Gestational Age Calculator (LMP Method)
Use the first day of your last menstrual period (LMP) to estimate gestational age, due date, and pregnancy progress.
What this LMP calculator does
This gestational age calculator by LMP estimates how far along a pregnancy is based on the first day of your last menstrual period. This is the same dating approach commonly used in early prenatal care and in many clinical records.
After you enter your LMP date, the calculator provides:
- Current gestational age in weeks and days
- Estimated due date (EDD)
- Approximate conception date
- Current trimester and overall pregnancy progress
How gestational age by LMP is calculated
Standard obstetric dating
In obstetrics, pregnancy is usually dated from LMP, not from conception. That means gestational age starts about two weeks before ovulation and fertilization in a typical 28-day cycle.
The basic formulas are:
- Gestational age (days) = As-of date − LMP date
- Due date = LMP + 280 days (40 weeks)
- Approximate conception date = LMP + (cycle length − 14) days
Why cycle length matters
If your cycle is regularly longer or shorter than 28 days, ovulation may occur later or earlier. Adjusting cycle length can shift the estimated due date by a few days. For example:
- 32-day cycle: ovulation tends to occur later, so due date may shift later
- 25-day cycle: ovulation tends to occur earlier, so due date may shift earlier
How to use this calculator correctly
- Enter the first day of your last period (not the day bleeding ended).
- Set the date you want to calculate from (today is auto-filled).
- Enter your typical cycle length if it is consistent.
- Click Calculate to view results instantly.
For the most reliable estimate, use dates that you are confident about. If your cycles are highly irregular, LMP dating may be less accurate.
Understanding your results
Weeks and days format
Gestational age is displayed as weeks + days (for example, 18 weeks 3 days). This is the standard format used by most OB/GYN clinics and ultrasound reports.
Estimated due date (EDD)
The due date is an estimate, not an exact prediction. Most deliveries occur within a window around the due date, not exactly on that day.
Trimester classification
- First trimester: 0 to 13 weeks 6 days
- Second trimester: 14 to 27 weeks 6 days
- Third trimester: 28 weeks and beyond
When LMP dating may be less accurate
LMP-based dating works best for people with regular cycles and known period dates. It may be less precise in situations such as:
- Irregular menstrual cycles
- Recent hormonal contraception changes
- Uncertain or unknown LMP date
- Breastfeeding with inconsistent cycles
- Bleeding that was not a true menstrual period
In these cases, an early ultrasound (especially in the first trimester) is often used by clinicians to confirm or revise dating.
Clinical note and safety reminder
This tool is for educational and planning use. It does not diagnose pregnancy health, viability, or complications. Always rely on your prenatal care provider for official dating and medical decisions.
If you have pain, bleeding, severe nausea/vomiting, fever, reduced fetal movement (later pregnancy), or any urgent concern, seek medical care promptly.
Quick FAQ
Is gestational age the same as fetal age?
No. Gestational age starts at LMP, while fetal age starts near conception, typically about two weeks later.
Can this calculator replace ultrasound dating?
No. Ultrasound, especially early ultrasound, is often more precise when dates are uncertain or cycles are irregular.
Why is my due date different from an app or doctor visit?
Differences can come from cycle assumptions, date rounding, time zone/date parsing issues in tools, or updated dating from ultrasound measurements.