In some respects, bringing home a new puppy is like bringing home a tiny newborn human baby.  Chihuahuas, like many other toy breeds, may be susceptible to a form of low blood sugar called hypoglycemia.
You probably won't experience low blood sugar (with your particular puppy) but, in the event that you do, it's an emergency.  Small dogs, especially chihuahuas have a very small fat reserve around the liver.  When they get stressed for some reason, (like going to a new home) or if they play too hard (using a lot of energy), or miss a meal, the fat reserve is used up and the body will start to draw on the blood sugar for energy.  If this condition goes unchecked, the puppy will grow weaker until it falls into a coma and eventually dies.  The good news is that your puppy should outgrow this phase by 16 weeks of age.  Your goal as a new owner, is to keep the puppies stress level as low as possible during this critical period. 

Often, the most dangerous period of time is between 7-10 weeks old because this is when the puppies are being weaned.  Also, the immunities that the puppy was getting through it's mother's milk drop off around 9 weeks of age and that will increase it's vulnerability to virus and bacteria.  During this time the puppy is being weaned, bathed, toenails clipped, and sometimes sent to a new owner.  All together these factors make for a stressful time for the little dog.  Some puppies breeze through this period and others have problems.
Signs of hypoglycemia may vary.  Some dogs get a forlorn look on it's face and then become inactive.  Next they will stagger and eventually fall down and fall into what looks like a false sleep.  This sleep-like coma will turn into teeth clenching seizures and spasms followed by a comatose condition and eventually death.  Not all symptoms may be present at the same time.  Watch for lack of coordination or lethargic behavior along with pale gums. If your puppy seems too sleepy, stand him up and make sure he can stand and walk normally.  You can also check their gums for lack of color, which is another typical sign.  If he lies back down, falls or staggers, then get some sugar into him right away.  Most people use Nutristat or Nutrical mixed with a few drops of water.  You then syringe it into the puppies mouth being careful not to release it all at once.  While you are doing this you can heat a bathroom towel in the microwave for about 45 seconds and wrap it around your pup.  This helps them maintain their body temperature and will bring it back up if it has begun to fall.  If you are unsure on how to proceed, then contact your breeder and/ or your veterinarian.
Hypoglycemia is seen most often in smaller chihuahua puppies but I caution all new puppy owners to be on the look out for any of these symptoms.  Once you take your new puppy home, it is your job to be their "Guardian".  While hypoglycemia poses a danger to young puppies, it can be easily treated if caught early enough.