When should I disclose my pregnancy? Small Business

I was recently offered a position with a small business and plan on accepting the offer. By the time I would start, I’ll be 21 weeks pregnant. The business is made up of about 7 people. From what I understand, to have anti-discrimination protection, the company has been compromised of at least 15 people. There is also a 90 day introductory period. So they could legally fire me because I’m pregnant.
The company does seems really great and everyone I have met, even the owners, are amazing and nice people. But I am obviously nervous to let them know and feel guilty coming into this small business with this news. Do any advice is greatly appreciated!