LMP Calculator
Enter the first day of your last menstrual period to calculate your due date, gestational age, current trimester, and upcoming pregnancy milestones.
How It Works
This calculator uses Naegele's Rule, the standard method used by healthcare providers worldwide. The formula is straightforward: add 280 days (40 weeks) to the first day of your last menstrual period (LMP).
While conception typically occurs around day 14 of a 28-day cycle (approximately 2 weeks after your LMP), gestational age is conventionally counted from the LMP date. This means that at the time of conception, you are already considered approximately 2 weeks pregnant.
The calculator provides:
- Gestational age in weeks and days since your LMP
- Due date based on Naegele's rule (LMP + 280 days)
- Current trimester — first (weeks 1-12), second (weeks 13-26), or third (weeks 27-40)
- Estimated conception date (LMP + 14 days)
- Key milestones including first heartbeat, anatomy scan, viability, and full term dates
Frequently Asked Questions
What is LMP and why is it used to calculate due dates?
LMP stands for Last Menstrual Period — specifically the first day of your most recent period before becoming pregnant. It is used because most women know their LMP date but not their exact conception date. Naegele's rule adds 280 days (40 weeks) to the LMP to estimate the due date. This method assumes a 28-day cycle with ovulation on day 14.
How accurate is the LMP-based due date?
LMP-based due dates are a good initial estimate but are most accurate for women with regular 28-day cycles. Only about 5% of babies are born on their exact due date. Most are born within 1-2 weeks before or after. If your cycles are irregular, your doctor may adjust the due date using an early ultrasound measurement.
What is gestational age vs. fetal age?
Gestational age is counted from the first day of your LMP, which is about 2 weeks before conception actually occurs. So at 4 weeks gestational age, the embryo is really only about 2 weeks old. Fetal age (also called conceptional age) is counted from the actual date of fertilization. Medical professionals use gestational age as the standard.
What if I don't remember my LMP date?
If you don't remember your exact LMP date, your doctor can estimate gestational age through an early ultrasound, typically performed in the first trimester. Crown-rump length (CRL) measurements in early pregnancy are very accurate for dating. You can also try our due date calculator which accepts ultrasound-based dates.
Medical Disclaimer
This calculator is for informational purposes only and is not a substitute for professional medical advice. Always consult your healthcare provider for medical decisions regarding your pregnancy.