Pregnancy is dated from the first day of your last menstrual period — so week 1 starts before conception actually happens. Forty weeks (280 days) is the standard length; the first trimester spans weeks 1–13, the second weeks 14–27, and the third weeks 28–40+. Counting from the due date instead: the current week is simply (due date − today) subtracted from 40. This tool does both, so you can start from whichever date you're sure of.
孕期从末次月经第一天起算——所以第 1 周在真正受孕之前就开始了。40 周（280 天）为标准长度；孕早期为 1–13 周、孕中期 14–27 周、孕晚期 28–40 周以上。如果从预产期反推：当前周数 = 40 −（预产期 − 今天）。本工具两种方式都支持，从你有把握的日期开始即可。
