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An oxy shot can be given before any pups are out.If one pup and no contractions or alot and the uterus is not contracting.
It all depends on your vet and what they find on the exam of the female in labor.SOmetimes an oxy shot is not what they need sometimes it is a calcium shot (blood work needs to be done for that).
I had a singleton litter last year.Female went into labor was all the way dialated and no contractions took her to the vet if he would not of given her an oxy shot we would of had a dead pup eventually and a dead mom for they both would of sat stalled were they were.10 min after the shot she started contracting and we had a little male.
It all depends on what is going on with the mother at the time of whelping with what drugs and procedures to do.