Canine Maternal Hydrops is a dangerous condition about which not a lot is known.

If you are planning on breeding your dog, be vigilant during her pregnancy for signs of an unusually large, hard belly.

Note this condition is more common in Retrievers, but is linked to large litters of 10 or more in all breeds.

It is a good idea to have an estimate early on in the pregnancy to see how many puppies are present, so you can decide if a planned C-section is advisable or not. 

No one knows the cause of CMH and none of the theories below have been proven.
It is thought it could be a mismatch of blood type between the mother and the puppies in the womb
Hereditary causes. as this accounts for a similar condition in horses and cattle
Infection, including exposure to chemicals and supplements.

Symptoms vary and bitches may show all or none of the symptoms below

Size and weight increase that is out of proportion to the expected number of puppies

Abdomen that is hard to the touch

Edema or swelling of the vulva, hocks, and feet
(If the vulva is swollen and more than double its normal size it is more than likely hydrops)

Leaking of clear fluid

Unclear, blurry x-ray at day 55-56

Ultrasound scan showing excess fluid

Heavy vomiting close to delivery

Early temperature drop and labour not starting

Decrease in condition in the last two weeks of pregnancy

There is two types of Hydrops, one with all over body edema and excess amniotic fluid and one with just too much amniotic fluid.

C-sections are necessary in all cases and sometimes a premature C-section must be performed to save the bitch, and the puppies. Most bitches will expel large amounts of fluid during c- section, which must be replaced by IV.

Treatment involves managing the symptoms systematically.

A low-sodium, Adkins-type diet and 500-1000 mg. of dandelion root twice daily can help. Weight gain should be controlled, but without starving the dam, and she can be given a spironolactone diuretic to decrease the fluid. 

Another common practice is injecting Dexamethasone to mature the lungs of the puppies faster, giving them a much great chance of survival should they be born early.  

Most importantly, the vet should give your bitch IV fluids during the C-section and give the puppies antibiotics.