Err. I dont know how to start it. Hmmm. Okay!

Doctors and experts say pregnancy period for human beings is 40 weeks or nine months.

But why in tamil nadu it is always mentioned as 10 months? I mean the pregnancy period.
We have lots of amma songs, which say that it is 10 months (Neeyum 10 maasam naanum 10 maasam, eeraindhu maadhangal karuvoadu enai thaangi etc).

Why this confusion? Or is like it in sanga kaalam it was 10 months and later it got reduced to 9 months?