A.P.
Well, exposure to some real sunlight during the day followed by all lights out at night helps boost melatonin levels, which are naturally high in children anyway (for the most part). But sometimes I think my kid's production falls off when her schedule is changed (or when I travel and she gets excited that I'm finally home.) So we boost her melatonin with certain foods.
I also mentioned the whole dried tart cherries/warm milk at night ritual, works pretty great for my kid (I have a night owl daughter, so I drug her with dried cherries and about 3 ounces of warm milk when she's going strong into the late late hours.)
Also, is she getting enough exercise? If you are due soon, she might not be getting as much exercise with you like she normally would.
If she doesn't seem to improve, or her personality alters, you should take her to the pediatrician. There may be some kind of underlying issue, like a virus or something that she can't quite kick all the way.