How long into pregnancy can women fly?

Pregnant women are allowed to fly as much as they please until the end of their 27th week of pregnancy. Between week 28 and week 35 of their pregnancy, they need a medical certification at check-in, stating fitness for travel on specified dates with WOW air and estimated date of birth. The certificate must be issued within 72 hours prior to the commencement of travel and cannot be older than 3 days. 

From week 36 flying is completely forbidden.   

Use this metric for guidance:   

• 1 - 27 weeks = Fly as much as you like.  
• 28 - 35 weeks = You can fly, but need a doctor's note with you 
• 36 - 40 weeks = Flying is forbidden.