What month did your wife get pregnant ?
If your wife got pregnant recently and she is March 2017 applicant who most likely will get PPR around Feb/Mar 2018.
Then you have to consider;
Assuming she gets PPR Feb/Mar 2018, will your wife likely deliver the baby before expected to land as PR in Canada (....PR visa has expiry date).
Some airlines may not allow her to fly if her pregnancy has reached advanced stage...like 5 to 6 months.
She submitted her medicals ahead which probably will expire in November 2017, if AVO does not ask her to re-medical, then good for her.
If AVO were to ask her to redo medical, she may have problem with the X-ray medical examination.......lets hope they skip that 'cos you had earlier done x-ray.
If you wife is unable to land as PR in Canada before baby birth, please do add the newborn to also receive PR visa as you do not want to sponsor your newborn later separately as a Dependent child.
The sponsoring processing timeline for Dependent is depressing......currently more than 12months...................
http://www.cic.gc.ca/english/information/times/index.asp
If you want to inform AVO that your wife is pregnant, you just need to send updated IRCC form IMM5532E - PART C - Q#10.
Since you were earlier Approved to Sponsor, not sure if this will have any effect on your Sponsor Approval.
The updated form could be sent through
http://www.cic.gc.ca/english/contacts/web-form.asp.
Additional info for Medical Exam:
http://www.cic.gc.ca/english/helpcentre/answer.asp?qnum=987&top=4
info:
https://www.canadavisa.com/canada-immigration-discussion-board/threads/how-to-add-a-new-born-to-spousal-sponsorship-application-for-permanent-residency.534211/