The bitch needs to be a fairly good example of the breed, study the breed Standard and talk to others in the breed. Can you find suitable homes for the puppies? However much people have admired your bitch and those who tell you how much they would like a puppy from her, they tend to disappear when the pups are on the ground. Have you got the energy and the facilities to accommodate 8 or so active mastiff puppies at 10, 12, 14 weeks, with large appetites. At birth when they are cuddled up to mum they don't take up much room but they grow at a rapid rate and by 3 weeks need a roomy area to live in.
Breeding a litter requires a lot of thought and dedication; don't be tempted to use the dog down the road. Study pedigrees, talk to your bitch’s breeder and other breeders and owners. When you have made a short list of suitable studs, try to see as many of his off spring as you can, study their temperaments, are they over timid or equally over aggressive? Make sure both sire and dam don't carry the same faults, for no matter how good you may think your bitch is she is bound to have some faults. The stud dog owner will more than likely want to see your bitch before agreeing to the mating. Do not leave this until the bitch is in season sort it all out well before hand.
Breeding a litter is not cheap, there is a stud fee to be paid. A purpose built whelping box is an asset, with pig rails, polyester bedding, extra food for your bitch, weaning foods for the puppies. Vets bills possibly, vaccinations. There are no end of books on breeding do read some of these It is advisable to have her vaccinations boosted before she comes in season, and worm her.
Make sure the stud fee arrangements are fully understood. It is always a good idea to have this in writing and signed by both parties.
Most bitches come into season every 6 months, some longer, some rather irregularly, you will notice the signs that she is coming in, keep a careful eye so that you can spot when the first bleeding starts, most mastiffs are ready between the 12-15days. If possible have 2 matings with a day in between, it is the custom to take the bitch to the dog and arrangements should be made with the stud dog's owner. The gestation period is approximately 63 days. Some go longer others shorter.
Do not coddle/pamper her once she has been mated, go on with life as usual. Giving extra food at this time can be a mistake wait until after she has whelped, though increasing the protein content of her diet is a good idea. i.e. fish cheese eggs. Split the meals into 3 smaller ones. A heavy bitch is not an easy whelper .
Mastiffs are very prone to giving out false information and there are often very disappointed owners who were convinced their bitch was in whelp, this happens just as often to experienced breeders as to novices. If you do want to make sure so that you can prepare the whelping area beforehand ask your vet for a scan it only takes a few minutes they probably wont be able to tell you numbers but that is not a problem at least you will know there are puppies there. A scan is usually done at 30 days.
Personally I have never had a bitch scanned, 9 weeks is not long to wait and I would prefer to omit anything that can cause stress.