Being as how I didn't start reading until I was practically menopausal... I have no clue.
I can see why it might be harder, because being pregnant tends to make you more emotional and takes a lot of energy from you... and it seems like the main focus is on baby,
but it seemed like my "sensitivity" to other things was much higher...and I won't call it intuition either because what I sensed was not that at all. I knew things before they happened and could sense when something wasn't right days before the event... and this seemed not to be as strong when I"m "normal".
But I was reading runes during my last pregnancy and didn't have any trouble with off readings or accuracy or anything like that. I do think that I probably read more into them because I was more emotional and susceptable to my emotional yo-yo'ness and probably over reacted to them.
Honestly, I don't think that it will harm you or the baby... I simply think it might be harder to focus... because as soon as you get comfortable, you suddenly need to get up and pee or the baby will decide it's time to practice sommersaults, ninja kicking or headbanging. It seems that they do want lots of attention even before they enter the world and demand it at the top of their wee little lungs, so everytime you try to focus on something else, they let you know... they are your prime concern.