Pregnancy Length: 9 Months or 10? Understanding Gestation

Question
I know the gestation period is 40-42 weeks. My girlfreind swears that that calculates out to 10 months. I'm telling her that not all months have 4 weeks - some have 4+ weeks while other months have 5. This calculates out to closer to 9 months. Who is right? Is pregnancy 9 or 10 months?

Answer
Hello Eric. A normal pregnancy is 10 lunar months and 9 calendar months. Midwives and doctors speak of pregnancy lasting 40 weeks- we do not use "months" at all for just the reason you have discovered. Get used to thinking of pregnancy as 40 weeks and nothing to do with months. Hope that helps- Cheri.