The Jack Russell was abandoned in a field in Chobham on June 26. She was close to death when she was found by a dogwalker who took her to RSPCA Millbrook Animal Centre.
The RSPCA rushed her to the vets where a scan showed she was expecting six puppies.
The dog, now called Penny, gave birth a day later but her puppies did not survive.
RSPCA Millbrook’s deputy manager Liz Wood said: “We were very worried for Penny when we saw her condition, she looked gravely ill and was hardly responding to us.
“We rushed her to a veterinary surgery where they discovered she was covered in fleas and maggots, was dehydrated and had critically low blood glucose levels.
It comes as the RSPCA is preparing for a surge in abandoned animals due to the coronavirus crisis.
The charity typically sees a spike in dumped pets in the summer but fears this year could be even worse as the lockdown eases and people struggle financially.
Dermot Murphy, head of the RSPCA’s animal rescue teams, said: “During lockdown we’ve seen pets become a source of comfort and support for people and it appears many people have taken on new animals.
“Fortunately during this time we’ve dealt with fewer abandoned pets however we are worried that as lockdown eases, people return to work, go on holidays or struggle financially we will be facing a massive surge of animal abandonments.
“Sadly summer tends to bring with it a surge in abandoned animals.
“We don’t know why but it may be a combination of the warmer weather making people feel less guilty about dumping a pet to fend for themselves and people doing away on holiday abandoning pets instead of arranging care for them.”
Since the lockdown came into force in March the RSPCA has received 3,492 reports of abandoned animals.
The huge total includes 1,509 dogs, 1,165 cats, 299 small animals such as hamsters, guinea pigs and ferrets and 275 exotic pets.
Mr Murphy added: “This is the toughest year yet for the RSPCA despite the huge challenges, our amazing teams have been continuing to rescue animals throughout this crisis.
“I’d urge anyone struggling with their pet to ask for help. Animals have been there to help us through the crisis, please don’t abandon them now.”
The RSPCA launched an emergency appeal in April to keep its rescuers on the frontline during the coronavirus pandemic.