uterine edema is usually graded on a scale of 0 to 4 or 0 to 5: 0 = no edema, 3 = maximum normal edema, and greater than or equal to 3.5 to 4 = excessive edema. Assessing the degree of edema has been used to determine the appropriate time to administer human chorionic gonadotropin (HCG) or gonadotropin-releasing hormone (GnRH);⁴  whether the mare is in estrus; if the follicle in the transitional mare is competent;³  diagnosis of inflammation of the uterus; and if the corpus luteum is regressing. 

Clinically there are mares that, for unknown reasons, continue to have a high edema score even at ovulation or after ovulation. Some have suggested that maximum or excessive edema at ovulation may be due to subclinical endometritis, excessive estrogen secretion, or failure of progesterone to increase.^3,5 Regardless, the practitioners must decide whether to ignore the presence of excess edema or treat the uterus. Apparently, there have been no studies to determine if high edema scores near ovulation adversely affect fertility or the incidence of early embryonic loss. This study tested the hypothesis that maximum or excessive uterine edema near ovulation lowered pregnancy rates and increased early embryonic loss in a large group of Thoroughbred mares.

2. Materials and Methods

Nine hundred and twenty Thoroughbred mares were examined with ultrasound for a total of 1127 cycles during the 2013 breeding season on day 2 (day prior to breeding and day ovulating inducing agent was usually given) and day 0 in relation to ovulation (day of OV = 0). Uterine edema was given a score of 0 to 4 with 0 = no edema, 3 = maximum, and 4 = excessive edema. Uterine fluid was also graded based on the vertical measurement of intrauterine fluid in cm. Other data included antibiotics (Y/N), uterine lavage (Y/N), and pregnancy at 14 and 40 to 50 days. Mares were classified in 3 age groups: young (2–9 years), middle-aged (10–16 years), and old (>16 years) and 3 reproductive statuses at the time of breeding: foaling, maiden or barren. Since there were only 4 mare cycles with grade 4 edema at the time of ovulation, the edema scores of 3 and 4 were combined for analyses

Risk factors for pregnancy status were identified in univariable analysis with an X² test to compare categorical data between groups. Multivariable logistic regression with the dependent variable of pregnancy rate/cycle was used to develop a model that included independent variables with a P ≤ 0.20 in a stepwise backward method using SAS ProcJMP^a; a priori values of  ≤ 0.05 were considered to be significant in the final model.

3. Results

The parameters that had significant effects in the logistic model on pregnancy rates per cycle and early embryonic losses were age (P < 0.03), reproductive status (P < 0.01), uterine lavage (P < 0.04), and age x status P < 0.001). The presence of uterine fluid greater than 1 cm tended (P < 0.09) to affect pregnancy rate/cycle. Edema scores of 3 and 4 prior to ovulation and at ovulation did not signifi- cantly affect the pregnancy rate or embryonic loss. Whether mares were given antibiotics also did not affect the outcome parameters (pregnancy rate/cycle or embryonic loss). Based on the X² analysis, the pregnancy rate per cycle was less for old mares (28/74, 38%) versus middle-aged mares (237/379, 63%) or young mares (480/673, 71%). Barren mares had greater P < 0.05) early embryonic losses between 14 to 50 days (29/217, 13%) than those for foaling (63/727, 8.7%) and maiden (10/182, 5.5%), but pregnancy rates were similar (P > 0.05) among mares of various reproductive statuses (66%, 144/ 217, 65%, 472/727, or 71%, 129/182 for barren, foaling, or maiden mares, respectively). When age was examined as a continuous variable and not categorical, there was an age x status interaction. Mares over 11 years old and barren had a lower pregnancy rate than those barren and less than 11 years. The presence of greater than <1 cm of uterine fluid at the time of ovulation increased (P < 0.02) embryonic loss (19/113, 17%) compared to those mares with <1 cm of uterine fluid (60/668, 8%) but the pregnancy rates were similar (58 vs 67%). This effect was primarily due to the presence of fluid in older mares (age x fluid). Based on the X² analysis, the presence of grade 3 or 4 edema at ovulation did not (P = 0.94) affect the pregnancy rate or embryonic loss. However, those mares that were lavaged had a greater embryonic loss (50/384, 13%) than those not lavaged (52/741, 7%).

4. Discussion

Edema was not a parameter in the logistic model that affected pregnancy rate or embryonic loss. Furthermore, if we examined the pregnancy rates and embryonic losses in mares with minimal edema (0 to 2) compared to those with maximal or excessive uterine edema (3–4, there was no effect on preg- nancy rate or early embryonic loss so this could be a reflection of the relatively small number of cycles [8] in which mares had excessive edema [grade 4]). Thus, we concluded that the presence of edema scores of 3 and 4 prior to and after ovulation had no apparent effect on the pregnancy rate per cycle. The overall incidence of mares with grades 3 and 4 uterine edema at ovulation was 73/1126 (6.5%). These mares also had a higher incidence of uterine fluid >1 cm (18%) than those mares with edema scores of 0 to 2 (9.5%). The fact that excessive edema did not lower pregnancy rates or increase embryonic losses was unexpected. Others have suggested that excessive edema at ovulation or after ovulation may be an indication of subclinical endometritis or perhaps an endocrine imbalance.⁵ Unfortunately, we were not able to determine the cause of excessive edema around the time of ovulation under the conditions of this study. In contrast, the presence of fluid > 1 cm did affect pregnancy rate and embryonic loss. Others have reported that the fluid during estrus decreases fertility, depending upon the amount of fluid and grade of fluid.^6,7 The echogenicity of uterine fluid was not available in this group of mares. Mares that had uterine lavage during estrus also had a higher embryonic loss rate. This is probably not a direct result of the lavage procedure but an indication that mares that needed a lavage were more than likely those with fluid, older mares, or those with some other reproductive problem.

It was not surprising that reproductive status and age affected pregnancy rates and the incidence of early embryonic loss. Others have reported lower fertility and higher embryonic loss for aged mares and for barren mares compared to younger maiden or foaling mares.⁸ There was, in fact, an interaction of age with reproductive status and age and the presence of uterine fluid. Mares over 11 years of age had a higher incidence of having moderate to large amounts of fluid and more barren mares were represented in the aged category than the other two age groups.

In summary, maximum or excessive uterine edema near the time of ovulation did not adversely affect pregnancy rates or the incidence of early embryonic loss. [...]