Anything is possible with young kittens. Infections, viruses, and yes birth defects can make a one month old kitten die. Are they crying or meowing alot like they are hungry? Do they have a temp? How is the mom's health? Are there cleaners, chemicals around? Do you keep them seperate from the rest of the house in a room that only the mom is allowed in? Many factors can be the cause of a death of a kitten. Lungs not developing the same with the heart. Have the kittens been sneezing? Are their eyes watery or have goop in them? I hope that you can get them to the vet before anything else happens. And YES, get the Mom spayed and the Dad neutered. Breeding has its rewards if you have healthy litters but then something happens and you have something like this happen. I have been breeding for a while and only once had a kitten fade at the age of 7 weeks old and it was his lungs were not developing as they should have. Mother nature can be cruel and wonderful at the same time. Good luck with the kittens and keep us posted on them.