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 say it's 18 weeks. I always thoguht it was 17 weeks, so I want to know hwat it really is. Thanks.