I'm pretty sure I've heard of this...oh to have my "What to expect when you're expecting" book with me at work! I am almost positive that the book specifically mentions brown spots on hands.... I did find this info though:
"A: During pregnancy, the hormone melanin, which tints the skin, is more active than normal. Melanin causes the linea nigra, a brown line that develops on the midline of the abdomen during pregnancy. It can also cause chloasma (also known as the mask of pregnancy), characterized by discoloration on the forehead, nose, and cheeks. Women with darker complexions are more prone to developing chloasma. Melanin can also darken other pigmented areas or spots on the body, such as birthmarks and freckles. Most of these pigmented areas will lighten again a few months after delivery, though they may not clear up completely. Stay out of the sun so that these areas don't darken even more. If you can't avoid the sun, wear hats, cover up, and use sunscreen."
I have heard from several sources that melanin is pretty wild for some pregnant women. This may be the cause of your brown spots...
Hope this helps!