it depends how you count it - 6 months is only 24 weeks if you count a month as 4 weeks. Which would make a 40 week pregnancy 10 months long.
In calendar months 6 months is about 24 weeks as there are 52 weeks in a year. In fact though, it depends which months you are using as some are obviously shorter than others - this is why pregnancy terminology is in weeks! Less ambiguous.
If a "normal" pregnancy is considered to be 40 weeks long, that 40 weeks includes the 2 weeks before conception.
Whether you then want to count in days, weeks, months, lunar months, up or down is up to you and how confused you want to make the person you're talking to (I went 10 days over and really started to regrett telling everyone my EDD)