Gestational Age Calculator
Gestational age is counted from the first day of your last menstrual period (LMP), about two weeks before conception. Enter that date (or a conception date) to see how many weeks and days pregnant you are today and the estimated due date, 280 days after LMP. An early ultrasound dates a pregnancy more reliably than LMP; if your clinician has given you a due date, use theirs.
Before you start
Method : Calendar count from LMP with the standard 280-day due date (Naegele's rule).
Validation : Date arithmetic tested with hand-worked cases. Not a substitute for ultrasound dating.
Review : not reviewed by a qualified clinician. Educational only — not a diagnosis or a medical test.
If you have bleeding, severe pain, reduced baby movements or fluid leaking, contact your midwife or maternity unit straight away.
Your answers stay in this browser tab; nothing is sent or stored.
How to Use This Calculator
- Choose LMP or conception date.
- Enter the date.
- Press Calculate.
Methodology & assumptions
Days since LMP are counted on the calendar in your own time zone and shown as completed weeks plus days. The due date is LMP + 280 days (Naegele's rule). A conception date is converted to LMP by subtracting 14 days, which assumes a 28-day cycle.
Trimesters: first up to 13w6d, second 14w0d–27w6d, third from 28w0d. Term labels follow ACOG: early term 37w0d–38w6d, full term 39w0d–40w6d, late term 41w0d–41w6d, post-term 42w0d+.
Worked examples
LMP January 1, 2026, checked October 5, 2026: 277 days → 39 weeks 4 days, full term; due date January 1 + 280 days = October 8, 2026.
Conception January 15, 2026: LMP treated as January 1, 2026 → same due date, October 8, 2026.
Understanding your result
This counts calendar days; it does not check the pregnancy. Ultrasound dating, IVF transfer dates or your clinician's due date take priority.
Sources & further reading
- ACOG Committee Opinion No. 700: Methods for Estimating the Due Date (2017).280-day rule, first-trimester ultrasound as most accurate dating.
- ACOG Committee Opinion No. 579: Definition of Term Pregnancy (2013).Early, full, late and post-term definitions.
- NHS. 20-week screening scan.Timing of the mid-pregnancy anatomy scan.
Frequently Asked Questions
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