Pregnancy Calculator
Calculate your due date, current week, trimester, and key pregnancy milestones
What Is a Pregnancy Calculator?
A pregnancy calculator estimates your due date, current pregnancy week, and trimester based on your last menstrual period (LMP), conception date, or a due date you already have. It also maps out key milestones across the pregnancy so you can see what is coming up next.
How the Dates Are Calculated
From LMP: Due Date = LMP + 280 days (adjusted for cycle length); Conception ≈ LMP + (cycle length − 14) days
From Conception Date: Due Date = Conception + 266 days; LMP ≈ Conception − 14 days
From Due Date: LMP ≈ Due Date − 280 days; Conception ≈ Due Date − 266 days
Explanation of Every Input
Calculation Method
Choose whichever date you know most reliably. LMP is the standard clinical default; Conception Date is more precise if known (for example, from ovulation tracking or IVF); Due Date is useful if a provider has already given you one.
Average Cycle Length
The standard 280-day estimate assumes a 28-day cycle with ovulation on day 14. If your average cycle is longer or shorter, this shifts both the estimated conception date and due date to better reflect your actual ovulation timing — used only with the LMP method.
Pregnancy Trimesters
First Trimester (Weeks 1-12)
Early development stage. Morning sickness and fatigue are common; major organs begin forming.
Second Trimester (Weeks 13-26)
Often the "honeymoon phase" — energy typically returns, and fetal movement usually becomes noticeable.
Third Trimester (Weeks 27-40)
Final stage — the baby gains weight rapidly and organs, especially the lungs, continue maturing for birth.
Advantages of Using This Calculator
- Supports three different calculation methods depending on what you know.
- Adjusts for your actual average cycle length, not just the 28-day default.
- Maps out key milestones (viability, full term, and more) with estimated dates.
- Visualizes your progress across all three trimesters at a glance.
Limitations
- All results are estimates — only about 5% of babies arrive on their exact due date.
- Cannot account for irregular cycles beyond the average length you provide.
- Does not replace ultrasound dating or professional prenatal care.
Common Mistakes to Avoid
- Treating the due date as a guaranteed delivery date rather than an estimate.
- Forgetting to adjust cycle length if your cycles are notably longer or shorter than 28 days.
- Using this calculator as a substitute for medical advice or diagnosis.
Worked Examples
Calculating from LMP
Input: LMP: January 15, 2025 (28-day cycle)
Estimated Conception: January 29, 2025
Estimated Due Date: October 22, 2025
The most common method — conception is estimated 14 days after the LMP, and the due date is 280 days after the LMP.
Calculating from Conception Date
Input: Conception Date: January 29, 2025
Estimated Conception: January 29, 2025
Estimated Due Date: October 22, 2025
If you know your conception date precisely, the due date is calculated as 266 days later — the same result as the LMP method above, just anchored to a different known date.
Calculating from a Known Due Date
Input: Due Date: October 22, 2025
Estimated Conception: January 29, 2025
Estimated Due Date: October 22, 2025
If your provider already gave you a due date (for example, from an early ultrasound), this method back-calculates the estimated LMP and conception date for reference.
Longer Cycle Adjustment
Input: LMP: January 15, 2025 (32-day cycle)
Estimated Conception: February 2, 2025
Estimated Due Date: October 26, 2025
A longer average cycle shifts ovulation later, so both the estimated conception date and due date move about 4 days later than the standard 28-day assumption.
Frequently Asked Questions
How accurate is the due date calculation?
Only about 5% of babies are born on their exact estimated due date — most arrive within two weeks before or after. This calculator provides a standard clinical estimate, not a guarantee, and your healthcare provider may adjust it based on ultrasound measurements.
What is the average length of pregnancy?
A full-term pregnancy is typically calculated as 280 days (40 weeks) from the first day of your last menstrual period (LMP), even though actual conception usually occurs about two weeks after the LMP date.
Which calculation method should I use?
Last Menstrual Period (LMP) is the most common method and what most healthcare providers use by default. Use Conception Date if you know it precisely (for example, from fertility tracking or IVF), or Due Date if you already have one confirmed by a provider and want to see your current week.
What does average cycle length change about my results?
The standard 280-day calculation assumes a 28-day cycle with ovulation around day 14. If your average cycle is longer or shorter, entering it in Advanced Options shifts the estimated conception date and due date accordingly, since ovulation timing shifts with cycle length.
What happens during the first trimester (weeks 1-12)?
The first trimester covers early fetal development, including formation of major organs and body structures. Morning sickness, fatigue, and breast tenderness are common during this stage for many pregnant people.
What happens during the second trimester (weeks 13-26)?
Often called the "honeymoon phase," energy levels frequently improve, nausea typically eases, and fetal movement usually becomes noticeable. The anatomy scan, which checks fetal development in detail, is commonly scheduled around week 20.
What happens during the third trimester (weeks 27-40)?
The third trimester is the final stretch, during which the baby gains most of its birth weight and the lungs continue maturing in preparation for birth. This stage often includes more frequent prenatal visits and preparation for delivery.
What does "full term" mean?
Full term is generally defined as 39 weeks 0 days through 40 weeks 6 days of pregnancy. Babies born from 37 weeks 0 days through 38 weeks 6 days are considered "early term," and there are corresponding categories for late-term and post-term pregnancies.
What is the significance of week 24 (viability)?
Around 24 weeks is commonly cited as the threshold of viability, meaning a baby born at this point has some chance of survival with intensive neonatal care, though outcomes improve substantially with each additional week. This is a general clinical reference point, not a personal prediction.
Can this calculator tell me if something is wrong with my pregnancy?
No — this calculator only estimates dates and milestones based on the information you enter. It cannot assess your health or your pregnancy's progress, so any concerns about symptoms or development should go to your doctor or midwife.
Why do my LMP-based and conception-based calculations differ slightly?
The LMP method assumes conception occurs about two weeks after your last period starts, while the conception-date method uses your actual known conception date directly. If your cycle is not exactly 28 days, entering your real cycle length narrows this gap.
How is "days pregnant" calculated?
Days pregnant is measured from the first day of your last menstrual period to today, not from the actual conception date, which is why pregnancy is described in weeks starting from before conception even technically occurred.
Conclusion
Whether you know your last menstrual period, your conception date, or an existing due date, this calculator gives you a clear estimate of your current week, trimester, and the milestones ahead. Use it as a helpful planning guide alongside — not instead of — regular checkups with your healthcare provider.