Gestational Age Calculator (LMP)
Enter the first day of your last menstrual period (LMP) to estimate current gestational age and due date.
Educational tool only. This does not replace medical advice or ultrasound-based dating from your clinician.
How a gestational age calculator LMP works
A gestational age calculator LMP estimates pregnancy length from the first day of your last menstrual period. In obstetrics, pregnancy is commonly dated from LMP, not from conception. That means when the calculator says “8 weeks pregnant,” actual fetal age is usually about two weeks less.
The standard model assumes a 28-day cycle and ovulation around day 14. If your cycle is longer or shorter, the due date can be adjusted by the same number of days away from 28.
Formula used in this calculator
- Gestational age (days) = As-of date − LMP date
- Gestational age (weeks + days) = total days ÷ 7
- Estimated due date (EDD) = LMP + 280 days + (cycle length − 28)
- Estimated conception date = LMP + (cycle length − 14)
Why LMP dating is useful
LMP-based dating is simple, fast, and widely used for first estimates. It helps with planning prenatal visits, screening windows, and broad trimester milestones.
- Quick estimate at home
- No special equipment required
- Useful before first ultrasound
- Gives an initial expected due date
When LMP can be less accurate
LMP estimates can be off when cycles are irregular, bleeding is not a true period, or ovulation happened earlier/later than expected. In many pregnancies, first-trimester ultrasound gives the most accurate dating and may revise the due date.
Common reasons for mismatch
- Irregular menstrual cycles
- Recent hormonal contraception use
- Breastfeeding-related cycle changes
- Polycystic ovary syndrome (PCOS)
- Uncertain LMP recall
Trimester guide by gestational age
- First trimester: 0 weeks to 13 weeks + 6 days
- Second trimester: 14 weeks to 27 weeks + 6 days
- Third trimester: 28 weeks onward
Frequently asked questions
Is gestational age the same as fetal age?
No. Gestational age starts at LMP; fetal age starts near conception and is usually about two weeks less.
Can this calculator diagnose anything?
No. It is for estimation only. Clinical decisions should rely on your healthcare provider, exam findings, and ultrasound when needed.
What if my due date changes after ultrasound?
That is common. Providers often use ultrasound-based dating (especially early ultrasound) if it differs significantly from LMP dating.
Important medical note
If you have pain, heavy bleeding, severe nausea/vomiting, dizziness, or any urgent concern, seek medical care promptly. Use this gestational age calculator LMP as a planning aid—not as a substitute for professional evaluation.