Can horses tell if you are pregnant?

I've actually seen anecdotal evidence of this - a horse that suddenly changed his behavior towards a woman when she got pregnant. In this case, he got highly anxious. There are other stories of horses being more considerate towards pregnant women.

The truth is, it might be possible for a horse to smell the hormonal changes involved in pregnancy, but it's more likely that they're picking up on the woman's emotions and behavior. A lot of women become more careful when pregnant - and horses, with their incredible sensitivity to body language, are likely to notice that and change their own behavior accordingly. Some horses will go "My rider is being cautious, I should be cautious." Others might respond with "My rider is nervous! What's to be nervous about, I can't tell, worryworryworry." And a third personality type might be "Oooh, she's being over-cautious, what can I get away with?"

So, the most likely answer is: Yes, your horse can tell that you are pregnant - because you're inadvertently telling them. Or rather, you're telling them something has changed and they're reaction to that.