I read an article that related different embryo behavior and growth to the amount of activity mares (female horses), i.e., the amount of exercise they get.
The article can be found here: http://www.journalofanimalscience.org/content/90/11/3770
In a practice known as embryo transfer, female competitive race horses are fertilized via traditional copulating with a male, and the very young embryo is then transferred 7-9 days later into a non-competing female. This allows racing females to compete even in the breeding season when breeders hope to breed them with males to produce a new generation of competitors.
Researchers in Florida set about determining the effects of exercising mares during the 'periovulatory period,' or the period in which an oocyte is discharged from the graafian follicles of the ovary, by exercising females at this stage at different amounts--none, some, and full--and analyzing embryo recovering rates. The concern is that competing horses, who are exercising strenuously, will produce inferior embryos that may not survive the transfer into another female.
In the study, the researchers found that exercise led to reduced embryo recovery rates when compared to the recovery rates of non-exercising females. They hypothesized that reduced hormone concentrations linked to exercise, as shown by blood analysis, may affect the competency, or structural integrity, of the female's oocytes (eggs). In particular, the stress of exercise increased cortisol amounts, which have been shown to have effects of reproduction. Compromised oocytes may be less likely to become fertilized, and if fertilized, the resulting embryos may not develop correctly.