Yesterday (3pm) when being scanned my mare had a 48mm follicle. Semen arrived this morning (10:45AM) and the follicle was 40mm. I have heard they can decrease in size before ovulating but I was thinking 1-2mm. Does this sound normal? Or does it sound like it was regressing?
Also, this mare usually ovulates at around 36-40mm and I have never heard of her follicle getting smaller before ovulating in the past and we tracked her all year. She was a very hard mare to breed last year until the vet figured out she ovulated so early (she is a big 16h warmblood so at 36mm the vet would just order the semen and she would ovulate before it arrived). Why did this follicle get SO big this time? And why did it shrink 8mm in 13 hours?
Follicles will vary in diameter. They are not all uniform. They do - as you note - reduce slightly in size prior to ovulation, but they also lose the spherical aspect (which in some cases wasn't there to begin with), so one may get a different angle with the ultrasound, and therefore apparently get a dfferent diameter. It is why it is always advisable to measure from several (minimum 2) directions and take an average.
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