There are a few things common in the Japanese Chin breed that you need to be aware of. These are not all serious concerns but your Vet may mention them and cause alarm. This is unnecessary and I will address each one below:
About 2/3 of Japanese Chin have umbilical hernia's. About the diameter of a woman's pinky finger is the average size on a 12 week old puppy. This is not due to a genetic issue but rather the pull on the umbilical cord during the birth process. If desired this can be fixed when the puppy is being spayed or neutered but we nor any other long term breeders we have known have ever seen an issue even when leaving them alone. Females for breeding have had litters with no issue with a small umbilical hernia.
Some puppies experience more tug and get a larger hernia. If this is much larger than the pinky finger then we have it fixed at age 10 weeks if the puppy is not too tiny (in those cases we wait longer). If your puppy had one corrected you will notice a slight scar about 3/4 to 1 inch long in the umbilical area. We do not get surgery on ones that will never be a potential issue as the anesthesia risk would not be warranted, this is why I say it can be corrected during spay or neuter if desired as they are already under anesthesia and also older at this point.
Japanese Chin can have a 'soft spot' just like a human baby until a few months old. It is usually closed by 12-18 weeks of age but can persist a little longer. Though some vets are not familiar with this breed and may feel this is a problem, it is not. It is not hydrocephalus!
Small breeds are especially prone to patella issues and though our breeding dogs are vet checked for this they can still produce this in puppy's at times. We almost never see worse than a grade 1 and that is so slight we have not ever seen nor heard back of any problems when grade 1. Anything grade 2 or higher would be disclosed before placement and if a deposit was made prior to this being diagnosed then a refund will be given or a price reduction (buyers choice). Grade 2 is still not too bad and most Japanese Chin will do just fine but grade 3 and above would be high chance of surgery needed. Fortunately we very rarely see any like this here.
Small breeds are more prone to hypoglycemia. If your puppy has had any episodes here we will alert you. However, especially when a puppy is very small (charting for under 6 pounds grown) we will be very cautious during the travel and adjustment period to their new home. This is an abundance of caution but it is far better to be safe than sorry. Hypoglycemia can be deadly if not caught in time. Karo Syrup is good to have on hand because even if not conscious; placing it on tongue and gums can revive them. Nutri-Stat (can order from Amazon) is good to give when they are not eating regularly to prevent hypoglycemia such as when traveling or if sick.