This tool is for informational purposes only and does not constitute medical advice. Always consult a healthcare professional.
How the Due Date Calculator Works
This pregnancy due date calculator uses Naegele's rule, the standard method used in obstetrics for over a century. It works by adding 280 days (40 weeks) to the first day of your last menstrual period (LMP). If your menstrual cycle is longer or shorter than the standard 28 days, the calculator adjusts the estimated due date by adding or subtracting the difference, since ovulation timing shifts with cycle length.
In addition to the estimated due date, the calculator shows your current gestational week, which trimester you are in, and key pregnancy milestones with their dates. This gives you a comprehensive timeline of your pregnancy at a glance. Remember that only about 4-5% of babies arrive on their exact due date, and a healthy full-term delivery can occur anywhere between 37 and 42 weeks.
How to Use This Tool
- Enter your LMP date — select the first day of your last menstrual period using the date picker.
- Enter your cycle length — the default is 28 days. Change it if your cycle is typically longer or shorter.
- Click Calculate Due Date — your estimated due date, current week, trimester, and milestones appear instantly.
- Review your milestones — see the dates for your 12-week scan, 20-week anatomy scan, third trimester start, and due date.
- Copy the result — click "Copy Result" to share your due date and current week information.
How It Works Behind the Scenes
The calculator takes your LMP date and adds 280 days to arrive at the estimated due date. If your cycle length differs from 28 days, the difference is added to (or subtracted from) the 280 days. For example, with a 35-day cycle, the calculator adds 287 days (280 + 7) because ovulation occurred approximately 7 days later than in a standard 28-day cycle.
The current gestational week is calculated by finding the number of days between the LMP and today, then dividing by 7. The trimester is determined based on the gestational week: first trimester is weeks 1-12, second is weeks 13-27, and third is week 28 onwards. Milestone dates are calculated by adding the appropriate number of weeks (in days) to the LMP date, adjusted for cycle length.
Frequently Asked Questions
How is the due date calculated?
The due date is calculated using Naegele's rule, which adds 280 days (40 weeks) to the first day of your last menstrual period (LMP). If your cycle length differs from the standard 28 days, the calculator adjusts by adding or subtracting the difference. This method has been the standard in obstetrics for over a century and provides a reliable estimated due date for most pregnancies.
How accurate is a due date calculator?
Due date calculators provide an estimated date based on averages. Only about 4-5% of babies are born on their exact due date, and most births occur within two weeks before or after the estimate (37-42 weeks is considered full term). An early ultrasound scan, particularly in the first trimester, can provide a more precise estimate. Always rely on your healthcare provider for the most accurate assessment of your specific pregnancy.
What are the three trimesters of pregnancy?
The first trimester spans weeks 1 through 12 and is when the baby's major organs begin to form. The second trimester covers weeks 13 through 27 and is often when parents start to feel the baby move and the anatomy scan takes place. The third trimester runs from week 28 until birth (around week 40) and is when the baby gains most of its weight and prepares for delivery.
Does cycle length affect the due date?
Yes, cycle length can affect the estimated due date. Naegele's rule assumes a 28-day cycle with ovulation occurring around day 14. If your cycle is longer, ovulation likely occurred later, which pushes the due date forward by the same number of extra days. For example, a 35-day cycle would shift the estimated due date 7 days later. Entering your actual average cycle length helps improve the accuracy of the calculation.
What are the key pregnancy milestones?
Key milestones include 12 weeks (end of first trimester, when the risk of miscarriage drops significantly and the first scan is typically scheduled), 20 weeks (anatomy scan, halfway point, when you may begin to feel the baby's movements), 28 weeks (start of the third trimester, a significant viability milestone), and 40 weeks (your estimated due date). Your healthcare provider will schedule regular appointments around these dates.
Is my data stored?
No. All calculations are performed entirely in your browser using JavaScript. Your last menstrual period date, cycle length, and all calculated results are never transmitted to any server or stored in any database. Once you close or refresh the page, all entered data is gone. Daytics takes your privacy seriously, especially with sensitive health information like pregnancy dates.
