Educational information only
whatislmp.com explains the conventions used to date a pregnancy, such as counting from the first day of the last menstrual period (LMP).1 Nothing on this site is medical advice, a diagnosis or a recommendation about your own care.
Do not use the information or the calculators to make decisions about your health or your pregnancy without talking to a doctor, midwife or other healthcare professional.
What the calculators do
Each calculator applies a fixed calendar convention to the one date you enter, the first day of your LMP:
- The Due Date Calculator adds 280 days.1
- The Pregnancy Weeks Calculator counts the weeks and days from that date to today's date on your device.
- The Conception Date Calculator adds 14 days, the usual assumption for a 28-day cycle with ovulation on day 14.1
Every result is an estimate based only on the date you enter. It is not your confirmed due date, your confirmed stage of pregnancy or the day conception happened.
Limits of these estimates
The conventions assume a regular 28-day cycle with ovulation on day 14. They do not account for an LMP date that is remembered inaccurately, for cycles of irregular length, or for ovulation that happens earlier or later than usual.1
- An estimated conception date cannot establish when conception actually happened, and it cannot establish paternity.
- A due date worked out from the LMP may be changed after a dating scan.2
- This site cannot interpret your scans, test results or medical records.
Only a healthcare professional can date your pregnancy.
How the content is prepared
Content is researched from published guidance and studies, for example from the American College of Obstetricians and Gynecologists (ACOG) and the NHS, and medical statements are linked to their sources on each page. The content is not reviewed by a medical professional.
If you find an error, please tell us through the contact form.
In an emergency
If you think you may have a medical emergency, contact your local emergency services.
If you have questions or concerns about your health or your pregnancy, contact your doctor, midwife or other healthcare professional. Do not delay seeking care because of something you read on this site.
Sources
- American College of Obstetricians and Gynecologists (ACOG). Methods for Estimating the Due Date (Committee Opinion No. 700). 2017; reaffirmed 2025.
- NHS (England). Ultrasound scans in pregnancy. reviewed November 2023.