This is the info provided to me by QR when I travelled last May of this year:

From the beginning of the 29th week up to the end of the 32nd week:
- For Single/Complicated free pregnancy, you will need a medical certificate from a qualified Doctor. (The medical certificate template can also be downloaded from www.qatarairways.com.)
- A Multiple or Complicated pregnancy would require medical clearance from Qatar Airways. Fill the Medical Information (MEDIF) form (Available at Qatar Airways office) by your attending Doctor and submit it to the local Qatar Airways office for processing.

From the beginning of the 33rd week up to the end of the 35th week:
- For Single/Complication free pregnancy, you will require medical clearance from Qatar Airways through MEDIF.
- A Multiple or Complicated pregnancy will not be accepted for travel on board Qatar Airways flights

Any pregnancy case, from the beginning of the 36th week will not be accepted for travel on Qatar Airways.

Hope this help. Regards.