Why "9 months" is really an approximation
Medical professionals track pregnancy in weeks because it's a more precise unit for monitoring fetal development and timing care milestones. Everyday conversation, on the other hand, defaults to months — but the two units don't divide into each other evenly, which is why "how many months is that?" doesn't have a perfectly round answer.
Months ≈ weeks ÷ 4.345
A full-term pregnancy at 40 weeks works out to roughly 9.2 months — slightly more than the commonly used "9 months" shorthand, which is a convenient rounding rather than an exact figure.
How trimesters line up
The first trimester runs through week 12, the second from week 13 through week 26, and the third from week 27 through delivery. These boundaries are widely used reference points for tracking which stage of development and which common symptoms are typical at any given point.