I found this a little alarming, and interesting (you have to scroll down to where they mention pregnancy).
I was always told that melatonin was safe to take during pregnancy, and while TTC - I have never taken it during either, but I know many women who have.
However, the National Institute of Health says no! It may put the baby at risk for certain developmental delays, and while TTC it may interfere with ovulation making it more difficult to become pregnant.
As always, you and your doctor should make the final call about medications during pregnancy (And TTC) - I just thought this was some interesting info from a reliable source that I would share. I've seen so many questions about melatonin during pregnancy, and I've always seen the answer "it's safe"...but maybe not?
http://www.nlm.nih.gov/medlineplus/druginfo/natural/940.html