How this due date calculator works
This due date calculator is based on the first day of your last menstrual period (LMP), which is the most common starting point used in early pregnancy dating. The standard method assumes a 28-day cycle and estimates delivery at 40 weeks (280 days) from the LMP date.
If your cycle is typically longer or shorter than 28 days, the estimate can be adjusted by adding or subtracting days. For example, if your average cycle is 30 days, ovulation often happens a little later, so the due date estimate shifts later as well.
How to use the calculator
- Enter the first day of your last period.
- Enter your usual cycle length (between 21 and 45 days).
- Click Calculate Due Date to see your estimated delivery date and pregnancy timeline.
You’ll also get useful details such as estimated conception date, current gestational age, trimester, and a full-term delivery window.
Understanding your result
Estimated due date (EDD)
Your EDD is a planning date, not a deadline. Most babies are born in a range around the due date, not exactly on that day. Being a week early or late can still be normal depending on your provider’s evaluation.
Gestational age vs. fetal age
Gestational age is measured from your LMP, while fetal age starts near conception (usually about two weeks later). Medical appointments, lab timing, and ultrasound references generally use gestational age.
Trimester tracking
- First trimester: Week 1 to 13
- Second trimester: Week 14 to 27
- Third trimester: Week 28 to birth
Why due dates can change
A due date based on LMP is often very useful, but it may be revised after an early ultrasound. Ultrasound dating in the first trimester can provide a more accurate estimate, especially when:
- Your cycle length varies from month to month.
- You don’t remember the exact LMP date.
- You recently stopped hormonal birth control.
- You have conditions that affect ovulation timing.
If your periods are irregular
If your cycles are irregular, an LMP-based calculator can still give a rough estimate, but confidence is lower. In these situations, an early prenatal visit and ultrasound are especially important for accurate dating.
If you track ovulation with LH tests, basal body temperature, or fertility apps, bring that information to your appointment. It can help your clinician compare cycle data with ultrasound findings.
Pregnancy milestone guide
- ~8 weeks: early prenatal bloodwork and first care plan discussions
- ~12 weeks: end of first trimester and common genetic screening window
- ~20 weeks: anatomy ultrasound for fetal growth and structure review
- ~24–28 weeks: glucose screening and routine monitoring
- ~36 weeks: final-month care planning and delivery preparation
Frequently asked questions
Is this due date exact?
No. It is an estimate. Birth can normally occur before or after the EDD. Your provider will track fetal growth and health to guide real-world decisions.
Can I calculate due date from conception instead?
Yes, but conception date is often uncertain unless conception timing is known (for example, through fertility treatment). LMP remains the standard starting point for most routine dating.
What if I ovulated late?
Late ovulation can push the true due date later than the basic 28-day assumption. That’s why cycle adjustment and early ultrasound are both helpful.
Important note
This page is educational and not a substitute for professional medical advice, diagnosis, or treatment. If you are pregnant or think you may be pregnant, schedule prenatal care early and follow your clinician’s recommendations.