Pregnancy Calculator (by Last Menstrual Period)
Enter the first day of your last period and your average cycle length to estimate your due date and current pregnancy week.
For educational use only. Always confirm dates with your healthcare provider.
How a pregnancy calculator based on last period works
A pregnancy calculator last period tool estimates your due date using the first day of your last menstrual period (LMP). This method is widely used because many people remember their period start date more easily than the exact conception date.
Most calculators use Naegele's rule: add 280 days (40 weeks) to your LMP for a 28-day cycle. If your cycle is longer or shorter, the estimate can be adjusted by adding or subtracting days.
What this calculator gives you
- Estimated due date (EDD)
- Estimated conception date
- Current gestational age in weeks and days
- Trimester milestones
- Full-term delivery window
Why gestational age starts before conception
Pregnancy dating starts on day 1 of your last period, not on the day of conception. That means when you are called "4 weeks pregnant," conception likely happened about 2 weeks earlier than that date count.
This can feel confusing at first, but it is the standard way obstetric providers track fetal growth and prenatal testing windows.
Cycle length and due date accuracy
If your cycle is close to 28 days
LMP-based due date calculations are usually a good starting estimate.
If your cycle is shorter or longer
Ovulation often happens earlier in shorter cycles and later in longer cycles. That shifts the conception date and may shift your expected due date as well.
- Shorter cycle (for example, 25 days): due date may be a few days earlier
- Longer cycle (for example, 32 days): due date may be a few days later
When ultrasound dating may change your estimate
Early ultrasound, especially in the first trimester, can provide a more precise estimate than LMP alone. Your provider may update your due date if scan measurements differ from the LMP calculation by a meaningful amount.
Important notes for irregular periods
If your periods are irregular, if you recently stopped hormonal birth control, or if you are unsure about your LMP date, this calculator can still provide a rough estimate, but clinical dating may differ.
In these cases, schedule early prenatal care. Your healthcare team can combine menstrual history, symptoms, and ultrasound findings for better accuracy.
Frequently asked questions
Is the due date exact?
No. It is an estimate. Only a small percentage of babies are born exactly on their due date. Most arrive within a normal window around that date.
What if my due date has passed?
Some pregnancies continue beyond 40 weeks. Your provider will monitor fetal well-being and discuss next steps if pregnancy goes post-term.
Can this calculator diagnose pregnancy?
No. It does not diagnose pregnancy and does not replace medical advice, lab testing, or prenatal appointments.
Bottom line
A pregnancy due date calculator from your last period is a practical first step for planning appointments, understanding trimesters, and tracking milestones. Use it for orientation, then confirm details with your OB-GYN or midwife for the safest, most accurate care.