Don't expect the sufferer to have fears and worries that are reasonable. When you are depressed quite small things can worry or upset you greatly. Do try to give her as much practical help as possible.
Depression makes a sufferer feel extremely tired and small tasks feel like huge ones.
Try to keep your patience even though it may be taxed. Don't point out shortcomings, unfinished jobs, unkempt appearance. You don't know how lucky you are. There are lots worse off than you. Don't leave her alone with the baby if you feel there is the slightest possibility of her doing harm to the child or herself.