Pregnancy Due Date & Gestational Age Calculator
Understanding Pregnancy Due Date and Gestational Age
This calculator estimates your baby’s due date, current gestational age, and trimester. It uses the same basic principles that clinicians use in everyday prenatal care: counting from the first day of your last menstrual period (LMP), adjusting for your usual cycle length, or using an early pregnancy ultrasound. It can also work from a known conception date in cases such as assisted reproduction.
Remember that a due date is an estimate, not a promise. Only a small minority of babies arrive exactly on their due date. Most are born within a window of a couple of weeks before or after. The goal of due date and gestational age calculations is to provide a shared timeline for monitoring fetal development, planning tests and visits, and preparing for birth.
Key Definitions
- Estimated Due Date (EDD): The predicted date when labor is most likely to begin, assuming an uncomplicated, singleton pregnancy.
- Gestational age: How far along the pregnancy is, usually expressed in weeks and days, counted from the first day of your last menstrual period.
- Fetal age: The age of the developing baby from conception. This is typically about two weeks less than gestational age in a regular 28-day cycle.
- Trimester: One of three broad phases of pregnancy, defined by week ranges. Clinical guidelines can vary slightly, but the week ranges below are widely used.
How This Due Date Calculator Works
The calculator supports three common methods:
- From Last Menstrual Period (LMP): Uses the first day of your last menstrual period and your usual cycle length.
- From Conception Date: Uses the date you believe conception occurred (for example, ovulation tracking or assisted reproduction procedures).
- From Ultrasound (Dating Scan): Uses the date of an early pregnancy ultrasound and the gestational age measured at that scan.
When you choose a method in the form, interpret the generic “Enter Date” field as follows:
- If you select LMP, enter the first day of your last menstrual period.
- If you select Conception Date, enter the date you believe conception occurred.
- If you select Ultrasound, enter the date of the ultrasound scan; the underlying calculation assumes that the scan provided an accurate gestational age on that date.
Naegele’s Rule for LMP-Based Due Dates
For pregnancies dated from LMP, a traditional method called Naegele’s rule is commonly used. It assumes a 28-day menstrual cycle with ovulation on day 14 and calculates a due date 280 days (40 weeks) after the first day of the last period.
The classic Naegele’s rule can be written as:
In calendar terms, clinicians often express this as: subtract three months from the LMP date, then add one year and seven days (or 14 days, depending on convention and month length). The calculator implements this logic and adjusts to the correct calendar date.
Adjusting for Menstrual Cycle Length
Not everyone has a 28-day cycle. If your natural cycle is longer or shorter, ovulation (and therefore conception) usually happens later or earlier than day 14. To approximate this, an adjustment can be added to Naegele’s rule based on your reported cycle length.
A simple way to represent this adjustment is:
Here, Cycle is your usual cycle length in days. If your cycle is 30 days, the calculator adds roughly two days to the standard 280 days. If it is 26 days, it subtracts about two days. This is still an approximation, because individual ovulation patterns and hormonal factors can vary.
From Conception Date
If you know a reliable conception date (for example, the date of egg retrieval and embryo transfer or a precisely tracked ovulation), the calculator counts forward about 266 days (38 weeks) from that date to estimate the due date. This reflects the typical length of pregnancy from conception to birth.
In formula form:
From Ultrasound (Dating Scan)
Early pregnancy ultrasound is often the most accurate way to estimate gestational age, especially in the first trimester. During a dating scan, measurements such as crown–rump length are compared with reference charts to estimate how many weeks pregnant you are on the day of the scan.
Conceptually, the calculator treats the ultrasound date as a fixed point with a known gestational age. It then adds the remaining weeks to reach 40 weeks.
If an early ultrasound (before about 13 weeks) differs from your LMP-based due date by more than a few days, many clinicians will rely on the ultrasound-based date instead, because it is less affected by uncertain LMP recall or irregular cycles.
Gestational Age, Trimesters, and Your Results
Once the tool has an estimated due date, it can work backward or forward from today’s date to estimate your current gestational age and trimester. Gestational age is calculated as the time between the start of the pregnancy (LMP or equivalent) and today.
Trimesters are broad groupings of weeks used to structure prenatal care and explain developmental milestones. A common breakdown is:
- First trimester: 0 weeks 0 days through 13 weeks 6 days
- Second trimester: 14 weeks 0 days through 27 weeks 6 days
- Third trimester: 28 weeks 0 days through delivery
The “Trimester Focus” option in the form lets you emphasize information from a particular trimester in the results, such as typical symptoms, appointment timing, or key tests, without changing the underlying due date calculation.
Interpreting Your Calculator Results
Your output will typically include:
- Estimated due date: The best estimate based on the method you selected.
- Current gestational age: For example, “21 weeks 3 days,” based on today’s date.
- Current trimester: First, second, or third, based on the week ranges above.
- Pregnancy timeline highlights: Approximate dates when trimesters start and end, and when common screening tests are often scheduled.
Treat this timeline as a planning guide, not as a rigid schedule. Your own clinician may adjust the plan based on your health history, ultrasound findings, and local guidelines.
Worked Example
Imagine your last menstrual period started on 5 May 2025, and your usual cycle length is 30 days.
- Start with Naegele’s rule: add 280 days to 5 May 2025. That gives an unadjusted due date around 9 February 2026 (depending on month length handling).
- Adjust for your 30-day cycle: this is 2 days longer than 28, so add 2 days. The adjusted due date becomes about 11 February 2026.
- If today is 1 September 2025, count the time from 5 May 2025 to 1 September 2025. That is about 17 weeks 6 days of gestation.
- At 17 weeks 6 days, you are in the second trimester by standard definitions.
The calculator automates all of these steps, but understanding the logic can make it easier to compare results you see in different apps, notes, or ultrasound reports.
Comparison of Dating Methods
| Method | What You Enter | Typical Use | Approximate Accuracy* | Best For |
|---|---|---|---|---|
| LMP + cycle length | First day of last period; usual cycle length | Standard starting point when LMP is known and cycles are fairly regular | Often within about 1 week if LMP is remembered accurately | People with predictable cycles and clear recall of their last period |
| Conception date | Estimated or known conception date | Assisted reproduction, ovulation tracking, or very precise cycle monitoring | Often within about 3–5 days when conception timing is certain | People who know the exact timing of ovulation or embryo transfer |
| Ultrasound dating scan | Date of scan; gestational age measured at scan | Used to confirm or refine LMP-based dates | ±3–5 days in first trimester, then less precise later | Irregular cycles, uncertain LMP, or conflicting dates |
*Accuracy ranges are approximate and vary between individuals and clinical settings.
Limitations, Assumptions, and Medical Disclaimer
This calculator makes several simplifying assumptions to provide a clear, easy-to-use estimate:
- It assumes a singleton pregnancy (one baby). Twins or higher-order multiples often deliver earlier than 40 weeks.
- It assumes there are no planned inductions or scheduled cesarean births that would change the timing of delivery.
- It assumes that your LMP date, cycle length, or conception date are entered accurately to the best of your knowledge.
- It uses average gestational lengths (280 days from LMP or 266 days from conception). Many healthy pregnancies end before or after these averages.
- For ultrasound-based estimates, it relies on typical accuracy ranges. Individual scans can vary depending on timing, equipment, and technique.
Important: This tool is for general education and planning only. It does not provide a medical diagnosis, does not replace prenatal care, and is not a substitute for advice from your own midwife, obstetrician, family doctor, or other qualified health professional.
Always discuss your estimated due date, ultrasound results, and any questions or concerns with your clinician, especially if:
- You have irregular or very long/short cycles.
- You are carrying twins or more.
- You have a history of preterm birth, pregnancy complications, or chronic medical conditions.
- Your ultrasound date and LMP date do not agree.
Common Questions
How accurate is my due date?
Even with the best methods, your due date is an estimate. Research suggests that only a small percentage of babies are born on their exact due date. Many arrive within about two weeks before or after. Due dates are most helpful as a way to schedule care and anticipate a general birth window.
Why is my due date different from my ultrasound?
Differences between LMP-based and ultrasound-based dates are common. LMP-based estimates can be off if your cycles are irregular, if ovulation happened earlier or later than expected, or if the LMP date is hard to remember. Early ultrasound measurements directly assess fetal size, which often gives a more reliable estimate in the first trimester. Your clinician may update your official due date if the difference is large enough.
Can my due date change later in pregnancy?
Once a due date is set (especially if it is based on an early ultrasound), many clinicians prefer not to change it unless there is a strong reason. Later ultrasounds are less accurate for dating because babies grow at different rates. Large differences between expected and measured size later in pregnancy may prompt further evaluation but do not necessarily mean the original due date was wrong.
What to Do After You Know Your Due Date
After you have an estimated due date and gestational age, you can use that information to plan your care and preparation:
- Schedule prenatal visits: Many care plans start with a first visit around 8–10 weeks, then become more frequent in the third trimester.
- Anticipate tests and screenings: Some blood tests and screening options are only available in certain week ranges. Knowing your gestational age helps time these appropriately.
- Track symptoms and changes: Common experiences such as nausea, feeling movement, or changes in sleep often follow trimester patterns, but every pregnancy is different.
- Plan work and family leave: Many people use their estimated due date as a reference point for discussing leave schedules and support plans.
Use this calculator as a starting point, and always pair it with ongoing conversations with your healthcare team. They can interpret your specific situation in light of guidelines from major obstetric organizations and your own medical history.