When should I tell my boss?

Laura β€’ πŸ’πŸ’‘ 01.18.2014 πŸŒˆπŸ‘ΆπŸŽ€ 9.20.2017 πŸ€°πŸ‘Ά due 8.30.2019
Luckily I do not work on Mondays, and all my appointments so far are being scheduled for Mondays. But when should I tell my boss? I'm kind of scared because I am a 1099 contract employee and because well, he doesn't have kids. Should I wait until after my ultrasound appointment on Oct 17? Or should i tell him right away?? I'm 4wks 6days and I feel like I could wait a little longer before spilling the beans...