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Due Date Calculator

Estimate the due date, current gestational age and trimester

Start From

Date

Enter a valid date

Cycle Length

days

Enter a cycle length (20-45)

Estimated Due Date

Gestational Age

Days Remaining

Trimester

Pregnancy Week

--0%--

Due Date Calculator — Complete Guide

An estimated due date is the single most quoted number of any pregnancy, and also the most misunderstood. It is a midpoint, not a deadline: the statistical window in which most babies arrive stretches from 37 to 42 weeks, and only about one baby in twenty-five is born exactly on the day.

What this calculator does

Choose whether to start from the first day of your last menstrual period (LMP) or from the conception date, and the tool returns the estimated due date, today's gestational age in weeks and days, the days remaining, the current trimester, and a progress bar of the 40-week journey.

The method

from LMP: due = LMP + 280 days (+ cycle adjustment) from conception: due = conception + 266 days gestational age = today − LMP (measured from the LMP, not conception)

The 280-day figure is Naegele's rule, in use since the 19th century: 40 weeks from the last period, because pregnancy is conventionally dated from the LMP even though conception happens about two weeks later. That is why the conception-based route adds only 266 days. If your cycle is not 28 days, the estimate shifts: a 32-day cycle means ovulation happened around day 18, so the calculator adds the extra 4 days.

A worked example

LMP on January 10 with a 28-day cycle: due = January 10 + 280 days = October 17. On April 15 you are 13 weeks and 6 days along — the last days of the first trimester — with 185 days remaining. If the cycle were 34 days, the estimate moves to October 23.

Start point	Cycle	Estimated due date
LMP January 10	28 days	October 17
LMP January 10	34 days	October 23
Conception January 24	—	October 17

Accuracy, ultrasound and red flags

First-trimester ultrasound (before 14 weeks) is the gold standard for dating and can override the LMP estimate by a week or more.
IVF pregnancies are dated from transfer date precisely, adding 266 days to the egg-retrieval/transfer arithmetic used by your clinic.
Irregular cycles, recent hormonal contraception and breastfeeding can make the LMP route unreliable — flag that to your doctor.
Call your clinician with any bleeding, severe pain or reduced movement, regardless of what any calculator says.

Treat the due date as the centre of a four-week window. Planning around "about two weeks either side" is both statistically honest and much less stressful than waiting for a single day.

预产期计算器完整使用指南

预产期大概是怀孕后被引用最多的数字，也是被误解最多的数字。它是一个中点，不是期限：多数宝宝出生的统计窗口横跨孕 37 到 42 周，大约每 25 个宝宝里才有一个恰好落在预产期当天。

这个计算器能做什么

选择从「末次月经第一天（LMP）」还是「受孕日」起算，工具会给出预估预产期、今天的孕周（几周零几天）、距预产期天数、所处的孕期阶段，以及 40 周孕程的进度条。

计算方法

从末次月经算：预产期 = LMP + 280 天（+ 周期修正）从受孕日算：预产期 = 受孕日 + 266 天孕周 = 今天 − 末次月经（从 LMP 起算，而非受孕日）

280 天来自十九世纪沿用至今的内格勒法则：从末次月经算 40 周——虽然受孕大约发生在两周之后，但产科惯例统一按 LMP 计孕周，这也是为什么从受孕日算只加 266 天。如果你的周期不是 28 天，估算会相应平移：32 天周期意味着排卵约在第 18 天，计算器会补上这 4 天。

算例演示

末次月经 1 月 10 日、28 天周期：预产期 = 1 月 10 日 + 280 天 = 10 月 17 日。到 4 月 15 日当天孕 13 周 6 天——孕早期的尾声，距预产期还有 185 天。若周期是 34 天，预产期则顺延到 10 月 23 日。

起算点	周期	预估预产期
LMP 1 月 10 日	28 天	10 月 17 日
LMP 1 月 10 日	34 天	10 月 23 日
受孕 1 月 24 日	—	10 月 17 日

准确性、超声与预警信号

孕早期超声（14 周前）是确定孕周的金标准，可能比 LMP 推算提前或推后一周以上。
试管婴儿（IVF）按移植日期精确推算，加 266 天，以生殖中心使用的取卵/移植时间表为准。
月经不规律、近期用过激素类避孕药或哺乳期，都会让 LMP 推算不可靠——记得告诉医生。
无论计算器结果如何，出现出血、剧烈疼痛或胎动明显减少，请立即联系医生。

把预产期理解为一个四周窗口的中心点最合理。按「前后各两周左右」来安排，既符合统计事实，也比盯着某一天等发动的焦虑轻松得多。

Frequently Asked Questions

How is the due date calculated?

The standard method (Naegele's rule) adds 280 days to the first day of your last period, assuming a 28-day cycle with ovulation on day 14. If your cycle is longer or shorter, each day of difference shifts the estimate by a day. From the conception date, add 266 days instead.

How accurate is the estimated due date?

It is an estimate, not a target. Only about 4% of babies are born exactly on their due date, and around 80% arrive within the window of 37 to 42 weeks. An ultrasound in the first trimester is the most accurate way to date a pregnancy.

What are the three trimesters?

The first trimester runs to week 13, the second from week 14 to 27, and the third from week 28 to birth. The calculator shows which one you are in, along with the current gestational age in weeks and days.

Last reviewed: October 5, 2026 · How we calculate · Sources: ACOG, NIH, WHO