Average length of pregnancy, ovulation to birth is 268 days -- 38.3 weeks or ~8.8 months.

But we typically count pregnancy from last period (this is easier), which makes it pretty close to the round 40 that's usually cited.

> 40 weeks is 10 months from conception to birth.

A month is ~4.35 (6957/1600) weeks, so 40 weeks is ~9.2 months (69400/6957).