Pregnancy dating blog
Plain-English articles on how a pregnancy is dated from the first day of the last menstrual period (LMP): how due dates and pregnancy weeks are worked out, how the LMP relates to conception and to ultrasound dating, and what to do when the date is uncertain.
All articles
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Due dates
How to calculate your due date from your LMP
The 280-day count and Naegele's rule step by step, with worked examples and how far to trust the result.
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Pregnancy weeks
How pregnancy weeks are counted
Why the count starts at your last period, what "36+3" means, and where the trimesters begin and end.
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Due dates
How to work out your LMP from your due date
Counting 280 days back from a due date, and why the date you get may not be the day your period started.
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Conception
LMP vs. conception date
Why conception is assumed to be two weeks after the LMP, and why the real gap varies.
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Ultrasound dating
Ultrasound dating vs. LMP dating
How a dating scan estimates a due date, why it can differ from your LMP date, and which date is used.
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Uncertain dates
Irregular periods and your due date
What LMP dating assumes about your cycle, and why irregular cycles make the estimate less reliable.
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Uncertain dates
What if you don't know your LMP?
What can and can't be worked out without the date, and what to tell your doctor or midwife.
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Due dates
What is a full-term pregnancy?
Early, full, late and postterm in weeks and days, and how the categories relate to your due date.
Start with a calculator
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Due Date Calculator
Estimate your due date from the first day of your last period.
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Pregnancy Weeks Calculator
See how many weeks and days pregnant you are today.
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Conception Date Calculator
Estimate the conventional conception date from your LMP.
How these articles are written
Each article cites the published guidance and research it relies on, with numbered sources in the text and a full list at the end. The articles are published by whatislmp.com and are not reviewed by a medical professional. They explain conventions and their limits; they are not medical advice, and only a healthcare professional can date your pregnancy.