Pregnancy Due Date Calculator (Based on Last Menstrual Period)
Enter the first day of your last period to estimate your due date, current gestational age, and key pregnancy milestones.
This tool provides an estimate only and does not replace medical advice. Always confirm dates with your obstetric provider.
How this last period calculator for pregnancy works
A pregnancy calculator based on your last menstrual period (LMP) uses a standard method called Naegele’s rule. In simple terms, it starts counting pregnancy from the first day of your last period and adds 280 days (40 weeks). This is the same starting point many clinicians use before ultrasound dating is available.
Because not everyone has a 28-day cycle, this calculator also lets you enter your average cycle length. If your cycle is longer or shorter, the estimated due date shifts slightly.
What your results mean
Estimated due date (EDD)
Your EDD is the day you are expected to reach 40 weeks pregnant. Most births happen within a range, not exactly on one day, so the calculator also gives an estimated birth window around that date.
Gestational age
Gestational age is shown in weeks and days from the first day of your last period. For example, 12 weeks and 3 days means 87 days have passed since your LMP.
Estimated conception date
Conception usually happens near ovulation, commonly around 14 days before your next period. If your cycle length differs from 28 days, ovulation timing can shift, and this date is adjusted accordingly.
When this pregnancy week calculator is most useful
- When you know the first day of your last period.
- When you want a quick due date estimate before your first prenatal visit.
- When tracking milestones like trimesters and term dates.
- When comparing your own timeline with appointment schedules and prenatal plans.
Limits of an LMP-based due date calculator
Even a high-quality due date calculator can only estimate. Real pregnancies vary. If your cycles are irregular, if you recently stopped hormonal contraception, or if ovulation happened earlier/later than expected, the LMP method can be off by several days (or more).
In many cases, first-trimester ultrasound provides the most accurate dating. If ultrasound and LMP dates differ, your provider may update your official due date.
Tips for better accuracy
- Use the exact first day of bleeding (not spotting) for your last period.
- Enter your true average cycle length, not your shortest or longest month.
- Update your timeline after any medical dating scan.
- Treat online calculators as planning tools, not a diagnosis.
Frequently asked questions
Is this the same as a conception date calculator?
Not exactly. A conception date calculator estimates likely ovulation and fertilization timing. An LMP calculator starts from period date and calculates forward. This tool includes both estimates.
Can I use this with irregular periods?
You can, but results are less precise. If your cycles vary a lot, ultrasound-based dating is especially important.
What if I am past my estimated due date?
That can be normal. Many pregnancies continue beyond 40 weeks. Your provider will monitor you and discuss safe next steps.
Final note
This last period calculator for pregnancy is designed to make early planning easier—whether you are trying to estimate your due date, check your current pregnancy week, or understand trimester milestones. For medical decisions, always rely on your prenatal care team.