Pregnancy Due Date Calculator (LMP)
Enter the first day of your last menstrual period (LMP) and your average cycle length to estimate your expected due date.
This tool gives an estimate only. Always confirm timing and prenatal milestones with your healthcare provider.
How this last period due date calculator works
This calculator estimates your expected due date (EDD) using the first day of your last menstrual period (LMP), which is the most common starting point in early pregnancy dating. Most pregnancy timelines are measured as 40 weeks (280 days) from LMP for a typical 28-day cycle.
If your cycle is longer or shorter than 28 days, the estimate can be shifted accordingly. For example, a 30-day cycle often ovulates later than a 28-day cycle, so the estimated due date may be pushed out slightly.
What your results mean
1) Estimated due date
The due date is a target date, not a guarantee. A full-term birth can happen over a range of dates, and only a small percentage of people deliver exactly on the due date.
2) Current gestational age
Gestational age counts from your LMP, not from conception. So if you are “8 weeks pregnant,” that generally means it has been about 8 weeks since the first day of your last period.
3) Estimated conception date
This is usually approximated around ovulation, which often occurs about 14 days before your next period. This date is only an estimate, especially if cycles are irregular.
4) Trimester markers
Pregnancy is commonly divided into three trimesters. The calculator provides useful milestone dates so you can plan appointments, tests, and personal timelines.
Formula used (Naegele's rule + cycle adjustment)
- Base method: LMP + 280 days
- Cycle adjustment: Add or subtract days based on cycle length difference from 28 days
- Example: If your cycle length is 31 days, add 3 days to the standard estimate
This is a practical method used widely in pregnancy due date calculators, but it is still an estimate. Early ultrasound may provide a more precise clinical dating in many cases.
Why estimated due dates can change
Irregular cycles
If your cycle varies from month to month, ovulation timing may also vary, reducing LMP-based accuracy.
Uncertain LMP
If you are not sure of the first day of your last period, dating can be less precise. In this case, your provider may rely more heavily on ultrasound.
Early ultrasound updates
During prenatal care, your due date may be adjusted based on ultrasound measurements, especially in the first trimester. This is common and does not necessarily indicate a problem.
Tips for using your due date estimate
- Track your appointments and screening windows early.
- Use your estimated date to plan maternity leave and support at home.
- Keep expectations flexible—delivery may happen before or after the exact date.
- Discuss any bleeding, pain, or concerning symptoms with your clinician immediately.
Frequently asked questions
Is this calculator accurate?
It is generally accurate for regular cycles and known LMP dates, but it remains an estimate. Clinical confirmation is essential.
Can I use this if my cycle is not 28 days?
Yes. Enter your average cycle length. The tool adjusts the due date based on the difference from a 28-day cycle.
What if my due date has already passed?
The calculator will still show the estimated date and indicate that the due date is in the past. Contact your provider for guidance if you are currently pregnant and past your due date.
Important note
This page is for educational use and personal planning. It does not diagnose, treat, or replace professional medical care. For individualized advice about pregnancy dating, prenatal testing, or labor timing, speak with your healthcare provider.