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Result
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Formula
Pregnancy weeks ≈ days since last menstrual period ÷ 7
Example
If LMP was 12 weeks ago, the estimate is around 12 weeks pregnant.
How are pregnancy weeks estimated from the last period?
Count the days from the last menstrual period to the reference date, divide by 7, and round the display to one decimal. If the period was 84 days ago, the estimate is 12.0 weeks. That matches the idea in the worked example.
Dating from the last period is a common teaching method. It assumes a regular cycle and a known first day. Ovulation, a long cycle, or an uncertain date will move the real gestational age.
The page does not calculate a due date, a trimester label, or a conception date. It does not store the dates you type.
What should this estimate not be used for?
Do not use it to diagnose a pregnancy, to decide on care, or to replace an ultrasound or a clinician’s date. Bleeding, pain, or uncertainty about dates needs personal medical advice, not a web formula.
If the reference date is before the period date, the day count is floored at zero, so the page will not show negative weeks.
How to use this calculator
- Enter the last menstrual period. Use the first day of that period as the start date. A later day will shift the estimate.
- Enter the reference date. This is the “as of” date, often today. The page counts forward from the period date.
- Read weeks, and treat them as approximate. The result is days ÷ 7. A clinician may date a pregnancy differently.
Questions about Pregnancy Weeks Calculator
Is this a due-date calculator?
No. It only divides days since the last menstrual period by 7. It does not add 280 days or show an estimated date of delivery.
Which day of the period should I enter?
The first day. Entering a later day makes the estimate shorter than the usual last-period method.
Why might a scan show a different week?
Ultrasound dating and an irregular cycle can disagree with a last-period count. The scan and the clinician’s note are the ones to follow for care.
Does the page give medical advice?
No. It is approximate arithmetic for learning. It is not a diagnosis or a care plan.
Read the guide
The pregnancy-weeks page counts days from the first day of the last menstrual period to a reference date, then divides by 7 and shows one decimal place. It does not calculate a due date, a trimester, or a conception day. It is not medical advice.
Open “Pregnancy weeks from the last menstrual period”What this helps you do
- Estimate Pregnancy Weeks from the inputs you already have.
- Check the result against the formula and the worked example.
- Change one value at a time and compare the new estimate.
Limits to keep in mind
- The figure is only as complete as the inputs on this form.
- Rounding, fees, and local rules can change the real-world number.
- Use it to learn and compare. Confirm important finance or health decisions with a qualified professional.