Yeah is 18weeks. All my books say it and also ive looked it up on the internet and it said 18weeks :-)
i was just at babycenter and they said 18 weeks (that is where i am so that was just a happy coincidence) my baby books all say that too. so i guess it depends where you look but technically if you say 4 weeks is a month then 5 months is 20 weeks but i don't know. perhaps ask your doctor if they could write it out for you so you would know in accordance to how they calculate things?