I had a bad cold when I was in my third trimester and took diphenhydramine. I got a great night's sleep, but the next day, I couldn't feel my baby moving. I panicked and went to the hospital. Everything turned out fine but I think the diphenhydramine didn't just make me drowsy and sleepy, it did the same to my baby.
If I have another child, I probably will not take diphenhydramine during pregnancy again because this was a scary incident for me.