its always better to be safe...if you feel for any reason there is any left over placenta oxy is a god send...Ive not had to use it to induce contractions but have used it within 24 hours of whelping...
My girl China... I had given her the oxy the very next morning by the evening she was not eating, by that next afternoon her temp escalated to over 105...my vet was able to massaging the piece of placenta down and we pulled it out, I had her paws on my shoulder her back feet were on the table while he massaged her lower belly.
Here is a drug list to keep handy
http://www.peteducation.com/article....&articleid=856
unfortunately what ever you give mom goes into her milk and the babies.