Nothing? there are multiple posts in this thread with really good information regarding the fact that it's definitely not 100% chance.
Males, both equine and human may produce a significantly higher % of one sex or the other, (sometimes greater than 90%) thereby affecting the odds of one gender over the other. In theory, it is 50/50, but in reality, anomalies always occur, and more frequently than one might think. Uterine environment, penetration depth, sperm count, breeding schedule, and diet are other major factors that do influence gender selection.