From the last period to a 40-week due date
Pregnancy weeks are counted from the first day of the last menstrual period, or LMP, rather than from conception. That convention is roughly two weeks ahead of the actual gestational age of the embryo, which is why a baby conceived only recently is already described as two or three weeks along. The estimated due date uses Naegele’s rule: add 280 days, or forty completed weeks, to the LMP. The tool does this with plain local calendar arithmetic, so leap years and short months are handled correctly and the date agrees with the paper wheel your midwife uses.
The current position is recomputed every time the page renders from the current calendar day, and it is shown in completed weeks plus days. Trimesters follow the clinical bands used by obstetricians: the first trimester spans weeks 0 to 12, the second runs from week 13 through week 27, and the third begins at week 28 and continues to the due date at 40 weeks. The ten milestone markers follow the typical prenatal schedule: the urine or blood confirmation around week 4, the first ultrasound with a visible heartbeat at about week 8, the end of the first trimester and NIPT screening around week 12, first felt movements (quickening) near week 16, the detailed anatomy scan around week 20, the oral glucose tolerance test in the late second trimester near week 24, the start of the third trimester at 28, the baby-position check around 32, full-term preparation at 36, and the estimated due date at 40.
Reading and printing the timeline
The poster lays the full forty weeks onto a single vertical spine with tick marks every four weeks. Each trimester is tinted as its own band, milestones alternate on either side so their bilingual labels never collide, and a prominent marker labelled YOU ARE HERE rides the spine at the exact live position — before the LMP it parks at the start, and after the due date it rests at week 40 to show the baby is overdue. Dates on the axis are formatted in the local timezone directly from the browser calendar, with no server involved. When you download, the timeline is repainted at 300 dots per inch in A4, A3 or larger gallery sizes, finished with a discreet Memory Poster Studio footer, and the entire file is generated offline with no account and no watermark.
从末次月经，到 40 周预产期
孕周从末次月经第一天（LMP）开始计算，而不是从受孕那天算起。这一口径比胚胎实际胎龄大约早两周，所以刚受孕不久的宝宝，在医学上已经被称作两三周大。预产期采用内格勒规则：在末次月经日期上加 280 天，也就是完整的 40 周。工具用浏览器本地日历直接做日期加减，闰年与大小月都能正确处理，与产检时医生使用的推算转盘结果一致。
当前位置在每次画面渲染时都按当天日期重新计算，以「周 + 天」的形式显示。三孕期按产科通行区间划分：一孕期为 0 至 12 周，二孕期为 13 至 27 周，三孕期从 28 周开始直到 40 周预产期。十个关键里程碑对应常规产检节奏：4 周左右验孕或抽血确认，8 周左右首次 B 超可见胎心胎芽，12 周前后一孕期结束并可做 NIPT 无创筛查，16 周前后初次感到胎动，20 周前后进行大排畸系统超声，24 周前后进入二孕期尾声并做口服葡萄糖耐量测试，28 周三孕期开始，32 周前后检查胎位，36 周做好足月准备，40 周到达预产期。
阅读与打印时间线
海报把完整 40 周铺在一条竖向主轴上，每四周打一个刻度；三个孕期用不同色带区分，里程碑在左右两侧交替排布，双语标签互不重叠，醒目的「你在这里」标记会沿主轴停在实时位置上——早于末次月经时停在起点，超过预产期则停在 40 周表示已过预产期。轴上日期直接按浏览器本地时区与日历格式化，不经过任何服务器。下载时时间线会按 A4、A3 或更大的画廊尺寸以每英寸 300 点重绘，页脚带有低调的 Memory Poster Studio 字样，整个文件离线生成，无需账号，也没有水印。
Why is the due date 280 days instead of nine exact months?
Because pregnancy weeks start from the last period, not ovulation. 280 days is the average length of a menstrual-dated pregnancy, and Naegele’s rule — add seven days, subtract three months, add a year — is simply the same 280-day rule in another form. Only about one in twenty babies is born exactly on the estimated date.
预产期为什么是 280 天，而不是整整九个月？
因为孕周从末次月经而非排卵算起。280 天是按末次月经口径统计的平均妊娠长度，内格勒规则（加七天、减三个月、加一年）与 280 天规则其实等价。真正在预产期当天出生的宝宝只有大约二十分之一。
My cycles are irregular. Is the timeline still useful?
Yes for milestone planning, but ask your clinician about a dating scan: an ultrasound in the first trimester may adjust the due date by a week or more, and later milestones should then be read from the corrected gestational age rather than the LMP.
月经不规律，这条时间线还准吗？
用于安排产检依然有用，但请咨询医生是否做早孕 B 超核定孕周：孕早期超声可能把预产期调整一周以上，之后的里程碑应按校正后的孕周而不是末次月经来读。
Does the marker update by itself over time?
It recomputes against your device’s current date every time the page opens or you edit the LMP, so opening the same poster page next week moves the YOU ARE HERE marker forward by a week. The downloaded PNG keeps the position from the moment of export, which makes a nice series when reprinted at each trimester.
「你在这里」标记会自己往前走吗？
每次打开页面或修改末次月经日期时，它都会按设备当天日期重新计算；下周再打开同一页面，标记就会向前移动一周。下载的 PNG 会保留导出那一刻的位置，每个孕期各印一张，正好组成一组成长记录。
