Cervix-deep rectal temperature differential on the day of ovulation is correlated with embryo recovery results

Abstract

In many species, the reproductive tract experiences a temperature decrease before ovulation to aid gamete maturation. For example, pre-ovulatory follicles are ≤1.0°C cooler than ovarian stroma in cattle, and a lower temperature differential (TD) between cervix-rectum at the time of insemination has been correlated with pregnancy in cows. The present study aimed to compare TDs (cervix-deep rectal) in positive versus negative embryo recovery cycles. Deep rectal and cervical temperatures were measured using a precision thermometer with a 30cm probe (Electronic Temperature Instruments Ltd, UK; resolution 0.01°C, accuracy ±0.05°C) in 19 mares over 32 estrous cycles. Mares were inseminated with a fertile stallion and excluded if they required any treatment other than oxytocin administration. Embryo flushing was undertaken on Day 7/8. Deep rectal and cervical temperatures were taken on 5 occasions: the day of deslorelin administration and insemination, the day before ovulation, the day of ovulation (Day 0), one day post ovulation and before embryo recovery on Day 7 or 8. Statistical analysis was undertaken using the Mann-Whitney Rank Sum test. The median TD (cervix-deep rectal), and cervical and deep rectal temperatures were compared between cycles with a positive versus negative embryo recovery. From the 32 cycles, 21 (65.6%)cycles produced an embryo. On the day of ovulation positive cycles showed a lower median TD than negatives ones (0.19 vs. 0.40 °C; p=0.001). For the 11 negatives cycles a lower median TD was seen in the two days prior to ovulation versus the day of ovulation and the day of embryo recovery (p≤0.023 in all cases). For positive cycles lower median TDs were seen at all time points from induction till one day after ovulation versus the day of embryo recovery (p<0.007 in all cases). For rectal or cervical temperatures per se no significant differences were seen between positive and negative cycles at any time point. However, in negative cycles median cervical temperature was lower the day before ovulation versus the day of embryo recovery (p=0.014). In positive, but not negative, cycles the median rectal temperatures on the day of induction, ovulation and the day after ovulation were higher than on the day of embryo recovery (p<0.035 for all days). In conclusion, as in cattle, a thermoregulatory process occurs at the time of ovulation in the mare in which a lower TD between cervix and rectum is related to a greater likelihood of pregnancy. Further characterization of TDs within the reproductive tract of the mare would increase our understanding of the conditions required for oocyte maturation.