18 weeks is 5 months?

Jane • FTM due on Feb 4,2016 with my princess <3 💖🌺🌸👨‍👩‍👧
So when I hit 18 weeks I started telling people I was 5 months, only because I've been reading what to expect when you're expecting(an old version I realized) it said the 5th month begins approximately 18 weeks. My friend who is due the same time I am practically kept shoving it down my throat that it starts at 20 weeks not 18 and I should stop bumping up how far along I am. I was kinda like "ummm but I wasn't :/" I didn't just pull it from out of the air, it was something I read from an old yet reliable book. I mean I'll post the chapter below, I just didn't think she needed to get so mad with me. She hasn't spoken to me since:

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