Due Date Calculator
Free online due date calculator based on your last menstrual period. Uses Naegele's Rule for accurate estimation, shows current pregnancy week, trimester timeline, and key milestone dates.
Due Date Calculator
What is a Due Date Calculator?
A due date calculator estimates the expected delivery date of your baby based on the first day of your Last Menstrual Period (LMP). It uses the medically established Naegele’s Rule, which calculates the due date by adding 280 days (40 weeks) to your LMP date.
For those with menstrual cycles that differ from the typical 28-day cycle, our calculator includes a cycle length adjustment feature to provide a more accurate estimate. Keep in mind that the actual delivery date typically falls within 2 weeks of the estimated due date in about 80% of pregnancies, while only 4-5% of babies are born exactly on their due date.
Who Is This Tool For?
- Newly Pregnant Women – Those who have just received a positive pregnancy test and want to estimate their due date
- Pre-Doctor Visit Preparation – Women who want to have an approximate due date before their first prenatal appointment
- Maternity Leave Planning – Working professionals who need to plan their maternity leave timing
- Family Planning – Couples considering how the due date aligns with holidays, anniversaries, or other family events
- Baby Preparation Shopping – Parents-to-be planning when to start purchasing baby essentials
Key Features
- Real-time Calculation – Your due date is calculated instantly as soon as you enter your LMP. No need to click any buttons – results appear immediately when you select a date.
- Cycle Length Adjustment – If your average cycle isn’t 28 days (ranging from 21-35 days), enter your actual cycle length for a more accurate due date estimate.
- Visual Pregnancy Timeline – See your current pregnancy progress and trimester at a glance with our intuitive timeline visualization.
- Key Milestone Dates – View important dates including when you can take a pregnancy test, trimester transitions, and when you reach full term.
- Save as Image – Download your results as a PNG image to share on social media or keep as a memory.
- Copy Results – Easily copy your results as text to share via messaging apps or save in your notes.
How to Use
- Enter Your LMP Date – Select the first day of your most recent menstrual period. If you’re unsure of the exact date, enter your best estimate.
- Adjust Cycle Length – The default is 28 days. If your average cycle differs, adjust the number (typically between 21-35 days).
- View Your Results – Your due date, current week, and days remaining will be displayed automatically. The timeline shows your current position in pregnancy.
- Save or Share – Save your results as an image or copy the text to share with family, friends, or healthcare providers.
How We Calculate Your Due Date
Naegele’s Rule
Proposed by German obstetrician Franz Naegele in 1812, this formula remains the worldwide standard for estimating due dates.
- Basic Formula: LMP (Last Menstrual Period) + 280 days (40 weeks)
- Quick Calculation: Subtract 3 months from LMP, add 7 days, then add 1 year
Cycle Length Adjustment
Naegele’s Rule assumes a 28-day cycle. For different cycle lengths, we apply this adjustment:
- Adjustment Formula: Due Date + (Actual Cycle Length – 28 days)
- Example: If your cycle is 32 days, add 4 days to the due date
- Example: If your cycle is 25 days, subtract 3 days from the due date
Understanding Trimesters
| Trimester | Weeks | Key Developments |
|---|---|---|
| First Trimester | 1-13 | Fertilization, implantation, organ formation, morning sickness |
| Second Trimester | 14-27 | Stable period, first movements felt, gender can be determined |
| Third Trimester | 28-40 | Rapid growth, final preparations, full term at 37 weeks |
How Accurate Is the Due Date?
A due date is an “estimate” rather than an exact prediction. Understanding the statistics around actual delivery dates can help set realistic expectations:
- Born exactly on due date: About 4-5%
- Within 1 week of due date: About 50%
- Within 2 weeks of due date: About 80%
- Within 3 weeks of due date: About 90% or more
First-time mothers tend to deliver slightly past their due date, while experienced mothers often deliver a bit earlier. An ultrasound-based due date may be more accurate than one calculated from LMP, so if your doctor provides a revised date based on ultrasound measurements, use that as your reference.
Frequently Asked Questions
What if I don’t remember my exact LMP date?
You can enter an approximate date to get an estimated due date. However, for a more accurate calculation, we recommend confirming your dates through an ultrasound at your prenatal appointment. Early ultrasounds (8-12 weeks) are particularly accurate at determining gestational age based on fetal measurements.
How do I calculate if my periods are irregular?
Try to average your cycle lengths from the past few months. For example, if your last three cycles were 28, 32, and 30 days, enter 30 days as your average. If your cycles vary significantly (e.g., from 21 to 40 days), an ultrasound-based due date will be more reliable than LMP-based calculations.
Is pregnancy always exactly 280 days?
Some studies suggest the average pregnancy is actually 282-283 days (40 weeks plus 2-3 days). However, the medical community uses 280 days (40 weeks) as the standard for consistency. A delivery anywhere between 37 and 42 weeks is considered within the normal range.
How is the due date calculated for IVF pregnancies?
For IVF (in vitro fertilization) pregnancies, the due date is calculated from the embryo transfer date. For a Day 3 embryo transfer, add 263 days; for a Day 5 blastocyst transfer, add 261 days. Your fertility clinic will provide the most accurate due date based on your specific treatment protocol.
Is gestational age the same as fetal age?
No, they’re different. Gestational age is counted from the first day of your LMP, while the actual conception (fertilization) occurs about 2 weeks later. So if you’re 10 weeks pregnant (gestational age), your baby’s actual age (since conception) is about 8 weeks. Healthcare providers typically use gestational age when discussing pregnancy progress.
What if my doctor’s due date is different from this calculator?
If your doctor has provided a due date based on ultrasound measurements, particularly the crown-rump length (CRL) measured in early pregnancy, use that date as your reference. Ultrasound-based dating can be more accurate than LMP calculations, especially if your cycle is irregular. This calculator provides a useful estimate, but your healthcare provider’s assessment should take priority.