I have a 13 year old Warmblood mare (with foal at side) that we bred with frozen semen this year. At her 14 day ultrasound, my vet detected a tiny black dot on the ultrasound (3mm!) and said he wanted to check her again before he could determine pregnancy or not. at 16 days she had an 11mm vesicle, that appeared healthy, although small for 16 days. We put her on progesterone and checked her again at 23 days. A definite embryo is developing and looks perfectly healthy. We will check her again in a few days to check for a heartbeat. While I do not suspect a second follicle having been ovulated (but I know it COULD be possible), what other reasons for delayed growth of the embryo? Can frozen semen have anything to do with this? If the embryo looks healthy and has a heartbeat at the next ultrasound, should I remained concerned over this pregnancy? THANKS!
For many years the hypothesis was that frozen semen conceptus' had a tendency to be smaller than fresh or cooled semen conceptus at the early pregnancy check stage. It was agreed however that the size difference was roughly a day at most. I am afraid I am a little skeptical about the detection of a 3 mm conceptus, as the smallest conceptus that is identifiable per ultrasound is a 9-day (and that in a lab setting with excellent equipment and the best of experts doing it) and that is considerably larger than 3 mm...!! At 14 days, the conceptus is about 13 mm in diameter (on average), so your "16 day" conceptus was probably about a 12 or 13 day conceptus.
Having said all of the above, a couple of years ago, Newcombe et al did a retrospective study on conceptus diameter with different breeding methods and concluded that the diameter was not affected by the type of semen, but rather by the timing of breeding - pre- or post-ovulation. I have reproduced the published abstract below.
In all probability you did not need to use Regumate - most mares that are given it do not need it - but now you are committed to using it and will need to do a serial of tests if you decide to take the mare off (3 or 4 is recommended) to determine endogenous progesterone levels as those levels vary from day to day (and within the day). The unfortunate thing about using Regumate is that it actually suppresses the mare's own production levels of Progesterone, so you actually create a situation where you need to use it (what a great marketing ploy!! )
Effect of type of semen, time of insemination relative to ovulation and embryo transfer on early equine embryonic vesicle growth as determined by ultrasound
J. Cuervo-Arangoa, J. Aguilar, J.R. Newcombe
Received 29 August 2008; received in revised form 15 December 2008; accepted 26 December 2008. published online 02 March 2009.
Embryonic vesicle growth in the mare is easily monitored by ultrasound. Apart from pregnancy diagnosis, assessment of the embryonic vesicle in practice is also useful to evaluate its viability. Although subject to individual variation, embryo growth rate follows a constant pattern in the early stages of development in relation to embryonic age. Previous studies have shown a significant effect of some factors routinely used in practice, such as post-ovulation insemination and embryo transfer, on embryonic growth and the time in which the vesicle is first detected. This study attempts to confirm previous results in different settings and characterise the causes for this delay in growth. A total of 159 pregnancies from different mating protocols: (1) pre-ovulation natural mating, (2) pre-ovulation natural mating and transfer into recipient mares, (3) post-ovulation natural mating, and (4) post-ovulation AI with frozen/thaw spermatozoa were evaluated ultrasonographically from day 12 to 19 of pregnancy and vesicle diameters recorded. Regression analysis between embryonic vesicle diameters and embryonic ages was performed for each group and mean vesicle diameter at different age periods among groups were tested for statistical difference with a general linear model of variance. There was no significant difference between groups 1 and 2 (P=0.73) or between groups 3 and 4 (P=0.71). However both pre-ovulation groups (1 and 2) had larger vesicle diameters (P<0.000) at any embryonic age analysed than either of the post-ovulation groups (3 and 4). In conclusion, post-ovulation inseminations produced pregnancies with smaller vesicle diameters equivalent to approximately 1 day's growth.
How interesting! Thank you for the information. I did forget to mention that she was indeed bred with one dose, post ovulation. Based on your information about the size being a 12-13 day conceptus at what was 16 days post breeding, am I right to conclude that the most likely answer is that she probably did have a secondary ovulation?
Hard to say. It might be that, or it might be that she didn't conceive immediately. Or both. The main thing though is that you've got a heartbeat!!
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