There is a lot of research about multiple equine reproduction topics and almost all refer to conception rates or pregnancy rates. A mare that gets in foal is considered pregnant. But, how long was she pregnant? Most studies do not differentiate. Many studies end after the mare is in foal.
But, the only thing that really counts is did she have a live, viable foal? UC Davis did a study in the 1980s about foal heat breeding. They found that mares bred before day 10 post foaling had a much lower live foal rate. Each day bred before day 10 lowered the live foal rate. We will admit if everything looks perfect at day 9, we might gamble on that, but we never breed a mare before day 9. We have had very few mares abort in decades and maybe that’s part of the reason.
Pay attention to live foal rates, not just pregnancy rates.