You can prevent some mental health conditions by caring properly for the mother, for example a mother struggling to pay rent or unable to access necessary medical care is naturally going to be at higher risk of conditions like depression and if you improve her circumstances it's possible that the depression might at least abate for a while. Lots of people who are managing mental health conditions will spiral if their circumstances get bad enough but will otherwise be OK.

While it's unwise to try and simply reduce all mental health conditions to 'circumstances' or 'chemical imbalance', there are lots of things you can do to keep a person in balance to the point that any latent conditions they have aren't enough of a problem to interfere with their daily lives. Motherhood is already very difficult so it is worthwhile to try and keep as many challenges out of a new mother's life as we can.