Estimate Due Date from Last Menstrual Period (LMP)
Enter the first day of your last period to estimate your due date, current gestational age, and trimester.
Important: This tool is for education only and does not replace medical care. Always confirm dates with your OB-GYN or midwife.
What is a pregnancy last period calculator?
A pregnancy last period calculator is a due date calculator that uses the first day of your last menstrual period (LMP) to estimate how far along you are and when your baby may be born. This method is widely used in early prenatal care because it is simple and fast.
Most calculators assume a 28-day cycle and ovulation around day 14. If your cycle is longer or shorter, the estimate can be adjusted for improved accuracy.
How the calculation works
Naegele's rule (standard method)
The classic approach is to add 280 days (40 weeks) to the first day of your LMP. In practical terms:
- Estimated due date = LMP + 280 days
- Estimated conception date ≈ LMP + 14 days
Cycle-length adjustment
If your cycle is not 28 days, this calculator adjusts your due date by adding or subtracting the difference:
- Longer cycle (for example, 32 days): due date shifts later
- Shorter cycle (for example, 25 days): due date shifts earlier
How to use this calculator correctly
- Use the first day of bleeding for your most recent normal period.
- Enter your average cycle length (28 if unsure).
- Review the estimated due date, pregnancy week, and trimester.
- Share these results with your healthcare provider at your first visit.
Understanding your results
Estimated due date
This is your best estimate for delivery based on menstrual dating. Only a small percentage of babies are born exactly on the due date.
Gestational age
Gestational age counts from your LMP, not conception. That is why early pregnancy is often said to be “2 weeks pregnant” before conception likely occurred.
Trimester breakdown
- First trimester: 0 to 13 weeks, 6 days
- Second trimester: 14 to 27 weeks, 6 days
- Third trimester: 28 weeks to birth
What can affect accuracy?
- Irregular menstrual cycles
- Uncertain LMP date
- Recent hormonal birth control use
- Breastfeeding-related cycle changes
- Ovulation that occurred earlier or later than expected
For many people, a first-trimester ultrasound provides a more precise pregnancy dating estimate than LMP alone.
When to contact a healthcare professional
Reach out promptly if you have:
- Severe pain, heavy bleeding, or dizziness
- Uncertainty about your dates and cycle history
- A history of ectopic pregnancy, miscarriage, or fertility treatment
- Questions about prenatal vitamins, medications, or symptoms
Frequently searched topics related to LMP dating
People looking for a pregnancy last period calculator often also search for: pregnancy due date calculator, gestational age calculator, how many weeks pregnant am I, and conception date estimator. These are all closely related tools, and this calculator combines the most useful outputs in one place.