Hemmorhoids are mostly noticeable in the third trimester and after delivery. They can show up anytime really though, because a there are a few causes. The first is constipation. I had horrible constipation at the end of my first trimester which gave me hemmerhoids-but they went away quickly on their own. Pushing too hard or lingering on the toilet too long are some reasons you could get hemmerhoids. Try to avoid constipation (i.e. drink lot of water..oh and don't sit or stand for long periods of time).
The second is the hormone progesterone can cause swelling (as we all know swelling is common in pregnancy!) and it might just so happen that the veins that become hemmorhoids decide to swell too-sorry, no way to avoid this!
Third is your growing uterus putting pressure in this vein (someone please remind me of the name of this vein) on your right side, which can restrict blood flow, and cause your veins to swell. This is why hemmorhoids are most common in the third trimester.
Last cause is pushing too hard when trying to push out the baby. This gave me hemmorhoids that were pretty awful, and took lots of Tuck pads and ice packs and 2 weeks to get rid of. BUT, I got her out in less that 15 minutes and just a few pushes!!!

So, I guess don't push so hard, but if you get a good epidural, you won't be able to tell anyways.
Hope you can avoid them. But if not, please don't be embarrassed. It happens all the time!