When does the 5th month start?

Does it start at 17 weeks or 18 weeks? I"m 16 weeks now, and I really wnat to know which week I"ll be starting my 5th month. I think it's 17 weeks, but I went to baby-gaga.com, and they enunciate it's 18 weeks. I always thoguht it was 17 weeks, so I want to know hwat it really is. Thanks.
It's at 18 weeks, this is what I use.

http://baby2see.com/development/week5.ht…
your 5 month start at 20weeks you are pregnant for 10 month not 9 month i am 37weeks i am 9 month and a weeks pregnant
Answers:    According to the book "what to expect when your expecting" tour fith month starts at 18 weeks and continues until the end of your 22 week. Month 6 starts at 23 weeks and continues till the run out of your 27 week. Month 7 is from 28 weeks to the end of 31 weeks. Month 8 is 32 weeks to the end of 35 weeks. And month 9 is 36 weeks until delivery. Hope that help.

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