Kittens, especially very young kittens in the first two weeks of life are very vulnerable. Kittens can fade and die extremely quickly, so it is always important for the carer to keep a close eye on the kittens and how they are thriving.
What are the causes of fading kitten syndrome?
There are many possible causes of fading syndrome which can be infectious or non-infectious. Some of which include:
Congenital defect. Cleft palate, flat chest, umbilical hernia and other abnormalities.
Environmental temperature too hot or too cold (hyperthermia/hypothermia). Especially in the first week because newborn kittens are unable to regulate body temperature by panting or shivering.
Maternal neglect. This could be due to the mother being nervous, inexperienced or sick.
Prematurity and or low birth weight. If just one kitten is undersized, then placental insufficiency is the likely cause, if the entire litter are undersized, a poorly nourished queen is a likely consideration. 
It is advisable to keep a good track of each kitten and weigh them every day to ensure they are gaining weight. They should put on around 7 – 10 grams per day (there may be a large weight gain one day, and a small one another, but be aware of overall weight gain). Below is a rough timeline for normal kitten development.
0 – 3 days: The umbilical cord is still attached.
0 – 10 days: The eyes are closed.
2 weeks: The teeth start coming in.
2 weeks: The kitten begins to stand, the eyes and ears open.
4 weeks: The kitten begins playing and exploring her environment.
How is fading kitten syndrome treated?
Your veterinarian will perform a physical examination of the kitten(s) and obtain a medical history from you. Questions asked may include maternal nutrition during and after pregnancy, the delivery, how she is mothering the kittens, possible exposures to chemicals, toxins, and infections. He will also be interested in the blood type of the queen if that is known.
If a cause is found, then treatment is aimed at addressing the underlying condition. This may include;
Antibiotics to treat bacterial infections.
Treatment of parasites.
Fluids to treat dehydration.
Bottle or tube feeding.
Even with aggressive treatment, often kittens are too weak to pull through. It is incredibly important to seek veterinary attention the moment you see a kitten acting out of sorts. If several kittens die it may be worth having a necropsy performed to see if a cause can be determined, especially if you plan to mate the mother again.
 Cat Owner’s Home Veterinary Handbook – Delbert G. Carlson and James M. Giffin.