Pregnancy Week Calculator

Gestational age, due date and trimester milestones

Quick Answer

Pregnancy Week Calculator: Gestational age, due date and trimester milestones. Weeks count from the first day of the last menstrual period (LMP).

Last menstrual period

Cycle length (days)

Estimated due date

2026-06-08

Steps (Naegele's rule)

1EDD = LMP + 280 days + (cycle − 28) = 2025-09-01 + 280 + (28 − 28)

Key milestones from LMP

6-8w	First ultrasound (heartbeat)	2025-10-13 ~ 2025-10-27
11-13w	NT scan	2025-11-17 ~ 2025-12-01
15-20w	Down syndrome screening / NIPT	2025-12-15 ~ 2026-01-19
20-24w	Anomaly scan (大排畸)	2026-01-19 ~ 2026-02-16
24-28w	Glucose tolerance (OGTT)	2026-02-16 ~ 2026-03-16
28-32w	Growth scan (小排畸)	2026-03-16 ~ 2026-04-13
37-42w	Full-term window	2026-05-18 ~ 2026-06-22

💡 Only ~4% of babies arrive exactly on their due date — full-term is 37-42 weeks, so treat the EDD as the center of a 5-week window. Naegele's rule assumes ovulation on day 14; longer cycles shift the EDD later (built into this tool). Early ultrasound (8-13w) dating is more accurate than LMP when they disagree by >7 days.

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How to Use

Enter body metrics: fill height, weight, age etc. as prompted by the Pregnancy Week Calculator.
Read value and grade: results appear instantly with an authority-based classification.
Check reference tables: confirm your range and healthy targets against the tables on this page.
Back-solve your goal: adjust inputs to see what it takes to reach a target.

Key Points

Pregnancy Week Calculator is a free online health tool — open and use, no sign-up.
All calculations run locally in your browser; your data never leaves the device.
Key capability: Gestational age, due date and trimester milestones.
Results come with formulas and reference tables so you can verify every number.

Formula & How It Works

Gestational age & due date (Naegele's rule)

Weeks count from the first day of the last menstrual period (LMP)

Due date = LMP + 280 days (40 weeks), or month −3, day +7

Trimesters: 1st 1–13 wk, 2nd 14–27 wk, 3rd 28–40 wk

Full term 37–42 weeks; preterm <37, post-term >42

Example: LMP Mar 1 → due date Dec 8; on Jun 15 you are 15 weeks + 1 day, early 2nd trimester.

FAQ

How do I use the Pregnancy Week Calculator?

Open the page and fill in the input fields — results update automatically. Gestational age, due date and trimester milestones.

Are the results accurate?

Results follow published international standards (WHO, ACE, IOM) and are for reference only.

Can this replace medical advice?

No. Always consult a qualified healthcare professional for health decisions.

Is it free? Do I need to sign up?

Completely free. No registration, login or download required.

Does it work on mobile?

Yes. The responsive design works on all devices.

Related Guides

Source & Updates

Based on WHO, ACE, IOM and other published international standards. Last updated: 2025-07

Authoritative sources:

Last updated: 2026-08-04

Formulas and content on this page are reviewed by the YiJianSuan team. For reference only — official sources prevail.

Cite this page: YiJianSuan. Pregnancy Week Calculator[EB/OL]. https://www.yijiansuan.com/en/c/health/pregnancy-week

Results provided by this tool are for reference only and do not constitute professional advice.