Find a different doctor. Postpartum depression can happen anytime during pregnancy and during the first year of your baby's life. There are safe options for medication during pregnancy. And your doctor is foolish for not putting you on something even though you are pregnant. In general people perceive the risks from medication to be greater than the risks of depression...and the truth is the risks of depression can be much worse!
Good for you for recoginzing the signs and talking to your doctor about it, but maybe find a psychiatrist or different OB that can help you. As well as medication, things that can make a big difference is 1) getting enough sleep 2) exercise 3) having some one to talk to about it!
Maybe if you can get some good information from a doctor/psychiatrist your DH could be more understanding once he is educated. Depression is something I like to compare to an uninvited guest at your doorstep...you don't want them there, you never know when they'll show up, and they just stand there yapping away and you just want them to LEAVE!
Hang in there and I hope you can find the help you need!